Provisional organs: definition, significance in the development of vertebrates. Yolk sac, amnion, allantois: structure and functions
Extra-embryonic (temporary, provisional) organs are organs that are formed during embryonic development outside the body of the embryo, but take an active part in the processes of growth and development of the embryo and cease to function at birth. It must be remembered that the differentiation of these organs occurs very early, they perform specific functions already at a time when the embryo itself is represented by still poorly differentiated embryonic rudiments.
YOLK SAC.
The wall of this provisional organ is formed by extra-embryonic endoderm and extra-embryonic mesenchyme. The human yolk sac does not contain yolk, but is filled with a liquid containing proteins and salts. At the same time, the yolk sac retains the role of the first hematopoietic organ: the first blood cells and blood vessels are located in the mesenchyme of the yolk sac wall; as well as the first sex cells. Moreover, at 3-4 weeks of development in the wall of the yolk sac, as a reflection of phylogenesis, the yolk circle of blood circulation is formed, which soon becomes empty. The intestinal stalk, which connected the yolk sac with the intestine, also overgrows. The yolk sac turns into a wrinkled elongated vesicle, which is part of the umbilical cord.
ALLANTOYS.
Allantois is represented by the extra-embryonic endoderm and the extra-embryonic mesoderm of the amniotic stalk. In human development, allantois does not play a significant role and remains underdeveloped. Its role is reduced to conducting vessels from the embryo along the amniotic stalk to the chorion.
Amnion appears on the 13-14th day of development. Its wall consists of extra-embryonic ectoderm and extra-embryonic mesoderm. At first, the amnion is just a dome over a flat germinal shield. When the embryo rises above the yolk sac and the intestinal tube closes, the body of the embryo is surrounded by the amniotic membrane on all sides. In this case, the epithelium of the amnion, as it were, passes to the surface of the embryo in the place where the yolk stalk enters through the abdominal opening into the intestine. The embryo grows, the amnion cavity increases, the abdominal opening narrows, its edges form the umbilical ring, and the amnion moves further and further away from the umbilical ring and surrounds the yolk stalk. The remnants of the allantois, the vessels running along the amniotic stalk, and the yolk stalk form the umbilical cord, which is covered by the amnion.
The amnion cavity enlarges until it fills the entire space of the former blastocyst. In this case, the mesoderm of the amnion wall adheres tightly to the mesoderm of the chorion and fuses with it. The chorionic plate (general connective tissue) is formed.
The part of the amnion that grows together with the smooth chorion performs the function of secreting amniotic fluid, and the part that is adjacent to the villous chorion and covers the placenta resorbs this fluid. Thus, the amnion resembles an aquarium with a constantly changing liquid, in which the human embryo repeats the aquatic lifestyle of its ancestors. Amniotic fluid serves as protection against mechanical damage, the delicate parts of the growing embryo are not injured against each other, do not dry out and do not grow together. The aquatic environment is more thermostable, various metabolic processes go better in it, the necessary pressure is created for the development of the oral, nasal cavities, lungs, etc. Substances that are secreted by the amniotic membrane are necessary for the formation of the functions of the abdominal organs.
NUTRITION OF THE GEM. CHORION. PLACENTA.
In the early stages of development (zygote, morula), the nutrition of the embryo is autotrophic - due to the substances that the egg contained, and then - due to the liquid secretion of the trophoblast filling the blastocyst cavity.
When the number of blastomeres reaches a critical mass and the fertilization membrane is destroyed (approximately 5-6 days of development), the embryo gets the opportunity to feed on the surrounding tissues - it switches to the histiotrophic type of nutrition. Substances diffuse through the trophoblast from the fluids of the mother's body, the secretion of the uterine glands, during implantation - these are destroyed endometrial cells.
When the syncytiotrophoblast destroys the walls of the vessels of the uterus, it gets the opportunity to receive nutrients directly from the maternal blood - the histiotrophic type of nutrition is replaced by the hematotrophic one, which the embryo will have to use throughout the intrauterine life.
The trophoblast strives to increase the amount of absorbed substances from the blood, therefore it forms outgrowths - primary villi, which increase the area of contact of the trophoblast with the mother's blood. Blood flows out of the destroyed vessels, forming small lakes - lacunae, which are separated from each other by partitions - intact areas of the endometrium. Villi are located in the lacunae. The trophoblast secretes a substance that prevents blood clotting and receives the necessary nutrients and oxygen.
When cells of the extraembryonic mesoderm are evicted from the germinal shield, they are located inside the blastoderm vesicle, under the cytotrophoblast, and subsequently form a connective tissue. The villi consisting of the trophoblast (epithelium) and the underlying extra-embryonic mesoderm (connective tissue) are called secondary or chorionic villi. The chorion is thus formed at the beginning of the third week.
Soon, along the amniotic stalk, the blood vessels of the embryo grow into the chorion, which follow the connective tissue of the villi and branch there. By the end of the third week, tertiary or true chorionic villi are formed, consisting of trophoblast epithelium, connective tissue and blood vessels. By the end of pregnancy, the total surface of the chorionic villi reaches 14.5 sq.m, which, in terms of the ratio of the exchange surface and body weight, exceeds the exchange surface of the respiratory section of the lungs of an adult by more than 3 times. The vessels of the villi are connected to the vessels of the embryo through the allantoic arteries and veins (which will then become part of the umbilical cord). The only thing left for the heart of the embryo is to begin blood circulation, which happens at the end of the third - the beginning of the fourth week of development. Fetal metabolites through the umbilical arteries enter the vessels of the villi, overcome the barrier consisting of the wall of the villus capillary, the connective tissue of the villus and the trophoblast, and enter the mother's blood; from there, crossing the same barrier, but in the opposite direction, nutrients and oxygen enter the capillaries of the villi; through the umbilical vein they are transferred to the body of the embryo.
Under the influence of the embryo, significant changes occur in the endometrium. Most of all, they are noticeable at the site of implantation, but in one way or another, the entire uterine mucosa reacts to implantation. Therefore, during childbirth, the functional layer of the endometrium is rejected, which is why it is called decidua during pregnancy. Of course, different parts of the endometrium play a different role in the nutrition of the embryo, so the entire uterine mucosa is divided into 3 sections. When the embryo is introduced into the wall of the uterus, it receives nutrition from all sides. With the growth of the embryo, the part of the endometrium lying above it begins to protrude into the uterine cavity, grow and stretch. This part of the mucosa is called the decidua capsularis, and the adjacent chorion gradually loses villi due to malnutrition and turns into a smooth chorion. The section of the endometrium, located under the bubble, takes on the main load on the nutrition of the embryo and is called decidua basalis. A villous chorion is formed here, consisting of several large anchor, strongly branching villi. The endometrium, which lines the rest of the uterine cavity, except for the site of attachment of the embryo, is called decidua parietalis. The villous chorion and the basal decidua make up the placenta.
The placenta is divided into maternal and fetal parts. The fetal part includes the villous chorion, the chorionic plate and the amnion covering the placenta. During pregnancy, the epithelium of the villi (trophoblast) undergoes changes: from day 7, syncytiotrophoblast predominates, from day 10 - cytotrophoblast, in the second half of pregnancy, cytotrophoblast almost completely disappears. In some places, the syncytiotrophoblast is partially destroyed, and a fibrionoid appears in its place. The connective tissue part of the villi is represented by fibroblasts, macrophages and peculiar large granular Kashchenko-Hofbauer cells, as well as reticular and a small amount of collagen fibers, and in the main substance - glycosaminoglycans, which have a low viscosity to facilitate metabolic processes.
The maternal part of the placenta is formed by the basal decidua. The outer layers of the decidua basalis are destroyed by the villi of the chorion, where lacunae are formed, filled with maternal blood. The deep layers remain intact, forming a basal plate, from which connective tissue septa extend to the chorion, dividing the spaces filled with blood into separate chambers containing a group of villi (cotyledon). The connective tissue itself is called decidual and is distinguished by the presence of blood vessels with a wide lumen, cells with a large amount of glycogen and lipids (decidual cells), a lower content of fibers, i.e. the functional layer of the uterus becomes thicker and looser. The marginal part of the decidua basalis is not destroyed by the villi and adheres to the chorion at the border between the villous and smooth chorion, thus preventing the outflow of blood from the lacunae. The placenta completes its formation by the end of the 12th week of embryo development.
The placenta performs many functions. First of all, it ensures the saturation of the fetal blood with oxygen and the transfer of carbon dioxide into the mother's blood due to the difference partial pressure these gases in the blood of the mother and fetus (respiratory function). The placenta provides the trophism of the fetus: through the syncytiotrophoblast, nutrients from the mother's blood come by diffusion, and back - the metabolic products of the fetus (trophic and excretory functions). Gases and nutrients pass through the placental barrier, which consists of the trophoblast, the underlying connective tissue, and the wall of the villus capillary. The same barrier performs a protective function: it prevents the penetration of certain microorganisms, a number of toxic substances, fetal antigens, etc. into the blood of the fetus. In addition, the human placenta produces hormones (endocrine function): chorionic gonadotropin, chorionic somatomammotropin, progesterone and estrogens. Chorionic gonadotropin is secreted by the trophoblast already on the 7th day of pregnancy and is of great importance in the early diagnosis of pregnancy and some of its complications. All hormones, with the exception of estrogens, are synthesized by the syncytiotrophoblast. Precursors of estrogens are synthesized by the fetus itself, and the placenta translates them into an active state. The relansan produced by the placenta causes relaxation of the pubic symphysis before childbirth. Histamine and acetylcholine are produced in the placenta, under the influence of which capillaries expand and smooth muscles contract - preparing the uterus for childbirth. Placental hormones ensure the development of all reactions that occur during pregnancy and childbirth in a woman's body: growth of the uterus and mammary glands, regulation of the contractile activity of the uterus, specific changes in metabolism, and also contribute to the growth and development of the fetus.
During human embryogenesis the following extra-embryonic organs are formed: amnion, yolk sac, allantois, chorion and placenta. All three germ layers, as well as tissues of the mother's body (the maternal part of the placenta) participate in their formation.
trophoblast. As a result of the first division of crushing of the zygote, unequal blastomeres are formed. In particular, small light blastomeres actively proliferate and relatively quickly create an outer covering for dark blastomeres, called the blastocyst trophectoderm (V.D. Novikov, 1998).
The latter is the source of development trophoblast, which occurs in the process of interaction of the embryo with the mucous membrane of the uterus. Trophectoderm from one layer of cells turns into a trophoblast. Its outer part is transformed into a symplast (symplastotrophoblast) - in this part, the intercellular boundaries disappear, and the cell nuclei find themselves in a common symplastic plasma.
Inner part trophoblast saves cellular structure, in connection with which it is called the cytotrophoblast (or Langgans layer). Cyto- and symplastotrophoblast are structurally and metabolically connected and, together with the mesenchyme, form chorionic villi, creating an external cell-symplastic coating for them.
trophoblast ensures the implantation of the embryo and the formation of the most important extra-embryonic (provisional) organ - the placenta.
Embryo implantation activates proliferative and migratory processes in the embryoblast. This leads to the development of other extra-embryonic organs - amnion, yolk sac, allantois and chorion (in the period from the 7th to the 14th day of embryogenesis).
Amnion.
Amnion(water, amniotic membrane), is a hollow organ (sac) filled with fluid (amniotic fluid), in which the embryo is located and develops. The main function of the amnion is the production of amniotic fluid, which provides an optimal environment for the development of the embryo and protects it from drying out and mechanical stress. The amnion arises from the material of the epiblast by the formation of a cavity in its thickness - the amniotic vesicle.
In the process amnion epithelium development(first single-layer flat) on the 3rd month of embryogenesis is transformed into a prismatic. The epithelium is located on the basement membrane, under which there is a denser layer of connective tissue. Next is a spongy layer of loose fibrous connective tissue, spatially associated with the stroma of a smooth and villous chorion.
Amnion epithelial cells have secretory (in the placental part) and suction (in the extra-placental part) activity. Amniotic fluid is constantly exchanged, has a complex chemical composition changing during fetal development. In addition to the above functions, amniotic fluid is important for shaping processes - the development of the oral and nasal cavities, respiratory organs, and digestion.
The amount of water during pregnancy increases and by childbirth reaches 0.5-1.5 l, correlating with the length and weight of the fetus and gestational age. In the amniotic fluid, cells of the epidermis, epithelium of the oral cavity and vaginal epithelium of the fetus, epithelium of the umbilical cord and amnion, secretion products of the sebaceous glands, and vellus hair can be determined.
Yolk sac
.Yolk sac in humans (umbilical, or umbilical vesicle) - a rudimentary formation that has lost the function of a receptacle for nutrients. Until the 7-8th week of embryogenesis, its main function is hematopoietic. In addition, primary germ cells appear in the wall of the yolk sac - gonoblasts, which migrate into it from the primary streak.
Sources of development The tissues of the yolk sac are extraembryonic endoderm and extraembryonic mesenchyme. The wall of the yolk sac is lined with yolk epithelium, a special subtype of intestinal-type epithelium. The epithelium consists of a single layer of cuboidal or squamous cells of endodermal origin with light cytoplasm and round, intensely stained nuclei. After the formation of the trunk fold, the yolk sac communicates with the midgut cavity through the yolk stalk. Later, the yolk sac is found in the umbilical cord in the form of a narrow tube.
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SBEE HPE "Volgograd State Medical University"
Ministry of Health and social development Russia
Department of Histology, Embryology, Cytology
Extra-embryonic organs and their functional significance
Completed by: student of the 1st year of the 5th group
Faculty of Dentistry
Dadykina A.V.
Checked by: Ph.D., senior lecturer
T.S. Smirnova
Volgograd-2014
Introduction
1. Development of extra-embryonic organs
2. Yolk sac
4.Allantois
6.Placenta
7. Mother-fetus system
Bibliography
Introduction
An important role in the development of the vertebrate embryo belongs to the extraembryonic membranes, or provisional organs. They are temporary organs and are absent in an adult organism. The provisional organs provide the most important functions of the developing embryo, but they are not part of its body, thus being extra-embryonic organs. These include the yolk sac, amnion, chorion, allantois, and placenta. The extraembryonic region of the embryonic layers of fish forms only the yolk sac. In amphibians, due to the complete division of the zygote, it does not develop. Unlike fish and amphibians (anamnia), in reptiles, birds and mammals (amniotes), in addition to the yolk sac, the amnion, chorion (serosa, serous membrane) and allantois develop.
The dynamics of the relationship of the embryo, extra-embryonic organs and uterine membranes: a- human embryo 9.5 weeks of development (micrograph): 1 - amnion; 2 - chorion; 3 - forming placenta; 4 - umbilical cord
Development of extra-embryonic organs in the human embryo (scheme):
1 - amniotic vesicle;
1a - amnion cavity;
2 - the body of the embryo;
3 - yolk sac;
4 - extraembryonic coelom;
5-primary villi of the chorion;
6 - secondary villi of the chorion;
7 - allantois stalk;
8 - tertiary villi of the chorion;
9 - allan-tois;
10 - umbilical cord;
11 - smooth chorion;
12 - cotyledons
1. Development of extra-embryonic organs
The sources of provisional organs are blastocyst structures, including hypoblast and trophoblast.
Hypoblast. The blastocyst consists of an inner cell mass (embryoblast) and a trophoblast. On the 8-9th day, the inner cell mass is stratified into epiblast (primary ectoderm) and hypoblast (primary endoderm). Hypoblast cells do not take part in the formation of fetal structures, their descendants are present exclusively in the provisional organs. The extraembryonic endoderm forms the inner layer of the yolk sac and allantois.
The extraembryonic ectoderm is involved in the formation of the inner layer of the amnion. The extraembryonic mesoderm is divided into inner and outer layers. The inner leaf, together with the trophoblast, form the chorion, while the cells of the extraembryonic mesoderm overgrow the trophoblast, forming the endocoelomic cavity, or chorion cavity. The outer layer of the extraembryonic mesoderm is involved in the formation of the outer layers of the amnion, yolk sac, and allantois.
trophoblast(Figure 3-22). In the trophoblast, a polar region is distinguished, covering the inner cell mass, and a parietal (mural) part, which forms the blastocoel. Mural trophoblast cells establish contact with maternal tissue in the implantation crypt of the uterine endometrium. Two layers develop in the trophoblast: inner (cytotrophoblast) and outer (syncytiotrophoblast).
¦ Cytotrophoblast(Langhans layer) consists of intensively multiplying cells. Their nuclei contain well-defined nucleoli, and their cells contain numerous mitochondria, a well-developed granular endoplasmic reticulum and the Golgi complex. The cytoplasm contains a mass of free ribosomes and glycogen granules.
¦ Syncytiotrophoblast- a highly ploid multinuclear structure, formed from cytotrophoblast cells and serves as a source of placental somatomammotropin (placental lactogen), chorionic gonadotropin (CGT) and estrogen.
2. Yolk sac
The yolk sac is the most ancient extra-embryonic organ in evolution, which arose as an organ that deposits nutrients (yolk) necessary for the development of the embryo. In humans, this is a rudimentary formation (yolk vesicle). It is formed by extra-embryonic endoderm and extra-embryonic mesoderm (mesenchyme). The yolk sac is the portion of the primary intestine that extends beyond the embryo.
