Organogenesis From what tissues are organs formed? Organogenesis (neurula, organ formation)
ORGANOGENESIS (organogenesis; Greek, organon tool, organ + genesis origin, origin) - a set of processes of formation and development of organs. The period of intensive