Appearing on the 2nd week of development in humans, the yolk vesicle takes part in the nutrition of the embryo for a very short time, since from the 3rd week of development a connection is established between the fetus and the mother's body, i.e., hematotrophic nutrition. During the period of the greatest development of the yolk sac, its blood vessels are separated from the uterine wall by a thin layer of tissue, which makes it possible to absorb nutrients and oxygen from the uterus. The extra-embryonic mesoderm serves as the site of embryonic hematopoiesis (hematopoiesis).
This is where blood islands form. In the extraembryonic endoderm of the yolk sac, primordial sex cells are temporarily (on the way of their migration to the rudiments of the gonads). After the formation of the trunk fold, the yolk sac is connected with the intestine yolk stalk.
The yolk sac itself is displaced into the space between the chorion mesenchyme and the amniotic membrane.
Later, the amnion folds compress the yolk sac; a narrow bridge is formed connecting it with the cavity of the primary intestine, - yolk stem. This structure elongates and comes into contact with the body stalk containing the allantois. The yolk stalk and the distal part of the allantois, together with their vessels, form umbilical cord, extending from the embryo in the region of the umbilical ring. The yolk stalk is usually completely overgrown by the end of the 3rd month of fetal development.
Functional value:
In the embryos of fish, reptiles and birds, it performs the functions of nutrition and respiration, in higher vertebrates it performs the functions of hematopoiesis and the formation of primary germ cells (gonoblasts), which then migrate to the embryo and contribute to the formation of an embryo of a certain sex.
In mammals, the yolk sac, which functions for only a few days, performs, along with the latter, also a trophic function, contributing to the absorption of the secretion of the glands of the uterus. The yolk sac of vertebrates is the first organ in the wall of which blood islands develop, forming the first blood cells and the first blood vessels that provide the fetus with transport of oxygen and nutrients.
As a hematopoietic organ, it functions until the 7-8th week, and then undergoes reverse development. Back at the end of the 19th century, the great French physiologist Claude Bernard noted that? ... in its biochemical activity, the yolk sac is in many ways reminiscent of the liver.
Hematopoiesis in the wall of the yolk sac. In humans, it begins at the end of the 2nd - beginning of the 3rd week of embryonic development. In the mesenchyme of the wall of the yolk sac, the rudiments of vascular blood are isolated, or blood islands.
In them, mesenchymal cells lose their processes, round off and transform into blood stem cells. The cells limiting the blood islands flatten, connect with each other and form the endothelial lining of the future vessel. Some HSCs differentiate into primary blood cells (blasts), large cells with basophilic cytoplasm and a nucleus, in which large nucleoli are clearly visible. Most primary blood cells mitotically divide and become primary erythroblasts, characterized by a large size (megaloblasts).
This transformation takes place in connection with the accumulation of embryonic hemoglobin in the cytoplasm of blasts; polychromatophilic erythroblasts, and then acidophilic erythroblasts with a high content of hemoglobin. In some primary erythroblasts, the nuclei undergo karyorrhexis and are removed from the cells; in other cells, the nuclei are preserved. As a result, nuclear-free and nucleated primary erythrocytes, characterized by a large size from acidophilic erythroblasts and therefore called megalo-cytes. This type of hematopoiesis is called megaloblastic. It is characteristic of the embryonic period, but may appear in the postnatal period with certain diseases (malignant anemia).
Along with megaloblastic, normoblastic hematopoiesis begins in the wall of the yolk sac, in which secondary erythroblasts form from blasts; first, as they accumulate hemoglobin in their cytoplasm, they turn into polychromatophilic erythroblasts, then into normoblasts, from which secondary erythrocytes (normocytes) are formed; the sizes of the latter correspond to erythrocytes (normocytes) of an adult . The development of red blood cells in the wall of the yolk sac occurs inside the primary blood vessels, i.e. intravascular.
Simultaneously, extravascularly, from the blasts located around the vessels, no a large number of granulocytes - neutrophils and eosinophils. Part of the HSC remains in an undifferentiated state and is carried by the blood flow to various organs of the embryo, where they are further differentiated into blood cells or connective tissue. After the reduction of the yolk sac, the main hematopoietic organ temporarily becomes the liver.
Until the 6th week of pregnancy, the yolk sac for a child plays the role of a primary liver and produces vital proteins: transferrins, alpha-fetoprotein, alpha2-microglobulin. The yolk sac has various functions that determine the viability of the fetus. It fully fulfills its role as a primary nutrient by the end of the 1st trimester, until the formation of the spleen, liver and reticuloendothelial system in the fetus (the system subsequently responsible for the development of macrophages - part of the immune system).
The yolk sac after 12-13 weeks of pregnancy ceases its functions, is drawn into the cavity of the embryo, contracts and remains in the form of a cystic formation - the yolk stalk, near the base of the umbilical cord. If premature reduction of the yolk sac occurs when the fetal organs (liver, spleen, reticuloendothelial system) are not yet sufficiently formed, then the outcome of the pregnancy will be unfavorable (spontaneous miscarriage, non-developing pregnancy).
Anomalies of the yolk sac:
Anomalies of the yolk sac are diverse: aplasia, doubling, premature reduction, increase, decrease in size, etc., and, as a rule, accompany various kinds of abnormalities in the development of the fetus and the course of pregnancy.
So, changes in size, doubling of the yolk sac in 20-80% of cases are observed with malformations and chromosomal syndromes in the fetus. Aplasia, hyperechoic content, premature reduction in 60--70% of cases are observed in non-developing pregnancy, and sometimes diagnosed 1-2 weeks before fetal death in the first trimester.
The conducted studies have proved the possibility of predicting more distant complications of pregnancy. It was established that the pathology of the yolk sac (reduction in size, premature reduction) in combination with a decrease in the volume of the chorionic cavity suggests the development of intrauterine growth retardation of the fetus (in the II-III trimesters) with a probability of 74%. With the pathological development of the yolk sac, the pregnancy may be non-developing, or a miscarriage will occur.
3. Amnion
Amnion - amniotic sac- a voluminous sac filled with amniotic fluid (amniotic fluid). It arose in evolution in connection with the release of vertebrates from water to land. In human embryogenesis, it appears at the second stage of gastrulation, first as a small vesicle as part of the epiblast. Simultaneously with the stratification of the inner cell mass into epiblast and hypoblast, an amniotic cavity is formed, bounded by the epiblast and extra-embryonic (amniotic) ectoderm. During gastrulation, cells of the extra-embryonic mesoderm overgrow the amniotic ectoderm, forming the outer layer of the amnion.
In the region of the umbilical ring, the amnion passes to the umbilical cord and further to the fetal part of the placenta, forming their epithelial cover. The embryonic (embryonic) and fetal periods of human development occur inside the fetal bladder.
The wall of the amniotic vesicle consists of a layer of cells of the extra-embryonic ectoderm and extra-embryonic mesenchyme, forms its connective tissue. The epithelium of the amnion in the early stages is single-layer flat, formed by large polygonal cells closely adjacent to each other, among which there are many mitotically dividing. At the 3rd month of embryogenesis, the epithelium is transformed into a prismatic one. On the surface of the epithelium there are microvilli.
The cytoplasm always contains small lipid droplets and glycogen granules. In the apical parts of the cells there are vacuoles of various sizes, the contents of which are released into the amnion cavity. The epithelium of the amnion in the area of the placental disc is single-layer prismatic, sometimes multi-row, performs a predominantly secretory function, while the epithelium of the extra-placental amnion mainly resorbs amniotic fluid.
In the connective tissue stroma of the amniotic membrane, there is a basement membrane, a layer of dense fibrous connective tissue and a spongy layer of loose fibrous connective tissue that connects the amnion with the chorion. In the layer of dense connective tissue, the acellular part lying under the basement membrane and the cellular part can be distinguished. The latter consists of several layers of fibroblasts, between which there is a dense network of thin bundles of collagen and reticular fibers tightly adjacent to each other, forming a lattice. irregular shape oriented parallel to the shell surface.
The spongy layer is formed by a loose mucous connective tissue with sparse bundles of collagen fibers, which are a continuation of those that lie in a layer of dense connective tissue, connecting the amnion with the chorion. This connection is very fragile, and therefore both shells are easy to separate from each other. The main substance of the connective tissue contains many glycosaminoglycans.
* Amniotic folds. At the cranial end, the amnion forms the head amniotic fold. With the increase in the size of the embryo, its head grows forward into the amniotic fold. Lateral amniotic folds are formed on both sides of the embryo due to the edges of the head fold. The caudal amniotic fold is formed at the caudal end of the embryo and grows in a cranial direction.
The head, lateral and caudal amniotic folds converge over the embryo and close the amniotic cavity. The junction of the amniotic folds is the amniotic suture; here a tissue strand disappearing subsequently is formed.
* amniotic fluid. The formed amniotic sac is filled with a fluid that protects the embryo during concussion, allows the fetus to move and prevents the growing parts of the body from sticking to each other and to surrounding tissues. 99% of the amniotic fluid consists of water, 1% is proteins, fats, carbohydrates, enzymes, hormones, inorganic salts, as well as epithelial cells of the amnion, skin, intestines, respiratory and urinary tract. By the end of pregnancy, the volume of fluid is 700-1000 ml.
The amnion rapidly increases, and by the end of the 7th week, its connective tissue comes into contact with the connective tissue of the chorion. At the same time, the amnion epithelium passes to the amniotic stalk, which later turns into the umbilical cord, and in the region of the umbilical ring it merges with the epithelial cover of the skin of the embryo.
The amniotic membrane forms the wall of a reservoir filled with amniotic fluid that contains the fetus. The main function of the amniotic membrane is the production of amniotic fluid, which provides an environment for the developing organism and protects it from mechanical damage. The epithelium of the amnion, facing its cavity, not only releases amniotic fluid, but also takes part in their reabsorption. The necessary composition and concentration of salts are maintained in the amniotic fluid until the end of pregnancy. Amnion also performs a protective function, preventing harmful agents from entering the fetus.
The amnion increases in size very quickly and by the end of the 7th week its connective tissue comes into contact with the connective tissue of the chorion. At the same time, the amnion epithelium passes to the amniotic stalk, which later turns into the umbilical cord, and in the region of the umbilical ring it merges with the ectodermal cover of the skin of the embryo.
Functional value:
The amniotic membrane forms the wall of the reservoir in which the fetus is located. Its main function is the production of amniotic fluid, which provides an environment for the developing organism and protects it from mechanical damage. The epithelium of the amnion, facing its cavity, secretes amniotic fluid, and also takes part in their reabsorption.
In the epithelium of the amnion covering the placental disc, secretion probably occurs predominantly, and in the epithelium of the extraplacental amnion, resorption of amniotic fluid occurs predominantly. The amniotic fluid creates the aquatic environment necessary for the development of the embryo, maintaining the necessary composition and concentration of salts in the amniotic fluid until the end of pregnancy. . The amount of amniotic fluid also changes to give the baby freedom of movement and protect him from external influences, such as when a pregnant woman falls. Sometimes the functions of the amnion various reasons are violated and these disorders are the cause of oligohydramnios or polyhydramnios. Amnion also performs a protective function, preventing harmful agents from entering the fetus.
Provides stable conditions for the development of the fetus. The amnion wall forms the amniotic membrane, which secretes amniotic fluid. She maintains the constancy of their composition. The water in the amniotic fluid has a high heat capacity, so its temperature does not change. The temperature of the mother's body can change during the day, but the temperature of the amniotic fluid will not change. Essentially, the amnion is a thermostat that ensures the development of the amnion and the fetus.
protective function. The amnion protects the fetus from the penetration of microbes from the vagina, and to a lesser extent from mechanical damage. However, it is minimal. Therefore, the main function of the amnion is to provide stable conditions for the development of the fetus.
The amnion, together with the smooth chorion, takes an active part in the exchange of amniotic fluid, as well as in the paraplacental exchange. By their own physical properties fetal membranes are different from each other. Since the amniotic membrane is very dense and withstands pressure several times greater than that of the smooth chorion, the rupture of the smooth chorion occurs earlier than the amnion during childbirth.
4. Allantois
The back wall of the yolk sac by the 16th day of development forms a small outgrowth - allantois (gr. alias, sausage-shaped), formed by extra-embryonic endoderm and mesoderm. . In humans, the allantois does not reach significant development, but its role in providing nutrition and respiration of the embryo is still great, since the vessels located in the umbilical cord grow along it towards the chorion. The proximal part of the allantois is located along the yolk stalk, and the distal part, growing, grows into the gap between the amnion and the chorion. In humans, the allantois is rudimentary, it does not function as a respiratory organ or a reservoir for the final metabolic products, but is important in embryonic hematopoiesis and angiogenesis.
At the 3-5th week of development, hematopoiesis occurs in the wall of the allantois and the blood vessels of the umbilical cord (two umbilical arteries and one umbilical vein) are formed. At the 7th week of embryogenesis, the urorectal septum separates the cloaca into the rectum and the urogenital sinus connected to the allantois. Therefore, the proximal allantois is related to the formation of the bladder. On the 2nd month of embryogenesis, the allantois degenerates, and in its place appears urachus- dense fibrous cord, stretching from the top of the bladder to the umbilical ring. In the postnatal period, the urachus is organized into the median umbilical ligament.
In birds, reptiles, and most lower mammals, the distal part of the allantoic diverticulum expands into a sac that protrudes into the extraembryonic coelom. The human allantois has only a rudimentary tubular lumen bordering the region of the ventral stalk, but its mesoderm and blood vessels grow far beyond its lumen, similar to the similar relationship of allantoid vessels in more primitive species that have a saccular allantois.
Regardless of the differences in the shape and size of the lumen, the allantois, increasing, eventually comes into contact and fuses with the inner surface of the serous membrane. The term chorion is applied to the germinal membrane, secondarily formed by the union of the allantois with the serous membrane. In species with a sac-like allantois (for example, the pig), the chorion is essentially a layer of the allantoid splanchnopleura, a fused mesodermal surface with a layer of serous somatopleura. In primate embryos, where the lumen of the allantois is rudimentary, the formation of the chorion differs in that the endoderm does not participate in it. However, the allantoid mesoderm and vessels continue distally beyond the vestigial lumen of the allantois and extend along the inner surface of the serosa in the same general manner as in less organized animals.
The size of the lumen of the allantois plays a secondary role, since the main functional significance of this fusion between the allantois and the serous membrane lies in the relationships between the vessels created in this case. In the lower mammals, to which we must turn our attention in order to understand the origin of these relationships, the serosa is a thin membrane extending relatively far away from its place of origin at the ventral wall of the body. She is very poor in blood vessels.
The method of formation of the amnion from the inner wings of the same folds from which the serous membrane arises leads to the creation of a very meager blood supply; when the amnion is isolated in the form of a separate sac, the initial connection of the serous membrane with the embryo decreases sharply and this creates mechanical difficulties for maintaining even small initial vascular connections. The presence of allantois creates a way out of this impasse. In the walls of the allantois formed from the hindgut, a dense plexus of vessels quickly develops. This plexus is connected through large arteries and veins directly to the main blood vessels of the embryo.
Therefore, the fusion of allantois with the inner surface of the serous membrane provides this poorly vascularized layer with an abundant blood supply. Different groups of animals differ in the relationship of the constituent parts of the chorion, and the chorion itself meets completely different environmental conditions. Nevertheless, the described mechanism of vascularization of the outermost membranes of the embryo is basically the same everywhere. Will it be a bird embryo depending on vascular system during gas exchange with the outside air through a porous membrane, or is it a mammalian embryo that depends on it for metabolism with the uterus - all this does not change the essence of the matter.
The outermost shell surrounding the embryo is the layer most favorable for exchange with the environment. In the interest of this exchange, the embryo must have an abundant vasculature communicating with the site where the exchange takes place. If, in considering the chorion, these characteristic vital vascular relationships and the manner in which these relationships are established are kept in mind, then the analogy between the human chorion and the more primitive type of allantoid chorion becomes quite obvious. If, however, one notices only such random phenomena as the difference in the size of the allantois lumen, then the clarity of these relationships must inevitably disappear.
Functional role of allantois:
1) in birds, the allantois cavity reaches a significant development and urea accumulates in it, therefore it is called the urinary sac;
2) a person does not need to accumulate urea, therefore the allantois cavity is very small and completely overgrown by the end of the 2nd month.
However, blood vessels develop in the mesenchyme of the allantois, which connect with the vessels of the body of the embryo at their proximal ends (these vessels appear in the mesenchyme of the body of the embryo later than in the allantois). With their distal ends, the allantois vessels grow into the secondary villi of the villous part of the chorion and turn them into tertiary ones. From the 3rd to the 8th week of intrauterine development, due to these processes, the placental circle of blood circulation is formed. The amniotic leg, together with the vessels, is pulled out and turns into the umbilical cord, and the vessels (two arteries and a vein) are called umbilical vessels.
The mesenchyme of the umbilical cord is transformed into a mucous connective tissue. The umbilical cord also contains the remains of allantois and the yolk stalk. The function of allantois is to contribute to the performance of the functions of the placenta.
Great importance is currently attached to the Doppler study of blood flow in the emerging mother-placenta-fetus system.
In recent years, it has been convincingly shown that the functional state plays the most important pathogenetic role in the basis of many complications of pregnancy. vascular wall especially the endothelium. The leading role in the development of uteroplacental circulation and, consequently, in the morphogenesis of the placenta is given to the spiral arteries.
The intervillous space, which is an important structural unit of the placenta, is filled with blood coming from the spiral arteries, in which functional changes gradually occur. Terminal
sections of these arteries by 13-14 weeks of gestation are characterized by endothelial hypertrophy, degeneration of the muscle layer, as a result of which the vessel wall is deprived of smooth muscle elements and loses its ability to contract and expand.
Under physiological conditions, upon completion of the process of trophoblast invasion (after 14 weeks of pregnancy), the blood flow in the intervillous space becomes constant. We conducted a prospective population study (1035 patients), starting from early pregnancy, which included the study of uteroplacental circulation by Doppler.
Pathological indicators of blood flow in the uterine and spiral arteries (at 10 weeks) in the form of an increase in the systole-diastolic ratio, pulsation index and resistance index were recorded in 140 pregnant women. Most of these patients (124 - 88.5%) were pregnant women who later (in the II-III trimesters) developed clinical signs of preeclampsia (complications of a normal pregnancy, characterized by a disorder of a number of organs and body systems. It is believed that the basis pathogenesis lies generalized vasospasm and subsequent changes associated with impaired microcirculation, hypoperfusion, hypovolemia).
5. Chorion
Chorion, or villous sheath, appears for the first time in mammals, develops from the trophoblast and extraembryonic mesoderm.
In the formation of the chorion, three periods are distinguished: previllous, the period of villus formation and the period of cotyledons. A three-week-old embryo at the gastrula stage.
The amnion cavity and the yolk sac are formed. The trophoblast cells that form the placenta come into contact with the blood vessels of the uterus. The embryo is associated with the trophoblast originating from the extraembryonic mesoderm of the body leg. Allantois grows into the peduncle of the body, angiogenesis proceeds here, and subsequently the umbilical cord is formed with umbilical (allantoic) vessels passing through it: two umbilical arteries and one umbilical vein.
* Previllous period. During implantation, trophoblast cells proliferate and form cytotrophoblast. As it interacts with the endometrium, the trophoblast begins to cytolytically destroy endometrial tissues, resulting in cavities (lacunae) filled with the mother's blood. The lacunae are separated by partitions of trophoblast cells, these are primary villi. After the appearance of lacunae, the blastocyst can be called a fetal bladder.
* Villus period. During this period, primary, secondary and tertiary villi are successively formed.
¦ Primary villi- Clusters of cytotrophoblast cells surrounded by syncytiotrophoblast.
¦ Secondary villi. On the 12-13th day, the extraembryonic mesoderm grows into the primary villi, which leads to the formation of secondary villi, evenly distributed over the entire surface of the fetal egg. The epithelium of the secondary villi is represented by light rounded cells with large nuclei. Above the epithelium is a syncytium with indistinct borders, dark granular cytoplasm, brush border and polymorphic nuclei.
¦ Tertiary villi. From the 3rd week of development, tertiary villi containing blood vessels appear. This period is called placentation. The villi facing the basal part of the decidua are supplied with blood not only from the vessels originating from the chorionic mesoderm, but also from the vessels of the allantois.
The period of connection of the branches of the umbilical vessels with the local circulatory network coincides with the onset of heart contractions (day 21 of development), and the circulation of embryonic blood begins in the tertiary villi. Vascularization of the chorionic villi ends at the 10th week of pregnancy. By this time, the placental barrier is formed. Not all chorionic villi are equally well developed. The villi facing the capsular part of the falling off membrane are poorly developed and gradually disappear. Therefore, the chorion in this part is called smooth.
* Cotyledon period. Cotyledon, a structural and functional unit of the formed placenta, is formed by the stem villus and its branches containing fetal vessels. By the 140th day of pregnancy, 10-12 large, 40-50 small and up to 150 rudimentary cotyledons were formed in the placenta. By the 4th month of pregnancy, the formation of the main structures of the placenta ends. The lacunae of a fully formed placenta contain about 150 ml of maternal blood, which is completely replaced 3-4 times per minute. The total surface of the villi reaches 14 m 2, which ensures a high level of exchange between the pregnant woman and the fetus.
The smooth chorion is located between the aqueous and decidual membranes and consists of four layers: cellular, reticular, pseudobasal membrane and trophoblast.
The cell layer is adjacent to the spongy layer of the amnion. It is well differentiated in the early stages of pregnancy and is almost not determined in mature membranes. The reticular (or fibrous) layer of the chorion is the most durable.
The trophoblast is indistinctly separated from the adjacent decidua. Its cells penetrate deep into, providing a close connection between the chorionic and decidual membranes, in connection with which some authors [Govorka E. 1970; Wulf KN, 1981] consider these layers as a single choriodecidual complex. The trophoblast consists of several rows of round or polygonal cells, one or more nuclei. Between the choriotrophoblasts there are tubules, bordered, like the tubules of the amnion, by microvilli and containing tissue fluid.
In the cytoplasm of trophoblast cells, microfibrils, desmosomes, large mitochondria, endoplasmic reticulum and other ultrastructures are well developed. High functional activity, including pinocytosis, is indicated by the presence of vacuoles. A high content of RNA, glycogen, protein, amino acids, mucoproteins and mucopolysaccharides, as well as phosphorus compounds and many enzymes, including thermostable alkaline phosphatase, was found here. A fibrinoid is deposited in the trophoblast, in which the remnants of villi are visible, devoid of epithelium and retaining only a fibrous fibrous stroma without vessels.
The functional activity of the smooth chorion is maintained until the end of pregnancy. There are indications of the synthesis in it of chorionic gonadotropin, AK.TG, prolactin and prostaglandins, the precursor of which - arachidonic acid - was found in high concentration in the chorion as part of phospholipids. There are no fetal group antigens in the chorionic membrane.
The physical properties of the fetal membranes differ from each other. The amniotic membrane has a high density and can withstand pressure 5 times greater than the chorion. The rupture of the smooth chorion during childbirth occurs earlier than the amnion. The experiment shows the possibility of regeneration of the membranes after their rupture.
Chorion pathologies:
It is also important to carefully study the size and structure of the chorion in the first trimester of pregnancy. Normally, from 8-9 weeks, the chorion ceases to be circular, part of it thickens and becomes the site of the formation of the fetal part of the placenta. The thickness of the chorion increases with the course of pregnancy, amounting to 7.5 mm at 7 weeks and 13.3 mm at 13 weeks. The pathology of the chorion, detected by echography in the first trimester, is represented by retrochorial hematomas (50%), structural heterogeneity (28%), and hypoplasia (22%).
According to many researchers, in the presence of retrochorial hematomas, the probability of spontaneous abortion exceeds 30%; hypoplasia of the chorion in 85--90% of cases precedes the death of the fetus (non-developing pregnancy); the heterogeneity of the chorion structure clearly correlates with intrauterine infection (up to 75%).
Section of the chorionic villus of a 17-day-old human embryo ("Crimea"). Micrograph: 1 - symplastotrophoblast; 2 - cytotrophoblast; 3 - chorion mesenchyme (according to N. P. Barsukov)
6. Placenta
Placenta (children's place) human belongs to the type of discoidal hemochorial villous placenta (see Fig. 21.16; Fig. 21.17). This is an important temporary organ with a variety of functions that provide a connection between the fetus and the mother's body. At the same time, the placenta creates a barrier between the blood of the mother and the fetus.
The placenta consists of two parts: germinal, or fetal (pars fetalis) and maternal (pars materna). The fetal part is represented by a branched chorion and an amniotic membrane adhering to the chorion from the inside, and the maternal part is a modified uterine mucosa that is rejected during childbirth (decidua basalis).
The development of the placenta begins on the 3rd week, when vessels begin to grow into the secondary villi and tertiary villi form, and ends by the end of the 3rd month of pregnancy.
On the 6-8th week, connective tissue elements differentiate around the vessels. Vitamins A and C play an important role in the differentiation of fibroblasts and the synthesis of collagen by them, without sufficient intake of which the strength of the bond between the embryo and the mother's body is disrupted and the threat of spontaneous abortion is created. embryo embryo vertebrate
The main substance of the connective tissue of the chorion contains a significant amount of hyaluronic and chondroitinsulfuric acids, which are associated with the regulation of placental permeability.
With the development of the placenta, the destruction of the uterine mucosa occurs, due to the proteolytic activity of the chorion, and the change of histiotrophic nutrition to hematotrophic. This means that the villi of the chorion are washed by the blood of the mother, which has poured out from the destroyed vessels of the endometrium into the lacunae. However, the blood of the mother and fetus under normal conditions never mixes.
hematochorionic barrier, separating both blood flows, consists of the endothelium of the fetal vessels, the connective tissue surrounding the vessels, the epithelium of the chorionic villi (cytotrophoblast and symplastotrophoblast), and in addition, of fibrinoid, which sometimes covers the villi from the outside.
germinal, or fetal, part placenta by the end of the 3rd month is represented by a branching chorionic plate, consisting of fibrous (collagenous) connective tissue, covered with cyto- and symplastotrophoblast (a multinuclear structure covering the reducing cytotrophoblast).
The branching villi of the chorion (stem, anchor) are well developed only on the side facing the myometrium. Here they pass through the entire thickness of the placenta and with their tops plunge into the basal part of the destroyed endometrium.
The chorionic epithelium, or cytotrophoblast, in the early stages of development is represented by a single-layer epithelium with oval nuclei. These cells reproduce by mitosis. They develop symplastotrophoblast.
The symplastotrophoblast contains a large number of various proteolytic and oxidative enzymes (ATPases, alkaline and acidic
There are slit-like submicroscopic spaces between the symplastotrophoblast and the cellular trophoblast, reaching in places up to the basement membrane of the trophoblast, which creates conditions for the bilateral penetration of trophic substances, hormones, etc.
In the second half of pregnancy and, especially, at the end of it, the trophoblast becomes very thin and the villi are covered with a fibrin-like oxyphilic mass, which is a product of plasma coagulation and the breakdown of the trophoblast (“Langhans fibrinoid”).
With an increase in the gestational age, the number of macrophages and collagen-producing differentiated fibroblasts decreases, and fibrocytes appear. The number of collagen fibers, although increasing, remains insignificant in most villi until the end of pregnancy. Most stromal cells (myofibroblasts) are characterized by an increased content of cytoskeletal contractile proteins (vimentin, desmin, actin and myosin).
The structural and functional unit of the formed placenta is cotyledon, formed by the stem ("anchor") villus and its secondary and tertiary (final) branches. The total number of cotyledons in the placenta reaches 200.
Placental barrier at the 28th week of pregnancy. Electron micrograph, magnification 45,000 (according to U. Yu. Yatsozhinskaya): 1 - symplastotrophoblast; 2 - cytotrophoblast; 3 - basement membrane of the trophoblast; 4 - basement membrane of the endothelium; 5 - endotheliocyte; 6 - erythrocyte in capillary
Hemochorionic placenta. The dynamics of the development of chorionic villi: a- the structure of the placenta (arrows indicate blood circulation in the vessels and in one of the gaps where the villus was removed): 1 - amnion epithelium; 2 - chorionic plate; 3 - villi; 4 - fibrinoid; 5 - yolk vesicle; 6 - umbilical cord; 7 - placental septum; 8 - lacuna; 9 - spiral artery; 10 - basal layer of the endometrium; 11 - myometrium; b- structure of the primary trophoblast villus (1st week); in- structure of the secondary epithelial-mesenchymal villus of the chorion (2nd week); G- the structure of the tertiary chorionic villus - epithelial-mesenchymal with blood vessels (3rd week); d- structure of the chorionic villus (3rd month); e- structure of chorionic villi (9th month): 1 - intervillous space; 2 - microvilli; 3 - symplastotrophoblast; 4 - symplastotrophoblast nuclei; 5 - cytotrophoblast; 6 - the nucleus of the cytotrophoblast; 7 - basement membrane; 8 - intercellular space; 9 - fibroblast;
10 - macrophages (Kashchenko-Hofbauer cells); 11 - endotheliocyte; 12 - lumen of a blood vessel; 13 - erythrocyte; 14 - basement membrane of the capillary (according to E. M. Schwirst)
Mother part placenta is represented by a basal plate and connective tissue septa that separate the cotyledons from each other, as well as gaps filled with maternal blood. Trophoblast cells (peripheral trophoblast) are also found at the points of contact between the stem villi and the sheath.
In the early stages of pregnancy, the chorionic villi destroy the layers of the main falling off uterine membrane closest to the fetus, and in their place lacunae filled with maternal blood are formed, into which the chorionic villi hang freely.
The deep undestroyed parts of the falling off membrane, together with the trophoblast, form the basal plate.
Basal layer of the endometrium (lamina basalis)- connective tissue of the uterine lining decidual cells. These large, glycogen-rich connective tissue cells are located in the deep layers of the uterine mucosa. They have clear boundaries, rounded nuclei and oxyphilic cytoplasm. During the 2nd month of pregnancy, decidual cells are significantly enlarged. In their cytoplasm, in addition to glycogen, lipids, glucose, vitamin C, iron, nonspecific esterases, dehydrogenase of succinic and lactic acids are detected. In the basal plate, more often at the site of attachment of the villi to the maternal part of the placenta, clusters of peripheral cytotrophoblast cells are found. They resemble decidual cells, but differ in a more intense basophilia of the cytoplasm. An amorphous substance (Rohr's fibrinoid) is located on the surface of the basal plate facing the chorionic villi. Fibrinoid plays an essential role in ensuring immunological homeostasis in the mother-fetus system.
Part of the main falling off shell, located on the border of the branched and smooth chorion, i.e., along the edge of the placental disc, is not destroyed during the development of the placenta. Tightly growing to the chorion, it forms end plate, preventing the outflow of blood from the lacunae of the placenta.
The blood in the lacunae circulates continuously. It comes from the uterine arteries, which enter here from the muscular membrane of the uterus. These arteries run along the placental septa and open into lacunae. Maternal blood flows from the placenta through veins that originate from the lacunae with large holes.
The formation of the placenta ends at the end of the 3rd month of pregnancy. The placenta provides nutrition, tissue respiration, growth, regulation of the rudiments of the fetal organs formed by this time, as well as its protection.
Functions of the placenta . The main functions of the placenta: 1) respiratory; 2) transport of nutrients; water; electrolytes and immunoglobulins; 3) excretory; 4) endocrine; 5) participation in the regulation of myometrium contraction.
Breath the fetus is provided by oxygen attached to maternal hemoglobin, which diffuses through the placenta into the fetal blood, where it combines with fetal hemoglobin
(HbF). The CO 2 associated with fetal hemoglobin in the blood of the fetus also diffuses through the placenta, enters the mother's blood, where it combines with maternal hemoglobin.
Transport all the nutrients necessary for the development of the fetus (glucose, amino acids, fatty acid, nucleotides, vitamins, minerals), comes from the mother's blood through the placenta into the blood of the fetus, and, conversely, metabolic products excreted from the body (excretory function) enter the mother's blood from the fetus's blood. Electrolytes and water pass through the placenta by diffusion and by pinocytosis.
Pinocytic vesicles of the symplastotrophoblast are involved in the transport of immunoglobulins. The immunoglobulin that enters the blood of the fetus passively immunizes it from the possible action of bacterial antigens that can enter during maternal diseases. After birth, maternal immunoglobulin is destroyed and replaced by newly synthesized in the child's body under the action of bacterial antigens on it. Through the placenta, IgG, IgA penetrate into the amniotic fluid.
endocrine function is one of the most important, since the placenta has the ability to synthesize and secrete a number of hormones that ensure the interaction of the embryo and the mother's body throughout pregnancy. The site of placental hormone production is the cytotrophoblast and especially the symplastotrophoblast, as well as decidual cells.
The placenta is one of the first to synthesize chorionic gonadotropin, the concentration of which rapidly increases at the 2-3rd week of pregnancy, reaching a maximum at the 8-10th week, and in the fetal blood it is 10-20 times higher than in the mother's blood. The hormone stimulates the production of adrenocorticotropic hormone (ACTH) by the pituitary gland, enhances the secretion of corticosteroids.
plays an important role in the development of pregnancy placental lactogen, which has the activity of prolactin and pituitary luteotropic hormone. It supports steroidogenesis in the corpus luteum of the ovary in the first 3 months of pregnancy, and also takes part in the metabolism of carbohydrates and proteins. Its concentration in the mother's blood progressively increases at the 3-4th month of pregnancy and then continues to increase, reaching a maximum by the 9th month. This hormone, together with maternal and fetal pituitary prolactin, plays a role in the production of pulmonary surfactant and fetoplacental osmoregulation. Its high concentration is found in the amniotic fluid (10-100 times more than in the mother's blood).
In the chorion, as well as in the decidua, progesterone and pregnandiol are synthesized.
Progesterone (produced first corpus luteum in the ovary, and from the 5-6th week in the placenta) inhibits uterine contractions, stimulates its growth, has an immunosuppressive effect, suppressing the fetal rejection reaction. About 3/4 of the progesterone in the mother's body is metabolized and transformed into estrogen, and part is excreted in the urine.
Estrogens (estradiol, estrone, estriol) are produced in the symplasto-trophoblast of placental (chorionic) villi in the middle of pregnancy, and by the end
Pregnancy their activity increases 10 times. They cause hyperplasia and hypertrophy of the uterus.
In addition, melanocyte-stimulating and adrenocorticotropic hormones, somatostatin, etc. are synthesized in the placenta.
The placenta contains polyamines (spermine, spermidine), which affect the enhancement of RNA synthesis in smooth muscle cells of the myometrium, as well as oxidases that destroy them. An important role is played by amine oxidases (histaminase, monoamine oxidase), which destroy biogenic amines - histamine, serotonin, tyramine. During pregnancy, their activity increases, which contributes to the destruction of biogenic amines and a decrease in the concentration of the latter in the placenta, myometrium and maternal blood.
During childbirth, histamine and serotonin, along with catecholamines (norepinephrine, adrenaline), are stimulants of the contractile activity of smooth muscle cells (SMC) of the uterus, and by the end of pregnancy, their concentration increases significantly due to a sharp decrease (by 2 times) in the activity of amino oxidases (histaminase, etc. .).
With weak labor activity, there is an increase in the activity of aminooxidases, for example, histaminase (5 times).
The normal placenta is not an absolute barrier to proteins. In particular, at the end of the 3rd month of pregnancy, fetoprotein penetrates in a small amount (about 10%) from the fetus into the mother's blood, but the maternal organism does not reject this antigen, since the cytotoxicity of maternal lymphocytes decreases during pregnancy.
The placenta prevents the passage of a number of maternal cells and cytotoxic antibodies to the fetus. The main role in this is played by fibrinoid, which covers the trophoblast when it is partially damaged. This prevents the entry of placental and fetal antigens into the intervillous space, and also weakens the humoral and cellular “attack” of the mother against the fetus.
In conclusion, we note the main features of the early stages of development of the human embryo: 1) asynchronous type of complete crushing and the formation of "light" and "dark" blastomeres; 2) early isolation and formation of extra-embryonic organs; 3) early formation of the amniotic vesicle and the absence of amniotic folds; 4) the presence of two mechanisms in the stage of gastrulation - delamination and immigration, during which the development of provisional organs also occurs; 5) interstitial type of implantation; 6) strong development of the amnion, chorion, placenta and weak development of the yolk sac and allantois.
7. Mother-fetus system
The mother-fetus system arises during pregnancy and includes two subsystems - the mother's body and the fetus's body, as well as the placenta, which is the link between them.
The interaction between the mother's body and the fetus's body is provided primarily by neurohumoral mechanisms. At the same time, the following mechanisms are distinguished in both subsystems: receptor, perceiving information, regulatory, processing it, and executive.
The receptor mechanisms of the mother's body are located in the uterus in the form of sensitive nerve endings, which are the first to perceive information about the state of the developing fetus. In the endometrium there are chemo-, mechano- and thermoreceptors, and in the blood vessels - baroreceptors. Receptor nerve endings of the free type are especially numerous in the walls of the uterine vein and in the decidua in the area of attachment of the placenta. Irritation of uterine receptors causes changes in the intensity of respiration, blood pressure in the mother's body, which provides normal conditions for the developing fetus.
The regulatory mechanisms of the mother's body include parts of the central nervous system (temporal lobe of the brain, hypothalamus, mesencephalic reticular formation), as well as the hypothalamic-endocrine system. An important regulatory function is performed by hormones: sex hormones, thyroxine, corticosteroids, insulin, etc. Thus, during pregnancy, there is an increase in the activity of the mother's adrenal cortex and an increase in the production of corticosteroids, which are involved in the regulation of fetal metabolism. The placenta produces chorionic gonadotropin, which stimulates the formation of pituitary ACTH, which activates the activity of the adrenal cortex and enhances the secretion of corticosteroids.
Regulatory neuroendocrine apparatus of the mother ensures the preservation of pregnancy, the necessary level of functioning of the heart, blood vessels, hematopoietic organs, liver and the optimal level of metabolism, gases, depending on the needs of the fetus.
The receptor mechanisms of the fetal body perceive signals about changes in the mother's body or their own homeostasis. They are found in the walls of the umbilical arteries and veins, in the mouths of the hepatic veins, in the skin and intestines of the fetus. Irritation of these receptors leads to a change in the heart rate of the fetus, the speed of blood flow in its vessels, affects the sugar content in the blood, etc.
Regulatory neurohumoral mechanisms of the fetal body are formed in the process of development. The first motor reactions in the fetus appear at the 2-3rd month of development, which indicates maturation nerve centers. The mechanisms regulating gas homeostasis are formed at the end of the second trimester of embryogenesis. The beginning of the functioning of the central endocrine gland - the pituitary gland - is noted at the 3rd month of development. The synthesis of corticosteroids in the adrenal glands of the fetus begins in the second half of pregnancy and increases with its growth. The fetus has increased insulin synthesis, which is necessary to ensure its growth associated with carbohydrate and energy metabolism.
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EMBRYOLOGY. Chapter 21. BASICS OF HUMAN EMBRYOLOGY
EMBRYOLOGY. Chapter 21. BASICS OF HUMAN EMBRYOLOGY
Embryology (from the Greek. embryonic- embryo, logos- doctrine) - the science of the laws of development of embryos.
Medical embryology studies the patterns of development of the human embryo. Particular attention is drawn to embryonic sources and regular processes of tissue development, metabolic and functional features of the mother-placenta-fetus system, and critical periods of human development. All this has great importance for medical practice.
Knowledge of human embryology is necessary for all doctors, especially those working in the field of obstetrics and pediatrics. This helps in diagnosing disorders in the mother-fetus system, identifying the causes of deformities and diseases in children after birth.
Currently, knowledge of human embryology is used to uncover and eliminate the causes of infertility, fetal organ transplantation, and the development and use of contraceptives. In particular, the problems of culturing eggs, in vitro fertilization and implantation of embryos in the uterus have become topical.
The process of human embryonic development is the result of a long evolution and to a certain extent reflects the features of the development of other representatives of the animal world. Therefore, some of the early stages of human development are very similar to similar stages in the embryogenesis of lower organized chordates.
Human embryogenesis is a part of its ontogenesis, including the following main stages: I - fertilization and zygote formation; II - crushing and formation of the blastula (blastocyst); III - gastrulation - the formation of germ layers and a complex of axial organs; IV - histogenesis and organogenesis of germinal and extra-embryonic organs; V - systemogenesis.
Embryogenesis is closely related to progenesis and the early postembryonic period. Thus, the development of tissues begins in the embryonic period (embryonic histogenesis) and continues after the birth of a child (postembryonic histogenesis).
21.1. PROGENESIS
This is the period of development and maturation of germ cells - eggs and sperm. As a result of progenesis, a haploid set of chromosomes appears in mature germ cells, structures are formed that provide the ability to fertilize and develop a new organism. The process of development of germ cells is considered in detail in the chapters on the male and female reproductive systems (see Chapter 20).
Rice. 21.1. The structure of the male germ cell:
I - head; II - tail. 1 - receptor;
2 - acrosome; 3 - "cover"; 4 - proximal centriole; 5 - mitochondrion; 6 - layer of elastic fibrils; 7 - axone; 8 - terminal ring; 9 - circular fibrils
Main characteristics of mature human germ cells
male reproductive cells
Human spermatozoa are produced during the entire active sexual period in large quantities. For a detailed description of spermatogenesis, see chapter 20.
Sperm motility is due to the presence of flagella. The speed of movement of spermatozoa in humans is 30-50 microns / s. Purposeful movement is facilitated by chemotaxis (movement towards or away from a chemical stimulus) and rheotaxis (movement against fluid flow). 30-60 minutes after intercourse, spermatozoa are found in the uterine cavity, and after 1.5-2 hours - in the distal (ampullar) part of the fallopian tube, where they meet with the egg and fertilization. Sperm retain their fertilizing capacity for up to 2 days.
Structure. Human male sex cells - spermatozoa, or sperm-mii, about 70 microns long, have a head and a tail (Fig. 21.1). The plasma membrane of the spermatozoon in the head area contains a receptor, through which interaction with the egg takes place.
The head of the spermatozoon includes a small dense nucleus with a haploid set of chromosomes. The anterior half of the nucleus is covered with a flat sac case sperm. In it is located acrosome(from Greek. asron- top, soma- body). The acrosome contains a set of enzymes, among which an important place belongs to hyaluronidase and proteases, which are capable of dissolving the membranes covering the egg during fertilization. The case and acrosome are derivatives of the Golgi complex.
Rice. 21.2. The cellular composition of human ejaculate is normal:
I - male sex cells: A - mature (according to L.F. Kurilo and others); B - immature;
II - somatic cells. 1, 2 - typical spermatozoon (1 - full face, 2 - profile); 3-12 - the most common forms of spermatozoa atypia; 3 - macro head; 4 - microhead; 5 - elongated head; 6-7 - anomaly in the shape of the head and acrosome; 8-9 - anomaly of the flagellum; 10 - biflagellated sperm; 11 - fused heads (two-headed sperm); 12 - anomaly of the neck of the sperm; 13-18 - immature male sex cells; 13-15 - primary spermatocytes in the prophase of the 1st division of meiosis - proleptoten, pachytene, diplotene, respectively; 16 - primary spermatocyte in the metaphase of meiosis; 17 - typical spermatids (a- early; b- late); 18 - atypical binuclear spermatid; 19 - epithelial cells; 20-22 - leukocytes
The human sperm nucleus contains 23 chromosomes, one of which is sexual (X or Y), the rest are autosomes. 50% of spermatozoa contain the X chromosome, 50% - the Y chromosome. The mass of the X chromosome is somewhat larger than the mass of the Y chromosome, therefore, apparently, spermatozoa containing the X chromosome are less mobile than spermatozoa containing the Y chromosome.
Behind the head there is an annular narrowing, passing into the tail section.
tail section (flagellum) The spermatozoon consists of a connecting, intermediate, main and terminal parts. In the connecting part (pars conjungens), or neck (cervix) centrioles are located - proximal, adjacent to the nucleus, and the remains of the distal centriole, striated columns. Here begins the axial thread (axonema), continuing in the intermediate, main and terminal parts.
Intermediate part (pars intermedia) contains 2 central and 9 pairs of peripheral microtubules surrounded by spirally arranged mitochondria (mitochondrial sheath - vagina mitochondrialis). Paired protrusions, or "handles", consisting of another protein, dynein, which has ATP-ase activity, depart from the microtubules (see Chapter 4). Dynein breaks down ATP produced by mitochondria and converts chemical energy into mechanical energy, due to which the movement of sperm is carried out. In the case of a genetically determined absence of dynein, sperm are immobilized (one of the forms of male sterility).
Among the factors affecting the speed of sperm movement, temperature, pH of the medium, etc. are of great importance.
main part (pars principalis) The structure of the tail resembles a cilium with a characteristic set of microtubules in the axoneme (9 × 2) + 2, surrounded by circularly oriented fibrils that give elasticity, and a plasmalemma.
Terminal, or final part sperm (pars terminalis) contains an axoneme that ends in disconnected microtubules and a gradual decrease in their number.
The movements of the tail are whip-like, which is due to the successive contraction of microtubules from the first to the ninth pair (the first is considered a pair of microtubules, which lies in a plane parallel to the two central ones).
In clinical practice, in the study of sperm, various forms of spermatozoa are counted, counting their percentage (spermogram).
According to the World Health Organization (WHO), the following indicators are normal characteristics of human sperm: sperm concentration - 20-200 million / ml, the content in the ejaculate is more than 60% of normal forms. Along with the latter, human sperm always contains abnormal ones - biflagellated, with defective head sizes (macro- and microforms), with an amorphous head, with fused
heads, immature forms (with remnants of the cytoplasm in the neck and tail), with flagellum defects.
In the ejaculate of healthy men, typical spermatozoa predominate (Fig. 21.2). Quantity various kinds atypical sperm should not exceed 30%. In addition, there are immature forms of germ cells - spermatids, spermatocytes (up to 2%), as well as somatic cells - epitheliocytes, leukocytes.
Among the spermatozoa in the ejaculate, living cells should be 75% or more, and actively mobile - 50% or more. The established normative parameters are necessary for assessing deviations from the norm when various forms male infertility.
In an acidic environment, spermatozoa quickly lose their ability to move and fertilize.
female sex cells
eggs, or oocytes(from lat. ovum- egg), ripen in an immeasurably smaller amount than spermatozoa. In a woman during the sexual cycle (24-28 days), as a rule, one egg matures. Thus, during the childbearing period, about 400 eggs are formed.
The release of an oocyte from an ovary is called ovulation (see Chapter 20). The oocyte released from the ovary is surrounded by a crown of follicular cells, the number of which reaches 3-4 thousand. The egg has a spherical shape, the volume of the cytoplasm is larger than that of the sperm, and does not have the ability to move independently.
The classification of oocytes is based on signs of presence, quantity and distribution. yolk (lecithos), which is a protein-lipid inclusion in the cytoplasm, used to nourish the embryo. Distinguish yolkless(alecital), small-yolk(oligolecital), medium yolk(mesolecithal), multiyolk(polylecital) eggs. Small-yolk eggs are divided into primary (in non-cranial, for example, lancelet) and secondary (in placental mammals and humans).
As a rule, in small-yolk eggs, yolk inclusions (granules, plates) are evenly distributed, so they are called isolecithal(gr. isos- equal). human egg secondary isolecithal type(as in other mammals) contains a small amount of yolk granules, more or less evenly spaced.
In humans, the presence of a small amount of yolk in the egg is due to the development of the embryo in the mother's body.
Structure. The human egg has a diameter of about 130 microns. A transparent (shiny) zone is adjacent to the plasma lemma (zona pellucida- Zp) and then a layer of follicular epithelial cells (Fig. 21.3).
The nucleus of the female reproductive cell has a haploid set of chromosomes with an X-sex chromosome, a well-defined nucleolus, and there are many pore complexes in the nucleus envelope. During the period of oocyte growth, intensive processes of mRNA and rRNA synthesis take place in the nucleus.
Rice. 21.3. The structure of the female reproductive cell:
1 - core; 2 - plasmalemma; 3 - follicular epithelium; 4 - radiant crown; 5 - cortical granules; 6 - yolk inclusions; 7 - transparent zone; 8 - Zp3 receptor
In the cytoplasm, the protein synthesis apparatus (endoplasmic reticulum, ribosomes) and the Golgi complex are developed. The number of mitochondria is moderate, they are located near the nucleus, where there is an intensive synthesis of the yolk, the cell center is absent. The Golgi complex in the early stages of development is located near the nucleus, and in the process of maturation of the egg, it shifts to the periphery of the cytoplasm. Here are the derivatives of this complex - cortical granules (granula corticalia), the number of which reaches 4000, and the size is 1 micron. They contain glycosaminoglycans and various enzymes (including proteolytic ones), participate in the cortical reaction, protecting the egg from polyspermy.
Of the inclusions, ovoplasms deserve special attention yolk granules, containing proteins, phospholipids and carbohydrates. Each yolk granule is surrounded by a membrane, has a dense central part, consisting of phosphovitin (phosphoprotein), and a looser peripheral part, consisting of lipovitellin (lipoprotein).
Transparent zone (zona pellucida- Zp) consists of glycoproteins and glycosaminoglycans - chondroitin sulfuric, hyaluronic and sialic acids. Glycoproteins are represented by three fractions - Zpl, Zp2, Zp3. The Zp2 and Zp3 fractions form filaments 2–3 µm long and 7 nm thick, which
interconnected using the Zpl fraction. Fraction Zp3 is receptor sperm cells, and Zp2 prevents polyspermy. The clear zone contains tens of millions of Zp3 glycoprotein molecules, each with more than 400 amino acid residues connected to many oligosaccharide branches. Follicular epithelial cells take part in the formation of the transparent zone: the processes of follicular cells penetrate the transparent zone, heading towards the plasmolemma of the egg. The plasmolemma of the egg, in turn, forms microvilli located between the processes of follicular epithelial cells (see Fig. 21.3). The latter perform trophic and protective functions.
21.2. Embryogenesis
Human intrauterine development lasts an average of 280 days (10 lunar months). It is customary to distinguish three periods: initial (1st week), embryonic (2-8th week), fetal (from the 9th week of development to the birth of a child). By the end of the embryonic period, the laying of the main embryonic rudiments of tissues and organs is completed.
Fertilization and zygote formation
Fertilization (fertilization)- the fusion of male and female germ cells, as a result of which the diploid set of chromosomes characteristic of this type of animal is restored, and a qualitatively new cell appears - a zygote (a fertilized egg, or a single-celled embryo).
In humans, the volume of ejaculate - erupted sperm - is normally about 3 ml. To ensure fertilization, the total number of spermatozoa in the semen must be at least 150 million, and the concentration - 20-200 million / ml. In the genital tract of a woman after copulation, their number decreases in the direction from the vagina to the ampullar part of the fallopian tube.
In the process of fertilization, three phases are distinguished: 1) distant interaction and convergence of gametes; 2) contact interaction and activation of the egg; 3) penetration of the sperm into the egg and subsequent fusion - syngamy.
First phase- distant interaction - is provided by chemotaxis - a set of specific factors that increase the likelihood of meeting germ cells. An important role in this is played gamons- chemical substances produced by sex cells (Fig. 21.4). For example, eggs secrete peptides that help attract sperm.
Immediately after ejaculation, sperm are not able to penetrate the egg until capacitation occurs - the acquisition of fertilizing ability by sperm under the action of the secret of the female genital tract, which lasts 7 hours. In the process of capacitation, glycoproteins and proteins are removed from the sperm plasmolemma in the acrosome seminal plasma, which contributes to the acrosomal reaction.
Rice. 21.4. Distant and contact interaction of sperm and egg: 1 - sperm and its receptors on the head; 2 - separation of carbohydrates from the surface of the head during capacitation; 3 - binding of sperm receptors to egg receptors; 4 - Zp3 (the third fraction of glycoproteins of the transparent zone); 5 - plasmomolema of the egg; GGI, GGII - gynogamons; AGI, AGII - androgamones; Gal - glycosyltransferase; NAG - N-acetylglucosamine
In the mechanism of capacitation, hormonal factors are of great importance, primarily progesterone (the hormone of the corpus luteum), which activates the secretion of the glandular cells of the fallopian tubes. During capacitation, sperm plasma membrane cholesterol binds to female genital tract albumin and germ cell receptors are exposed. Fertilization occurs in the ampulla of the fallopian tube. Fertilization is preceded by insemination - the interaction and convergence of gametes (distant interaction), due to chemotaxis.
Second phase fertilization - contact interaction. Numerous sperm cells approach the egg and come into contact with its membrane. The egg begins to rotate around its axis at a speed of 4 revolutions per minute. These movements are caused by the beating of the sperm tails and last about 12 hours. Spermatozoa, when in contact with the egg, can bind tens of thousands of Zp3 glycoprotein molecules. This marks the start of the acrosomal reaction. The acrosomal reaction is characterized by an increase in the permeability of the sperm plasmolemma to Ca 2 + ions, its depolarization, which contributes to the fusion of the plasmolemma with the anterior acrosome membrane. The transparent zone is in direct contact with acrosomal enzymes. Enzymes destroy it, sperm passes through the transparent zone and
Rice. 21.5. Fertilization (according to Wasserman with changes):
1-4 - stages of acrosomal reaction; 5 - zone pellucida(transparent zone); 6 - perivitelline space; 7 - plasma membrane; 8 - cortical granule; 8a - cortical reaction; 9 - penetration of sperm into the egg; 10 - zone reaction
enters the perivitelline space, located between the transparent zone and the plasmolemma of the egg. After a few seconds, the properties of the plasmolemma of the egg cell change and the cortical reaction begins, and after a few minutes the properties of the transparent zone change (zonal reaction).
The initiation of the second phase of fertilization occurs under the influence of sulfated polysaccharides of the zona pellucida, which cause the entry of calcium and sodium ions into the head, sperm, replacing them with potassium and hydrogen ions and rupture of the acrosome membrane. Attachment of the sperm to the egg occurs under the influence of the carbohydrate group of the glycoprotein fraction of the transparent zone of the egg. Sperm receptors are a glycosyltransferase enzyme located on the surface of the acrosome of the head, which
Rice. 21.6. Phases of fertilization and the beginning of crushing (scheme):
1 - ovoplasm; 1a - cortical granules; 2 - core; 3 - transparent zone; 4 - follicular epithelium; 5 - sperm; 6 - reduction bodies; 7 - completion of the mitotic division of the oocyte; 8 - tubercle of fertilization; 9 - fertilization shell; 10 - female pronucleus; 11 - male pronucleus; 12 - syncarion; 13 - the first mitotic division of the zygote; 14 - blastomeres
"recognizes" the receptor of the female germ cell. Plasma membranes at the site of contact of germ cells merge, and plasmogamy occurs - the union of the cytoplasms of both gametes.
In mammals, only one sperm enters the egg during fertilization. Such a phenomenon is called monospermy. Fertilization is facilitated by hundreds of other sperm involved in the insemination. Enzymes secreted from acrosomes - spermolysins (trypsin, hyaluronidase) - destroy the radiant crown, break down glycosaminoglycans of the transparent zone of the egg. Separated follicular epithelial cells stick together into a conglomerate, which, following the egg, moves along the fallopian tube due to the flickering of cilia epithelial cells mucous membrane.
Rice. 21.7. Human egg and zygote (according to B.P. Khvatov):
a- human egg after ovulation: 1 - cytoplasm; 2 - core; 3 - transparent zone; 4 - follicular epithelial cells forming a radiant crown; b- human zygote in the stage of convergence of male and female nuclei (pronuclei): 1 - female nucleus; 2 - male core
Third phase. The head and the intermediate part of the caudal region penetrate into the ovoplasm. After the entry of the spermatozoon into the egg, on the periphery of the ovoplasm, it becomes denser (zone reaction) and forms fertilization shell.
Cortical reaction- fusion of the plasmolemma of the egg with the membranes of the cortical granules, as a result of which the contents of the granules enter the perivitelline space and act on the glycoprotein molecules of the transparent zone (Fig. 21.5).
As a result of this zone reaction, Zp3 molecules are modified and lose their ability to be sperm receptors. A fertilization shell 50 nm thick is formed, which prevents polyspermy - the penetration of other sperm.
The mechanism of the cortical reaction involves the influx of sodium ions through the segment of the spermatozoon plasmalemma, which is embedded in the egg cell plasmalemma after completion of the acrosomal reaction. As a result, the negative membrane potential of the cell becomes weakly positive. The influx of sodium ions causes the release of calcium ions from intracellular depots and an increase in its content in the hyaloplasm of the egg. This is followed by exocytosis of the cortical granules. The proteolytic enzymes released from them break the bonds between the transparent zone and the plasmolemma of the egg, as well as between sperm and the transparent zone. In addition, a glycoprotein is released that binds water and attracts it into the space between the plasmalemma and the transparent zone. As a result, a perivitelline space is formed. Finally,
a factor is released that contributes to the hardening of the transparent zone and the formation of a fertilization shell from it. Due to the mechanisms of preventing polyspermy, only one haploid nucleus of the spermatozoon gets the opportunity to merge with one haploid nucleus of the egg, which leads to the restoration of the diploid set characteristic of all cells. Penetration of the spermatozoon into the egg after a few minutes significantly enhances the processes of intracellular metabolism, which is associated with the activation of its enzymatic systems. The interaction of spermatozoa with the egg can be blocked by antibodies against substances included in the transparent zone. On this basis, methods of immunological contraception are being sought.
After the convergence of the female and male pronuclei, which lasts for about 12 hours in mammals, a zygote is formed - a unicellular embryo (Fig. 21.6, 21.7). At the zygote stage, presumptive zones(lat. presumptio- probability, assumption) as sources of development of the corresponding sections of the blastula, from which germ layers are subsequently formed.
21.2.2. Cleavage and formation of the blastula
Splitting up (fissio)- sequential mitotic division of the zygote into cells (blastomeres) without the growth of daughter cells to the size of the mother.
The resulting blastomeres remain united into a single organism of the embryo. In the zygote, a mitotic spindle is formed between the receding
Rice. 21.8. The human embryo in the early stages of development (according to Hertig and Rock):
a- stage of two blastomeres; b- blastocyst: 1 - embryoblast; 2 - trophoblast;
3 - blastocyst cavity
Rice. 21.9. Cleavage, gastrulation and implantation of the human embryo (scheme): 1 - crushing; 2 - morula; 3 - blastocyst; 4 - blastocyst cavity; 5 - embryo-blast; 6 - trophoblast; 7 - germinal nodule: a - epiblast; b- hypoblast; 8 - fertilization shell; 9 - amniotic (ectodermal) vesicle; 10 - extra-embryonic mesenchyme; 11 - ectoderm; 12 - endoderm; 13 - cytotrophoblast; 14 - symplastotrophoblast; 15 - germinal disk; 16 - gaps with maternal blood; 17 - chorion; 18 - amniotic leg; 19 - yolk vesicle; 20 - mucous membrane of the uterus; 21 - oviduct
moving towards the poles by centrioles introduced by the spermatozoon. Pronuclei enter the prophase stage with the formation of a combined diploid set of egg and sperm chromosomes.
After passing through all the other phases of mitotic division, the zygote is divided into two daughter cells - blastomeres(from Greek. blastos- germ, meros- part). Due to the virtual absence of the G 1 period, during which the cells formed as a result of division grow, the cells are much smaller than the maternal one, therefore, the size of the embryo as a whole during this period, regardless of the number of its constituent cells, does not exceed the size of the original cell - the zygote. All this made it possible to call the described process crushing(i.e., grinding), and the cells formed in the process of crushing - blastomeres.
Cleavage of the human zygote begins by the end of the first day and is characterized as full non-uniform asynchronous. During the first days it occurred
walks slowly. The first crushing (division) of the zygote is completed after 30 hours, resulting in the formation of two blastomeres covered with a fertilization membrane. The stage of two blastomeres is followed by the stage of three blastomeres.
From the very first crushing of the zygote, two types of blastomeres are formed - “dark” and “light”. "Light", smaller, blastomeres are crushed faster and are arranged in one layer around the large "dark", which are in the middle of the embryo. From the superficial "light" blastomeres, subsequently arises trophoblast, connecting the embryo with the mother's body and providing its nutrition. Internal, "dark", blastomeres form embryoblast, from which the body of the embryo and extraembryonic organs (amnion, yolk sac, allantois) are formed.
Starting from the 3rd day, cleavage proceeds faster, and on the 4th day the embryo consists of 7-12 blastomeres. After 50-60 hours, a dense accumulation of cells is formed - morula, and on the 3rd-4th day, the formation begins blastocysts- a hollow bubble filled with liquid (see Fig. 21.8; Fig. 21.9).
The blastocyst moves through the fallopian tube to the uterus within 3 days and enters the uterine cavity after 4 days. The blastocyst is free in the uterine cavity (loose blastocyst) within 2 days (5th and 6th days). By this time, the blastocyst increases in size due to an increase in the number of blastomeres - embryoblast and trophoblast cells - up to 100 and due to increased absorption of the secretion of the uterine glands by the trophoblast and active production of fluid by trophoblast cells (see Fig. 21.9). The trophoblast during the first 2 weeks of development provides nutrition to the embryo due to the decay products of maternal tissues (histiotrophic type of nutrition),
The embryoblast is located in the form of a bundle of germ cells ("germ bundle"), which is attached internally to the trophoblast at one of the poles of the blastocyst.
21.2.4. Implantation
Implantation (lat. implantation- ingrowth, rooting) - the introduction of the embryo into the mucous membrane of the uterus.
There are two stages of implantation: adhesion(adhesion) when the embryo attaches to the inner surface of the uterus, and invasion(immersion) - the introduction of the embryo into the tissue of the mucous membrane of the uterus. On the 7th day, changes occur in the trophoblast and embryoblast associated with the preparation for implantation. The blastocyst retains the fertilization membrane. In the trophoblast, the number of lysosomes with enzymes increases, which ensure the destruction (lysis) of the tissues of the uterine wall and thereby contribute to the introduction of the embryo into the thickness of its mucous membrane. Microvilli appearing in the trophoblast gradually destroy the fertilization membrane. The germinal nodule flattens and becomes
in germinal shield, in which preparations for the first stage of gastrulation begin.
Implantation lasts about 40 hours (see Fig. 21.9; Fig. 21.10). Simultaneously with implantation, gastrulation (the formation of germ layers) begins. it first critical period development.
In the first stage trophoblast is attached to the epithelium of the uterine mucosa, and two layers are formed in it - cytotrophoblast and symplastotrophoblast. In the second stage symplastotrophoblast, producing proteolytic enzymes, destroys the uterine mucosa. At the same time, the villi trophoblast, penetrating into the uterus, sequentially destroy its epithelium, then the underlying connective tissue and vessel walls, and the trophoblast comes into direct contact with the blood of the maternal vessels. Formed implantation fossa, in which areas of hemorrhages appear around the embryo. The nutrition of the embryo is carried out directly from the maternal blood (hematotrophic type of nutrition). From the mother's blood, the fetus receives not only all the nutrients, but also the oxygen necessary for breathing. At the same time, in the uterine mucosa from connective tissue cells rich in glycogen, the formation of decidual cells. After the embryo is completely immersed in the implantation fossa, the hole formed in the uterine mucosa is filled with blood and tissue destruction products of the uterine mucosa. Subsequently, the mucosal defect disappears, the epithelium is restored by cellular regeneration.
The hematotrophic type of nutrition, replacing the histiotrophic one, is accompanied by a transition to a qualitatively new stage of embryogenesis - the second phase of gastrulation and the laying of extra-embryonic organs.
21.3. GASTRULATION AND ORGANOGENESIS
Gastrulation (from lat. gaster- stomach) - a complex process of chemical and morphogenetic changes, accompanied by reproduction, growth, directed movement and differentiation of cells, resulting in the formation of germ layers: outer (ectoderm), middle (mesoderm) and inner (endoderm) - sources of development of the complex of axial organs and embryonic tissue buds.
Gastrulation in humans occurs in two stages. First stage(deeds-nation) falls on the 7th day, and second stage(immigration) - on the 14-15th day of intrauterine development.
At delamination(from lat. lamina- plate), or splitting, from the material of the germinal nodule (embryoblast), two sheets are formed: the outer sheet - epiblast and internal - hypoblast, facing into the cavity of the blastocyst. Epiblast cells look like pseudostratified prismatic epithelium. Hypoblast cells - small cubic, with foamy cyto-
Rice. 21.10. Human embryos 7.5 and 11 days of development in the process of implantation in the uterine mucosa (according to Hertig and Rocca):
a- 7.5 days of development; b- 11 days of development. 1 - ectoderm of the embryo; 2 - endoderm of the embryo; 3 - amniotic vesicle; 4 - extra-embryonic mesenchyme; 5 - cytotrophoblast; 6 - symplastotrophoblast; 7 - uterine gland; 8 - gaps with maternal blood; 9 - epithelium of the mucous membrane of the uterus; 10 - own plate of the mucous membrane of the uterus; 11 - primary villi
plasma, form a thin layer under the epiblast. Part of the epiblast cells later form a wall amniotic sac, which begins to form on the 8th day. In the area of the bottom of the amniotic vesicle, a small group of epiblast cells remains - the material that will go to the development of the body of the embryo and extra-embryonic organs.
Following delamination, cells are evicted from the outer and inner sheets into the blastocyst cavity, which marks the formation extraembryonic mesenchyme. By the 11th day, the mesenchyme grows up to the trophoblast and the chorion is formed - the villous membrane of the embryo with primary chorionic villi (see Fig. 21.10).
Second stage gastrulation occurs by immigration (movement) of cells (Fig. 21.11). The movement of cells occurs in the area of the bottom of the amniotic vesicle. Cellular flows arise in the direction from front to back, towards the center and in depth as a result of cell reproduction (see Fig. 21.10). This results in the formation of a primary streak. At the head end, the primary streak thickens, forming primary, or head, knot(Fig. 21.12), from where the head process originates. The head process grows in the cranial direction between the epi- and hypoblast and further gives rise to the development of the notochord of the embryo, which determines the axis of the embryo, is the basis for the development of the bones of the axial skeleton. Around the hora, the spinal column is formed in the future.
Cellular material that moves from the primary streak into the space between the epiblast and hypoblast is located parachordally in the form of meso-dermal wings. Part of the epiblast cells is introduced into the hypoblast, participating in the formation of the intestinal endoderm. As a result, the embryo acquires a three-layer structure in the form of a flat disc, consisting of three germ layers: ectoderm, mesoderm and endoderm.
Factors affecting the mechanisms of gastrulation. The methods and rate of gastrulation are determined by a number of factors: the dorsoventral metabolic gradient, which determines the asynchrony of cell reproduction, differentiation, and movement; surface tension of cells and intercellular contacts that contribute to the displacement of cell groups. An important role is played by inductive factors. According to the theory of organizational centers proposed by G. Spemann, inductors (organizing factors) arise in certain parts of the embryo, which have an inducing effect on other parts of the embryo, causing their development in a certain direction. There are inductors (organizers) of several orders acting sequentially. For example, it has been proven that the first order organizer induces the development of the neural plate from the ectoderm. In the neural plate, an organizer of the second order appears, which contributes to the transformation of a section of the neural plate into an eye cup, etc.
At present, the chemical nature of many inductors (proteins, nucleotides, steroids, etc.) has been elucidated. The role of gap junctions in intercellular interactions has been established. Under the action of inductors emanating from one cell, the induced cell, which has the ability to respond specifically, changes the path of development. A cell that is not subjected to induction action retains its former potencies.
Differentiation of the germ layers and mesenchyme begins at the end of the 2nd - beginning of the 3rd week. One part of the cells is transformed into the rudiments of tissues and organs of the embryo, the other - into extra-embryonic organs (see Chapter 5, Scheme 5.3).
Rice. 21.11. The structure of a 2-week-old human embryo. The second stage of gastrulation (scheme):
a- transverse section of the embryo; b- germinal disc (view from the side of the amniotic vesicle). 1 - chorionic epithelium; 2 - chorion mesenchyme; 3 - gaps filled with maternal blood; 4 - base of the secondary villi; 5 - amniotic leg; 6 - amniotic vesicle; 7 - yolk vesicle; 8 - germinal shield in the process of gastrulation; 9 - primary strip; 10 - rudiment of intestinal endoderm; 11 - yolk epithelium; 12 - epithelium of the amniotic membrane; 13 - primary knot; 14 - prechordal process; 15 - extraembryonic mesoderm; 16 - extraembryonic ectoderm; 17 - extraembryonic endoderm; 18 - germinal ectoderm; 19 - germinal endoderm
Rice. 21.12. Human embryo 17 days ("Crimea"). Graphic reconstruction: a- embryonic disc (top view) with projection of axial anlages and definitive cardiovascular system; b- sagittal (middle) section through the axial tabs. 1 - projection of the bilateral bookmarks of the endocardium; 2 - projection of bilateral anlages of the pericardial coelom; 3 - projection of bilateral anlages of corporal blood vessels; 4 - amniotic leg; 5 - blood vessels in the amniotic leg; 6 - blood islands in the wall of the yolk sac; 7 - allantois bay; 8 - cavity of the amniotic vesicle; 9 - cavity of the yolk sac; 10 - trophoblast; 11 - chordal process; 12 - head knot. Conventions: primary strip - vertical shading; the primary cephalic nodule is indicated by crosses; ectoderm - without shading; endoderm - lines; extra-embryonic mesoderm - points (according to N. P. Barsukov and Yu. N. Shapovalov)
Differentiation of the germ layers and mesenchyme, leading to the appearance of tissue and organ primordia, occurs non-simultaneously (heterochronously), but interconnected (integratively), resulting in the formation of tissue primordia.
21.3.1. Ectoderm differentiation
As the ectoderm differentiates, they form embryonic parts - dermal ectoderm, neuroectoderm, placodes, prechordal plate, and extra-germ ectoderm, which is the source of the formation of the epithelial lining of the amnion. Smaller part of the ectoderm located above the notochord (neuroectoderm), gives rise to differentiation neural tube and neural crest. Skin ectoderm gives rise to stratified squamous epithelium of the skin (epidermis) and its derivatives, the epithelium of the cornea and conjunctiva of the eye, the epithelium of the oral cavity, enamel and cuticle of the teeth, the epithelium of the anal rectum, the epithelial lining of the vagina.
Neurulation- the process of formation of the neural tube - proceeds differently in time in different parts of the embryo. Neural tube closure begins at cervical region, and then spreads posteriorly and somewhat more slowly in the cranial direction, where brain bubbles. Approximately on the 25th day, the neural tube is completely closed, only two non-closed openings at the anterior and posterior ends communicate with the external environment - anterior and posterior neuropores(Fig. 21.13). Posterior neuropore corresponds neurointestinal canal. After 5-6 days, both neuropores overgrow. From the neural tube, neurons and neuroglia of the brain and spinal cord, the retina of the eye and the organ of smell are formed.
With the closing of the side walls of the neural folds and the formation of the neural tube, a group of neuroectodermal cells appears, which are formed in the junction of the neural and the rest (skin) ectoderm. These cells, first arranged in longitudinal rows on either side between the neural tube and the ectoderm, form neural crest. Neural crest cells are capable of migration. In the trunk, some cells migrate in the surface layer of the dermis, others migrate in the ventral direction, forming neurons and neuroglia of parasympathetic and sympathetic nodes, chromaffin tissue and adrenal medulla. Some cells differentiate into neurons and neuroglia of the spinal nodes.
Cells are released from the epiblast prechordal plate, which is included in the composition of the head of the intestinal tube. From the material of the prechordal plate, the stratified epithelium of the anterior part of the digestive tube and its derivatives subsequently develops. In addition, the epithelium of the trachea, lungs and bronchi, as well as the epithelial lining of the pharynx and esophagus, derivatives of gill pockets - the thymus, etc., is formed from the prechordal plate.
According to A. N. Bazhanov, the source of formation of the lining of the esophagus and respiratory tract serves as the endoderm of the head intestine.
Rice. 21.13. Neurulation in the human embryo:
a- view from the back; b- cross sections. 1 - anterior neuropore; 2 - posterior neuropore; 3 - ectoderm; 4 - neural plate; 5 - neural groove; 6 - mesoderm; 7 - chord; 8 - endoderm; 9 - neural tube; 10 - neural crest; 11 - brain; 12 - spinal cord; 13 - spinal canal
Rice. 21.14. The human embryo at the stage of formation of the trunk fold and extra-breathing organs (according to P. Petkov):
1 - symplastotrophoblast; 2 - cytotrophoblast; 3 - extra-embryonic mesenchyme; 4 - place of the amniotic leg; 5 - primary intestine; 6 - amnion cavity; 7 - amnion ectoderm; 8 - extra-embryonic amnion mesenchyme; 9 - cavity of the yolk vesicle; 10 - endoderm of the yolk vesicle; 11 - extra-embryonic mesenchyme of the yolk sac; 12 - allantois. The arrows indicate the direction of formation of the trunk fold
As part of the germinal ectoderm, placodes are laid, which are the source of the development of epithelial structures. inner ear. From the extra-breathing ectoderm, the epithelium of the amnion and umbilical cord is formed.
21.3.2. Endoderm differentiation
Differentiation of the endoderm leads to the formation of the endoderm of the intestinal tube in the body of the embryo and the formation of the extraembryonic endoderm, which forms the lining of the vitelline vesicle and allantois (Fig. 21.14).
Isolation of the intestinal tube begins with the appearance of the trunk fold. The latter, deepening, separates the intestinal endoderm of the future intestine from the extraembryonic endoderm of the yolk sac. In the posterior part of the embryo, the resulting intestine also includes that part of the endoderm from which the endodermal outgrowth of the allantois arises.
From the endoderm of the intestinal tube, a single-layer integumentary epithelium of the stomach, intestines and their glands develops. In addition, from this
dermis develop epithelial structures of the liver and pancreas.
The extraembryonic endoderm gives rise to the epithelium of the yolk sac and allantois.
21.3.3. mesoderm differentiation
This process begins on the 3rd week of embryogenesis. The dorsal sections of the mesoderm are divided into dense segments lying on the sides of the chord - somites. The process of segmentation of the dorsal mesoderm and the formation of somites begins in the head of the embryo and rapidly spreads caudally.
The embryo on the 22nd day of development has 7 pairs of segments, on the 25th - 14, on the 30th - 30, and on the 35th - 43-44 pairs. Unlike somites, the ventral sections of the mesoderm (splanchnotome) are not segmented, but split into two sheets - visceral and parietal. A small section of the mesoderm, connecting the somites with the splanchnotome, is divided into segments - segmental legs (nephrogonotome). At the posterior end of the embryo, segmentation of these divisions does not occur. Here, instead of segmental legs, there is a non-segmented nephrogenic rudiment (nephrogenic cord). The paramesonephric canal also develops from the mesoderm of the embryo.
Somites differentiate into three parts: the myotome, which gives rise to striated skeletal muscle tissue, the sclerotome, which is the source of the development of bone and cartilage tissues, and the dermatome, which forms the connective tissue basis of the skin - the dermis.
From segmental legs (nephrogonotomes) the epithelium of the kidneys, gonads and vas deferens develops, and from the paramesonephric canal - the epithelium of the uterus, fallopian tubes (oviducts) and the epithelium of the primary lining of the vagina.
The parietal and visceral sheets of the splanchnotome form the epithelial lining of the serous membranes - the mesothelium. From a part of the visceral layer of the mesoderm (myoepicardial plate), the middle and outer shells of the heart develop - the myocardium and epicardium, as well as the adrenal cortex.
The mesenchyme in the body of the embryo is the source of the formation of many structures - blood cells and hematopoietic organs, connective tissue, blood vessels, smooth muscle tissue, microglia (see Chapter 5). From the extra-embryonic mesoderm, the mesenchyme develops, giving rise to the connective tissue of extra-embryonic organs - amnion, allantois, chorion, yolk vesicle.
The connective tissue of the embryo and its provisional organs is characterized by a high hydrophilicity of the intercellular substance, a richness of glycosaminoglycans in the amorphous substance. The connective tissue of the provisional organs differentiates faster than in the organ rudiments, which is due to the need to establish a connection between the embryo and the mother's body and
ensuring their development (for example, the placenta). Differentiation of the chorion mesenchyme occurs early, but does not occur simultaneously over the entire surface. The process is most active in the development of the placenta. The first fibrous structures also appear here, which play an important role in the formation and strengthening of the placenta in the uterus. With the development of the fibrous structures of the stroma of the villi, argyrophilic pre-collagen fibers are successively formed, and then collagen fibers.
At the 2nd month of development in the human embryo, differentiation of the skeletal and skin mesenchyme, as well as the mesenchyme of the heart wall and large blood vessels, begins first of all.
The arteries of the muscular and elastic type of human embryos, as well as the arteries of the stem (anchor) villi of the placenta and their branches, contain desmin-negative smooth myocytes, which have the property of faster contraction.
At the 7th week of development of the human embryo in the skin mesenchyme and mesenchyme internal organs small lipid inclusions appear, and later (8-9 weeks) the formation of fat cells occurs. Following the development of connective tissue of cardio-vascular system the connective tissue of the lungs and digestive tube differentiates. The differentiation of the mesenchyme in human embryos (11-12 mm long) at the 2nd month of development begins with an increase in the amount of glycogen in the cells. In the same areas, the activity of phosphatases increases, and later, in the course of differentiation, glycoproteins accumulate, RNA and protein are synthesized.
fruitful period. The fetal period begins from the 9th week and is characterized by significant morphogenetic processes occurring in the body of both the fetus and the mother (Table 21.1).
Table 21.1. A brief calendar of intrauterine development of a person (with additions according to R. K. Danilov, T. G. Borovoy, 2003)
Continuation of the table. 21.1
Continuation of the table. 21.1
Continuation of the table. 21.1
Continuation of the table. 21.1
Continuation of the table. 21.1
Continuation of the table. 21.1
Continuation of the table. 21.1
The end of the table. 21.1
21.4. EXTRA-GERMAL ORGANS
Extra-embryonic organs that develop in the process of embryogenesis outside the body of the embryo perform a variety of functions that ensure the growth and development of the embryo itself. Some of these organs surrounding the embryo are also called embryonic membranes. These organs include the amnion, yolk sac, allantois, chorion, placenta (Fig. 21.15).
The sources of development of tissues of extra-embryonic organs are the troph-ectoderm and all three germ layers (Scheme 21.1). General properties fabric-
Rice. 21.15. The development of extra-embryonic organs in the human embryo (scheme): 1 - amniotic vesicle; 1a - amnion cavity; 2 - the body of the embryo; 3 - yolk sac; 4 - extraembryonic coelom; 5 - primary villi of the chorion; 6 - secondary villi of the chorion; 7 - allantois stalk; 8 - tertiary villi of the chorion; 9 - allan-tois; 10 - umbilical cord; 11 - smooth chorion; 12 - cotyledons
Scheme 21.1. Classification of tissues of extra-embryonic organs (according to V. D. Novikov, G. V. Pravotorov, Yu. I. Sklyanov)
her extra-embryonic organs and their differences from the definitive ones are as follows: 1) the development of tissues is reduced and accelerated; 2) connective tissue contains few cellular forms, but a lot of amorphous substance rich in glycosaminoglycans; 3) aging of tissues of extra-embryonic organs occurs very quickly - by the end of fetal development.
21.4.1. Amnion
Amnion- a temporary organ that provides an aquatic environment for the development of the embryo. It arose in evolution in connection with the release of vertebrates from water to land. In human embryogenesis, it appears at the second stage of gastrulation, first as a small vesicle as part of the epiblast.
The wall of the amniotic vesicle consists of a layer of cells of the extra-embryonic ectoderm and extra-embryonic mesenchyme, forms its connective tissue.
The amnion rapidly increases, and by the end of the 7th week, its connective tissue comes into contact with the connective tissue of the chorion. At the same time, the amnion epithelium passes to the amniotic stalk, which later turns into the umbilical cord, and in the region of the umbilical ring it merges with the epithelial cover of the skin of the embryo.
The amniotic membrane forms the wall of the reservoir filled with amniotic fluid, in which the fetus is located (Fig. 21.16). The main function of the amniotic membrane is the production of amniotic fluid, which provides an environment for the developing organism and protects it from mechanical damage. The epithelium of the amnion, facing its cavity, not only releases amniotic fluid, but also takes part in their reabsorption. The necessary composition and concentration of salts are maintained in the amniotic fluid until the end of pregnancy. Amnion also performs a protective function, preventing harmful agents from entering the fetus.
The epithelium of the amnion in the early stages is single-layer flat, formed by large polygonal cells closely adjacent to each other, among which there are many mitotically dividing. At the 3rd month of embryogenesis, the epithelium is transformed into a prismatic one. On the surface of the epithelium there are microvilli. The cytoplasm always contains small lipid droplets and glycogen granules. In the apical parts of the cells there are vacuoles of various sizes, the contents of which are released into the amnion cavity. The epithelium of the amnion in the area of the placental disc is single-layer prismatic, sometimes multi-row, performs a predominantly secretory function, while the epithelium of the extra-placental amnion mainly resorbs amniotic fluid.
In the connective tissue stroma of the amniotic membrane, a basement membrane, a layer of dense fibrous connective tissue and a spongy layer of loose fibrous connective tissue are distinguished, connecting
Rice. 21.16. The dynamics of the relationship of the embryo, extra-embryonic organs and uterine membranes:
a- human embryo 9.5 weeks of development (micrograph): 1 - amnion; 2 - chorion; 3 - forming placenta; 4 - umbilical cord
common amnion with chorion. In the layer of dense connective tissue, the acellular part lying under the basement membrane and the cellular part can be distinguished. The latter consists of several layers of fibroblasts, between which there is a dense network of thin bundles of collagen and reticular fibers tightly adjacent to each other, forming an irregularly shaped lattice oriented parallel to the surface of the shell.
The spongy layer is formed by a loose mucous connective tissue with sparse bundles of collagen fibers, which are a continuation of those that lie in a layer of dense connective tissue, connecting the amnion with the chorion. This connection is very fragile, and therefore both shells are easy to separate from each other. The main substance of the connective tissue contains many glycosaminoglycans.
21.4.2. Yolk sac
Yolk sac- the most ancient extra-embryonic organ in evolution, which arose as an organ that deposits nutrients (yolk) necessary for the development of the embryo. In humans, this is a rudimentary formation (yolk vesicle). It is formed by extra-embryonic endoderm and extra-embryonic mesoderm (mesenchyme). Appearing on the 2nd week of development in humans, the yolk vesicle in the nutrition of the embryo takes
Rice. 21.16. Continuation
b- diagram: 1 - muscular membrane of the uterus; 2- decidua basalis; 3 - amnion cavity; 4 - cavity of the yolk sac; 5 - extraembryonic coelom (chorionic cavity); 6- decidua capsularis; 7 - decidua parietalis; 8 - uterine cavity; 9 - cervix; 10 - embryo; 11 - tertiary villi of the chorion; 12 - allantois; 13 - mesenchyme of the umbilical cord: a- blood vessels of the chorionic villus; b- lacunae with maternal blood (according to Hamilton, Boyd and Mossman)
participation is very short, since from the 3rd week of development, a connection between the fetus and the mother's body is established, i.e., hematotrophic nutrition. The yolk sac of vertebrates is the first organ in the wall of which blood islands develop, forming the first blood cells and the first blood vessels that provide oxygen and nutrients to the fetus.
As the trunk fold is formed, which lifts the embryo above the yolk sac, an intestinal tube is formed, while the yolk sac is separated from the body of the embryo. The connection of the embryo with the yolk sac remains in the form of a hollow funiculus called the yolk stalk. As a hematopoietic organ, the yolk sac functions until the 7-8th week, and then undergoes reverse development and remains in the umbilical cord in the form of a narrow tube that serves as a conductor of blood vessels to the placenta.
21.4.3. Allantois
Allantois is a small finger-like process in the caudal part of the embryo, growing into the amniotic stalk. It is derived from the yolk sac and consists of the extraembryonic endoderm and the visceral mesoderm. In humans, the allantois does not reach significant development, but its role in providing nutrition and respiration of the embryo is still great, since the vessels located in the umbilical cord grow along it towards the chorion. The proximal part of the allantois is located along the yolk stalk, and the distal part, growing, grows into the gap between the amnion and the chorion. It is an organ of gas exchange and excretion. Oxygen is delivered through the vessels of the allantois, and metabolic products of the embryo are released into the allantois. At the 2nd month of embryogenesis, allantois is reduced and turns into a cord of cells, which, together with the reduced vitelline vesicle, is part of the umbilical cord.
21.4.4. umbilical cord
The umbilical cord, or umbilical cord, is an elastic cord that connects the embryo (fetus) to the placenta. It is covered by an amniotic membrane surrounding a mucous connective tissue with blood vessels (two umbilical arteries and one vein) and vestiges of the yolk sac and allantois.
Mucous connective tissue, called "Wharton's jelly", ensures the elasticity of the cord, protects the umbilical vessels from compression, thereby ensuring a continuous supply of nutrients and oxygen to the embryo. Along with this, it prevents the penetration of harmful agents from the placenta to the embryo by extravascular means and thus performs a protective function.
Immunocytochemical methods have established that in the blood vessels of the umbilical cord, placenta and embryo there are heterogeneous smooth muscle cells (SMCs). In veins, in contrast to arteries, desmin-positive SMCs were found. The latter provide slow tonic contractions of the veins.
21.4.5. Chorion
Chorion, or villous sheath, appears for the first time in mammals, develops from the trophoblast and extraembryonic mesoderm. Initially, the trophoblast is represented by a layer of cells that form primary villi. They secrete proteolytic enzymes, with the help of which the uterine mucosa is destroyed and implantation is carried out. On the 2nd week, the trophoblast acquires a two-layer structure due to the formation in it of the inner cell layer (cytotrophoblast) and the symplastic outer layer (symplastotrophoblast), which is a derivative of the cell layer. The extra-embryonic mesenchyme that appears along the periphery of the embryoblast (in humans at the 2-3rd week of development) grows to the trophoblast and forms secondary epitheliomesenchymal villi with it. From this time on, the trophoblast turns into a chorion, or villous membrane (see Fig. 21.16).
At the beginning of the 3rd week, blood capillaries grow into the villi of the chorion and tertiary villi form. This coincides with the beginning of hematotrophic nutrition of the embryo. The further development of the chorion is associated with two processes - the destruction of the uterine mucosa due to the proteolytic activity of the outer (symplastic) layer and the development of the placenta.
21.4.6. Placenta
Placenta (children's place) human belongs to the type of discoidal hemochorial villous placenta (see Fig. 21.16; Fig. 21.17). This is an important temporary organ with a variety of functions that provide a connection between the fetus and the mother's body. At the same time, the placenta creates a barrier between the blood of the mother and the fetus.
The placenta consists of two parts: germinal, or fetal (pars fetalis) and maternal (pars materna). The fetal part is represented by a branched chorion and an amniotic membrane adhering to the chorion from the inside, and the maternal part is a modified uterine mucosa that is rejected during childbirth (decidua basalis).
The development of the placenta begins on the 3rd week, when vessels begin to grow into the secondary villi and tertiary villi form, and ends by the end of the 3rd month of pregnancy. On the 6-8th week around the vessels
Rice. 21.17. Hemochorionic placenta. The dynamics of the development of chorionic villi: a- the structure of the placenta (arrows indicate blood circulation in the vessels and in one of the gaps where the villus was removed): 1 - amnion epithelium; 2 - chorionic plate; 3 - villi; 4 - fibrinoid; 5 - yolk vesicle; 6 - umbilical cord; 7 - placental septum; 8 - lacuna; 9 - spiral artery; 10 - basal layer of the endometrium; 11 - myometrium; b- structure of the primary trophoblast villus (1st week); in- structure of the secondary epithelial-mesenchymal villus of the chorion (2nd week); G- the structure of the tertiary chorionic villus - epithelial-mesenchymal with blood vessels (3rd week); d- structure of the chorionic villus (3rd month); e- structure of chorionic villi (9th month): 1 - intervillous space; 2 - microvilli; 3 - symplastotrophoblast; 4 - symplastotrophoblast nuclei; 5 - cytotrophoblast; 6 - the nucleus of the cytotrophoblast; 7 - basement membrane; 8 - intercellular space; 9 - fibroblast; 10 - macrophages (Kashchenko-Hofbauer cells); 11 - endotheliocyte; 12 - lumen of a blood vessel; 13 - erythrocyte; 14 - basement membrane of the capillary (according to E. M. Schwirst)
connective tissue elements are differentiated. Vitamins A and C play an important role in the differentiation of fibroblasts and the synthesis of collagen by them, without sufficient intake of which the strength of the bond between the embryo and the mother's body is disrupted and the threat of spontaneous abortion is created.
The main substance of the connective tissue of the chorion contains a significant amount of hyaluronic and chondroitinsulfuric acids, which are associated with the regulation of placental permeability.
With the development of the placenta, the destruction of the uterine mucosa occurs, due to the proteolytic activity of the chorion, and the change of histiotrophic nutrition to hematotrophic. This means that the villi of the chorion are washed by the blood of the mother, which has poured out from the destroyed vessels of the endometrium into the lacunae. However, the blood of the mother and fetus under normal conditions never mixes.
hematochorionic barrier, separating both blood flows, consists of the endothelium of the fetal vessels, the connective tissue surrounding the vessels, the epithelium of the chorionic villi (cytotrophoblast and symplastotrophoblast), and in addition, of fibrinoid, which sometimes covers the villi from the outside.
germinal, or fetal, part placenta by the end of the 3rd month is represented by a branching chorionic plate, consisting of fibrous (collagenous) connective tissue, covered with cyto- and symplastotrophoblast (a multinuclear structure covering the reducing cytotrophoblast). The branching villi of the chorion (stem, anchor) are well developed only on the side facing the myometrium. Here they pass through the entire thickness of the placenta and with their tops plunge into the basal part of the destroyed endometrium.
The chorionic epithelium, or cytotrophoblast, in the early stages of development is represented by a single-layer epithelium with oval nuclei. These cells reproduce by mitosis. They develop symplastotrophoblast.
The symplastotrophoblast contains a large number of various proteolytic and oxidative enzymes (ATPases, alkaline and acidic
Rice. 21.18. Section of the chorionic villus of a 17-day-old human embryo ("Crimea"). Micrograph:
1 - symplastotrophoblast; 2 - cytotrophoblast; 3 - chorion mesenchyme (according to N. P. Barsukov)
- total about 60), which is associated with its role in the metabolic processes between the mother and fetus. Pinocytic vesicles, lysosomes and other organelles are detected in the cytotrophoblast and in the symplast. Starting from the 2nd month, the chorionic epithelium becomes thinner and is gradually replaced by symplastotrophoblast. During this period, the symplastotrophoblast exceeds the cytotrophoblast in thickness. On the 9th-10th week, the symplast becomes thinner, and the number of nuclei in it increases. On the surface of the symplast facing the lacunae, numerous microvilli appear in the form of a brush border (see Fig. 21.17; Fig. 21.18, 21.19).
There are slit-like submicroscopic spaces between the symplastotrophoblast and the cellular trophoblast, reaching in places up to the basement membrane of the trophoblast, which creates conditions for the bilateral penetration of trophic substances, hormones, etc.
In the second half of pregnancy and, especially, at the end of it, the trophoblast becomes very thin and the villi are covered with a fibrin-like oxyphilic mass, which is a product of plasma coagulation and the breakdown of the trophoblast (“Langhans fibrinoid”).
With an increase in the gestational age, the number of macrophages and collagen-producing differentiated fibroblasts decreases, appearing
Rice. 21.19. Placental barrier at the 28th week of pregnancy. Electron micrograph, magnification 45,000 (according to U. Yu. Yatsozhinskaya):
1 - symplastotrophoblast; 2 - cytotrophoblast; 3 - basement membrane of the trophoblast; 4 - basement membrane of the endothelium; 5 - endotheliocyte; 6 - erythrocyte in capillary
fibrocytes. The number of collagen fibers, although increasing, remains insignificant in most villi until the end of pregnancy. Most stromal cells (myofibroblasts) are characterized by an increased content of cytoskeletal contractile proteins (vimentin, desmin, actin and myosin).
The structural and functional unit of the formed placenta is cotyledon, formed by the stem (“anchor”) villus and its
secondary and tertiary (final) branches. The total number of cotyledons in the placenta reaches 200.
Mother part placenta is represented by a basal plate and connective tissue septa that separate the cotyledons from each other, as well as gaps filled with maternal blood. Trophoblast cells (peripheral trophoblast) are also found at the points of contact between the stem villi and the sheath.
In the early stages of pregnancy, the chorionic villi destroy the layers of the main falling off uterine membrane closest to the fetus, and in their place lacunae filled with maternal blood are formed, into which the chorionic villi hang freely.
The deep undestroyed parts of the falling off membrane, together with the trophoblast, form the basal plate.
Basal layer of the endometrium (lamina basalis)- connective tissue of the uterine lining decidual cells. These large, glycogen-rich connective tissue cells are located in the deep layers of the uterine mucosa. They have clear boundaries, rounded nuclei and oxyphilic cytoplasm. During the 2nd month of pregnancy, decidual cells are significantly enlarged. In their cytoplasm, in addition to glycogen, lipids, glucose, vitamin C, iron, nonspecific esterases, dehydrogenase of succinic and lactic acids are detected. In the basal plate, more often at the site of attachment of the villi to the maternal part of the placenta, clusters of peripheral cytotrophoblast cells are found. They resemble decidual cells, but differ in a more intense basophilia of the cytoplasm. An amorphous substance (Rohr's fibrinoid) is located on the surface of the basal plate facing the chorionic villi. Fibrinoid plays an essential role in ensuring immunological homeostasis in the mother-fetus system.
Part of the main falling off shell, located on the border of the branched and smooth chorion, i.e., along the edge of the placental disc, is not destroyed during the development of the placenta. Tightly growing to the chorion, it forms end plate, preventing the outflow of blood from the lacunae of the placenta.
The blood in the lacunae circulates continuously. It comes from the uterine arteries, which enter here from the muscular membrane of the uterus. These arteries run along the placental septa and open into lacunae. Maternal blood flows from the placenta through veins that originate from the lacunae with large holes.
The formation of the placenta ends at the end of the 3rd month of pregnancy. The placenta provides nutrition, tissue respiration, growth, regulation of the rudiments of the fetal organs formed by this time, as well as its protection.
Functions of the placenta. The main functions of the placenta: 1) respiratory; 2) transport of nutrients; water; electrolytes and immunoglobulins; 3) excretory; 4) endocrine; 5) participation in the regulation of myometrium contraction.
Breath the fetus is provided by oxygen attached to maternal hemoglobin, which diffuses through the placenta into the fetal blood, where it combines with fetal hemoglobin
(HbF). The CO 2 associated with fetal hemoglobin in the blood of the fetus also diffuses through the placenta, enters the mother's blood, where it combines with maternal hemoglobin.
Transport of all the nutrients necessary for the development of the fetus (glucose, amino acids, fatty acids, nucleotides, vitamins, minerals) comes from the mother's blood through the placenta into the fetal blood, and, conversely, metabolic products excreted from the mother's blood enter the mother's blood from his body (excretory function). Electrolytes and water pass through the placenta by diffusion and by pinocytosis.
Pinocytic vesicles of the symplastotrophoblast are involved in the transport of immunoglobulins. The immunoglobulin that enters the blood of the fetus passively immunizes it from the possible action of bacterial antigens that can enter during maternal diseases. After birth, maternal immunoglobulin is destroyed and replaced by newly synthesized in the child's body under the action of bacterial antigens on it. Through the placenta, IgG, IgA penetrate into the amniotic fluid.
endocrine function is one of the most important, since the placenta has the ability to synthesize and secrete a number of hormones that ensure the interaction of the embryo and the mother's body throughout pregnancy. The site of placental hormone production is the cytotrophoblast and especially the symplastotrophoblast, as well as decidual cells.
The placenta is one of the first to synthesize chorionic gonadotropin, the concentration of which rapidly increases at the 2-3rd week of pregnancy, reaching a maximum at the 8-10th week, and in the fetal blood it is 10-20 times higher than in the mother's blood. The hormone stimulates the production of adrenocorticotropic hormone (ACTH) by the pituitary gland, enhances the secretion of corticosteroids.
plays an important role in the development of pregnancy placental lactogen, which has the activity of prolactin and pituitary luteotropic hormone. It supports steroidogenesis in the corpus luteum of the ovary in the first 3 months of pregnancy, and also takes part in the metabolism of carbohydrates and proteins. Its concentration in the mother's blood progressively increases at the 3-4th month of pregnancy and then continues to increase, reaching a maximum by the 9th month. This hormone, together with maternal and fetal pituitary prolactin, plays a role in the production of pulmonary surfactant and fetoplacental osmoregulation. Its high concentration is found in the amniotic fluid (10-100 times more than in the mother's blood).
In the chorion, as well as in the decidua, progesterone and pregnandiol are synthesized.
Progesterone (produced first by the corpus luteum in the ovary, and from the 5-6th week in the placenta) inhibits uterine contractions, stimulates its growth, has an immunosuppressive effect, suppressing the fetal rejection reaction. About 3/4 of the progesterone in the mother's body is metabolized and transformed into estrogen, and part is excreted in the urine.
Estrogens (estradiol, estrone, estriol) are produced in the symplasto-trophoblast of placental (chorionic) villi in the middle of pregnancy, and by the end
Pregnancy their activity increases 10 times. They cause hyperplasia and hypertrophy of the uterus.
In addition, melanocyte-stimulating and adrenocorticotropic hormones, somatostatin, etc. are synthesized in the placenta.
The placenta contains polyamines (spermine, spermidine), which affect the enhancement of RNA synthesis in smooth muscle cells of the myometrium, as well as oxidases that destroy them. An important role is played by amine oxidases (histaminase, monoamine oxidase), which destroy biogenic amines - histamine, serotonin, tyramine. During pregnancy, their activity increases, which contributes to the destruction of biogenic amines and a decrease in the concentration of the latter in the placenta, myometrium and maternal blood.
During childbirth, histamine and serotonin, along with catecholamines (norepinephrine, adrenaline), are stimulants of the contractile activity of smooth muscle cells (SMC) of the uterus, and by the end of pregnancy, their concentration increases significantly due to a sharp decrease (by 2 times) in the activity of amino oxidases (histaminase, etc. .).
With weak labor activity, there is an increase in the activity of aminooxidases, for example, histaminase (5 times).
The normal placenta is not an absolute barrier to proteins. In particular, at the end of the 3rd month of pregnancy, fetoprotein penetrates in a small amount (about 10%) from the fetus into the mother's blood, but the maternal organism does not reject this antigen, since the cytotoxicity of maternal lymphocytes decreases during pregnancy.
The placenta prevents the passage of a number of maternal cells and cytotoxic antibodies to the fetus. The main role in this is played by fibrinoid, which covers the trophoblast when it is partially damaged. This prevents the entry of placental and fetal antigens into the intervillous space, and also weakens the humoral and cellular “attack” of the mother against the fetus.
In conclusion, we note the main features of the early stages of development of the human embryo: 1) asynchronous type of complete crushing and the formation of "light" and "dark" blastomeres; 2) early isolation and formation of extra-embryonic organs; 3) early formation of the amniotic vesicle and the absence of amniotic folds; 4) the presence of two mechanisms in the stage of gastrulation - delamination and immigration, during which the development of provisional organs also occurs; 5) interstitial type of implantation; 6) strong development of the amnion, chorion, placenta and weak development of the yolk sac and allantois.
21.5. MOTHER-FETUS SYSTEM
The mother-fetus system arises during pregnancy and includes two subsystems - the mother's body and the fetus's body, as well as the placenta, which is the link between them.
The interaction between the mother's body and the fetus's body is provided primarily by neurohumoral mechanisms. At the same time, the following mechanisms are distinguished in both subsystems: receptor, perceiving information, regulatory, processing it, and executive.
The receptor mechanisms of the mother's body are located in the uterus in the form of sensitive nerve endings, which are the first to perceive information about the state of the developing fetus. In the endometrium there are chemo-, mechano- and thermoreceptors, and in the blood vessels - baroreceptors. Receptor nerve endings of the free type are especially numerous in the walls of the uterine vein and in the decidua in the area of attachment of the placenta. Irritation of the uterine receptors causes changes in the intensity of respiration, blood pressure in the mother's body, which provides normal conditions for the developing fetus.
The regulatory mechanisms of the mother's body include parts of the central nervous system (temporal lobe of the brain, hypothalamus, mesencephalic reticular formation), as well as the hypothalamic-endocrine system. An important regulatory function is performed by hormones: sex hormones, thyroxine, corticosteroids, insulin, etc. Thus, during pregnancy, there is an increase in the activity of the mother's adrenal cortex and an increase in the production of corticosteroids, which are involved in the regulation of fetal metabolism. The placenta produces chorionic gonadotropin, which stimulates the formation of pituitary ACTH, which activates the activity of the adrenal cortex and enhances the secretion of corticosteroids.
Regulatory neuroendocrine apparatus of the mother ensures the preservation of pregnancy, the necessary level of functioning of the heart, blood vessels, hematopoietic organs, liver and the optimal level of metabolism, gases, depending on the needs of the fetus.
The receptor mechanisms of the fetal body perceive signals about changes in the mother's body or their own homeostasis. They are found in the walls of the umbilical arteries and veins, in the mouths of the hepatic veins, in the skin and intestines of the fetus. Irritation of these receptors leads to a change in the heart rate of the fetus, the speed of blood flow in its vessels, affects the sugar content in the blood, etc.
Regulatory neurohumoral mechanisms of the fetal body are formed in the process of development. The first motor reactions in the fetus appear at the 2-3rd month of development, which indicates the maturation of the nerve centers. The mechanisms regulating gas homeostasis are formed at the end of the second trimester of embryogenesis. The beginning of the functioning of the central endocrine gland - the pituitary gland - is noted at the 3rd month of development. The synthesis of corticosteroids in the adrenal glands of the fetus begins in the second half of pregnancy and increases with its growth. The fetus has increased insulin synthesis, which is necessary to ensure its growth associated with carbohydrate and energy metabolism.
The action of the neurohumoral regulatory systems of the fetus is directed to the executive mechanisms - the organs of the fetus, which provide a change in the intensity of respiration, cardiovascular activity, muscle activity, etc., and to the mechanisms that determine the change in the level of gas exchange, metabolism, thermoregulation and other functions.
In providing connections in the mother-fetus system, a particularly important role is played by placenta, which is able not only to accumulate, but also to synthesize the substances necessary for the development of the fetus. The placenta performs endocrine functions, producing a number of hormones: progesterone, estrogen, human chorionic gonadotropin (CG), placental lactogen, etc. Through the placenta, humoral and neural connections are made between the mother and the fetus.
There are also extraplacental humoral connections through the fetal membranes and amniotic fluid.
The humoral communication channel is the most extensive and informative. Through it, the flow of oxygen and carbon dioxide, proteins, carbohydrates, vitamins, electrolytes, hormones, antibodies, etc. (Fig. 21.20). Normally, foreign substances do not penetrate the mother's body through the placenta. They can begin to penetrate only in conditions of pathology, when the barrier function placenta. An important component of humoral connections are immunological connections that ensure the maintenance of immune homeostasis in the mother-fetus system.
Despite the fact that the organisms of the mother and fetus are genetically foreign in protein composition, immunological conflict usually does not occur. This is ensured by a number of mechanisms, among which the following are essential: 1) proteins synthesized by the symplastotrophoblast, which inhibit the immune response of the mother's organism; 2) chorionic gonadotropin and placental lactogen, which are in high concentration on the surface of the symplastotrophoblast; 3) the immunomasking effect of glycoproteins of the pericellular fibrinoid of the placenta, charged in the same way as the lymphocytes of the washing blood, is negative; 4) the proteolytic properties of the trophoblast also contribute to the inactivation of foreign proteins.
Amniotic waters, which contain antibodies that block antigens A and B, characteristic of the blood of a pregnant woman, also take part in the immune defense, and do not allow them to enter the blood of the fetus.
Maternal and fetal organisms are a dynamic system of homologous organs. The defeat of any organ of the mother leads to a violation of the development of the organ of the same name of the fetus. So, if a pregnant woman suffers from diabetes, in which insulin production is reduced, then the fetus has an increase in body weight and an increase in insulin production in the pancreatic islets.
In an animal experiment, it has been established that the blood serum of an animal from which a part of an organ has been removed stimulates proliferation in the organ of the same name. However, the mechanisms of this phenomenon are not well understood.
Nerve connections include placental and extraplacental channels: placental - irritation of baro- and chemoreceptors in the vessels of the placenta and umbilical cord, and extraplacental - entry into the mother's central nervous system of irritations associated with fetal growth, etc.
The presence of neural connections in the mother-fetus system is confirmed by data on the innervation of the placenta, a high content of acetylcholine in it,
Rice. 21.20. Transport of substances across the placental barrier
fetal development in the denervated uterine horn of experimental animals, etc.
In the process of formation of the mother-fetus system, there are a number of critical periods, the most important for establishing interaction between the two systems, aimed at creating optimal conditions for the development of the fetus.
21.6. CRITICAL PERIODS OF DEVELOPMENT
During ontogenesis, especially embryogenesis, there are periods of higher sensitivity of developing germ cells (during progenesis) and the embryo (during embryogenesis). This was first noticed by the Australian physician Norman Gregg (1944). The Russian embryologist P. G. Svetlov (1960) formulated the theory of critical periods of development and tested it experimentally. The essence of this theory
consists in the statement of the general position that each stage of development of the embryo as a whole and its individual organs begins with a relatively short period of a qualitatively new restructuring, accompanied by the determination, proliferation and differentiation of cells. At this time, the embryo is most susceptible to damaging effects of various nature (X-ray exposure, medicines and etc.). Such periods in progenesis are spermiogenesis and ovogenesis (meiosis), and in embryogenesis - fertilization, implantation (during which gastrulation occurs), differentiation of the germ layers and laying of organs, the period of placentation (final maturation and formation of the placenta), the formation of many functional systems, birth.
Among the developing human organs and systems, a special place belongs to the brain, which in the early stages acts as the primary organizer of the differentiation of surrounding tissue and organ primordia (in particular, sensory organs), and later is characterized by intensive cell reproduction (about 20,000 per minute), which requires optimal trophic conditions.
In critical periods, damaging exogenous factors can be chemicals, including many drugs, ionizing radiation (for example, x-rays in diagnostic doses), hypoxia, starvation, drugs, nicotine, viruses, etc.
chemicals and medications, penetrating the placental barrier, are especially dangerous for the fetus in the first 3 months of pregnancy, as they are not metabolized and accumulate in high concentrations in its tissues and organs. Drugs interfere with brain development. Starvation, viruses cause malformations and even intrauterine death (Table 21.2).
So, in human ontogenesis, several critical periods of development are distinguished: in progenesis, embryogenesis and postnatal life. These include: 1) the development of germ cells - ovogenesis and spermatogenesis; 2) fertilization; 3) implantation (7-8 days of embryogenesis); 4) development of axial rudiments of organs and formation of the placenta (3–8 weeks of development); 5) the stage of enhanced brain growth (15-20 weeks); 6) formation of the main functional systems of the body and differentiation of the reproductive apparatus (20-24 weeks); 7) birth; 8) neonatal period (up to 1 year); 9) puberty (11-16 years).
Diagnostic methods and measures for the prevention of human developmental anomalies. In order to identify anomalies in human development modern medicine has a number of methods (non-invasive and invasive). So, all pregnant women twice (at 16-24 and 32-36 weeks) are ultrasound procedure, which allows to detect a number of anomalies in the development of the fetus and its organs. At the 16-18th week of pregnancy using the method of determining the content alpha-fetoprotein in the mother's blood serum, malformations of the central nervous system can be detected (in case of an increase in its level by more than 2 times) or chromosomal abnormalities, for example, Down syndrome - trisomy of chromosome 21 or
Table 21.2. The timing of the occurrence of some anomalies in the development of embryos and human fetuses
other trisomy (this is evidenced by a decrease in the level of the test substance by more than 2 times).
Amniocentesis- an invasive method of research, in which through abdominal wall mothers take amniotic fluid (usually at the 16th week of pregnancy). In the future, a chromosomal analysis of amniotic fluid cells and other studies are performed.
Visual monitoring of fetal development is also used using laparoscope, introduced through the abdominal wall of the mother into the uterine cavity (fetoscopy).
There are other ways to diagnose fetal anomalies. However, the main task of medical embryology is to prevent their development. For this purpose, methods of genetic counseling and selection of married couples are being developed.
Artificial insemination methods germ cells from obviously healthy donors allow avoiding the inheritance of a number of unfavorable traits. The development of genetic engineering makes it possible to correct local damage to the genetic apparatus of the cell. So, there is a method, the essence of which is to obtain a testicular biopsy from
men with a genetically determined disease. The introduction of normal DNA into the spermatogonia, and then the transplantation of the spermatogonia into the previously irradiated testicle (to destroy genetically defective germ cells), the subsequent reproduction of the transplanted spermatogonia leads to the fact that the newly formed spermatozoa are freed from the genetically determined defect. Therefore, such cells can produce normal offspring when a female reproductive cell is fertilized.
Sperm cryopreservation method allows you to maintain the fertilizing ability of spermatozoa for a long time. This is used to preserve the germ cells of men associated with the danger of exposure, injury, etc.
Method of artificial insemination and embryo transfer(in vitro fertilization) is used to treat both male and female infertility. Laparoscopy is used to obtain female germ cells. A special needle is used to pierce the ovary membrane in the area of the vesicular follicle, aspirate the oocyte, which is subsequently fertilized by sperm. Subsequent cultivation, as a rule, up to the stage of 2-4-8 blastomeres and the transfer of the embryo to the uterus or fallopian tube ensures its development in the conditions of the maternal organism. In this case, it is possible to transplant the embryo into the uterus of a "surrogate" mother.
Improving methods of infertility treatment and prevention of human developmental anomalies are closely intertwined with moral, ethical, legal, social problems, the solution of which largely depends on the established traditions of a particular people. This is the subject of a special study and discussion in the literature. At the same time, advances in clinical embryology and reproduction cannot significantly affect population growth due to the high cost of treatment and methodological difficulties in working with germ cells. That is why the basis of activities aimed at improving the health and numerical growth of the population is the preventive work of a doctor, based on knowledge of the processes of embryogenesis. For the birth of healthy offspring, it is important to lead healthy lifestyle life and give up bad habits, as well as to carry out a set of those activities that are within the competence of medical, public and educational institutions.
Thus, as a result of studying the embryogenesis of humans and other vertebrates, the main mechanisms for the formation of germ cells and their fusion with the emergence of a unicellular stage of development, the zygote, have been established. The subsequent development of the embryo, implantation, the formation of germ layers and embryonic rudiments of tissues, extra-embryonic organs show a close evolutionary relationship and continuity in the development of representatives of various classes of the animal world. It is important to know that there are critical periods in the development of the embryo, when the risk of intrauterine death or development according to pathological conditions increases sharply.
way. Knowledge of the basic regular processes of embryogenesis makes it possible to solve a number of problems in medical embryology (prevention of fetal development anomalies, treatment of infertility), to implement a set of measures that prevent the death of fetuses and newborns.
test questions
1. Tissue composition of the child and maternal parts of the placenta.
2. Critical periods of human development.
3. Similarities and differences in the embryogenesis of vertebrates and humans.
4. Sources of tissue development of provisional organs.
Histology, embryology, cytology: textbook / Yu. I. Afanasiev, N. A. Yurina, E. F. Kotovsky and others. - 6th ed., revised. and additional - 2012. - 800 p. : ill.
The development of the human embryo is a complex process. And an important role in the correct formation of all organs and the viability of the future person belongs to extra-embryonic organs, which are also called provisional. What are these organs? When do they form and what role do they play? What is the evolution of human extra-embryonic organs?
Item specifics
In the second or third week of the existence of the human embryo, the formation of extra-embryonic organs begins, in other words, the membranes of the embryo.
The embryo has five yolk sac, amnion, chorion, allantois and placenta. All these are temporary formations, which neither a born child nor an adult will have. In addition, extra-embryonic organs are not part of the body of the embryo itself. But their functions are varied. The most important of them - extra-embryonic human organs play a significant role in providing nutrition and regulating the processes of interaction between the embryo and the mother.
Evolutionary digression
Extra-embryonic organs appeared on the stage of evolution as an adaptation of vertebrates to living on land. The most ancient shell - the yolk sac appeared in fish. Initially, its main function was to store and store nutrients for the development of the embryo (yolk). Later, the role of provisional authorities expanded.
Following in birds and mammals, an additional shell is formed - the amnion. The extra-embryonic organs, the chorion and the placenta, are the privilege of mammals. They provide a link between the mother's organism and the embryo, through which the latter is provided with nutrients.
Provisional human organs
Extra-embryonic organs include:
- Yolk sac.
- Amnion.
- Chorion.
- Allantois.
- Placenta.
In general, the functions of extra-embryonic organs are reduced to the creation of an aqueous environment around the embryo - the most favorable for its development. But they also perform protective, respiratory and trophic functions.
The oldest fetal membrane
The yolk sac appears in humans at 2 weeks and is a vestigial organ. It is formed from the extra-embryonic epithelium (endoderm and mesoderm) - in fact, it is part of the primary intestine of the embryo, which is taken out of the body. It is thanks to this membrane that the transport of nutrients and oxygen from the uterine cavity is possible. Its existence lasts about a week, since from the 3rd week the embryo is introduced into the walls of the uterus and switches to hematotrophic nutrition. But during the period of its existence, it is this fetal membrane that gives rise to embryonic processes of hematopoiesis (blood islands) and primary germ cells (gonoblasts), which later migrate into the body of the embryo. Later, this membrane will be squeezed by later-formed fetal membranes, turning into a vitelline stalk, which will completely disappear by the 3rd month of embryo development.
Water shell - amnion
The aqueous membrane appears in the early stages of gastrulation and is a sac filled with fluid. It is formed by connective tissue - it is its remains that are called the "shirt" in a newborn. This shell is filled with liquid, and therefore its function is to protect the embryo from concussions and to prevent the growing parts of its body from sticking together. Amniotic fluid is 99% water and 1% organic and inorganic matter.
Allantois
This fetal membrane is formed by the 16th day of development of the embryo from the sausage-like outgrowth of the posterior wall of the yolk sac. In many ways, it is also a rudimentary organ that performs the functions of nutrition and respiration of the embryo. During 3-5 weeks of development, the blood vessels of the umbilical cord form in the allantois. At week 8, it degenerates and turns into a strand connecting bladder and umbilical ring. After that, the allantois combines with the serous layers and forms the chorion - a choroid with many villi.
Chorion
The chorion is a sheath with many villi pierced by blood vessels. It is formed in three stages:
- Anterior villous - the membrane destroys the mucous endometrium of the uterus with the formation of gaps filled with maternal blood.
- The formation of villi of primary, secondary and tertiary orders. Tertiary villi with blood vessels mark the period of placentation.
- Stage of cotyledons - structural units of the placenta, which are stem villi with branches. By the 140th day of pregnancy, about 12 large, up to 50 small and 150 rudimentary cotyledons are formed.
The activity of the chorion is maintained until the end of pregnancy. In this fetal membrane, the synthesis of gonadotropin, prolactin, prostaglandin and other hormones occurs.
Children's place
An important temporary organ for the development of the fetus is the placenta (from the Latin placenta - “cake”) - the place where the blood vessels of the chorion and endometrium of the uterus intertwine (but do not merge). In the places of these plexuses, gas exchange and the penetration of nutrients from the mother's body to the fetus occur. The location of the placenta often does not affect the course of pregnancy and fetal development. Its formation ends by the end of the first trimester, and by the time of birth it has a diameter of up to 20 centimeters and a thickness of up to 4 centimeters.
It is difficult to overestimate the importance of the placenta - it provides gas exchange and nutrition, performs hormonal regulation of the course of pregnancy, performs a protective function, passing maternal blood antibodies, and forms immune system fetus.
The placenta has two parts:
- fetal (from the side of the embryo),
- uterine (from the side of the uterus).
Thus, a stable mother-fetus interaction system is formed.
linked by the same placenta
The body of the mother and child, together with the placenta, form the mother-fetus system, regulated by neurohumoral mechanisms: receptor, regulatory and executive.
Receptors are located in the uterus, which are the first to receive information about the development of the fetus. They are represented by all types: chemo-, mechano-, thermo- and baroreceptors. In the mother, when they are irritated, the intensity of breathing changes, arterial pressure and other indicators.
Regulatory functions are provided by calving of the central nervous system - the hypothalamus, the reticular formation, the hypothalamic-endocrine system. These mechanisms ensure the safety of pregnancy and the functional work of all organs and systems, depending on the needs of the fetus.
The receptors of the temporary organs of the fetus respond to changes in the state of the mother, and regulatory mechanisms mature in the process of development. The development of the nerve centers of the fetus is evidenced by motor reactions that appear at 2-3 months.
The weakest link
In the described system, the placenta becomes such a link. It is the pathologies of its development that most often lead to abortion. There may be the following problems with the development of the placenta:
Pathologies of development of fetal membranes
In addition to the placenta, the amnion and chorion also play their role in ensuring the normal course of pregnancy. Particularly dangerous pathologies of the chorion in the first trimester (formation of hematomas - 50% of pathologies, heterogeneous structure - 28% and hypoplasia - 22%), they increase the likelihood of spontaneous abortion from 30 to 90% depending on the pathology.
Finally
The organisms of the mother and fetus during pregnancy are a system of dynamic connection. And violations in any of its links lead to irreparable consequences. Violations in the work of the mother's body clearly correlate with similar disorders in the functioning of the fetal systems. For example, increased insulin production in a pregnant woman with diabetes leads to various pathologies in the formation of the pancreas in the fetus. That is why it is very important for all pregnant women to monitor their health and not neglect preventive examinations, because any deviation from the norm can signal an unfavorable development of the fetus.
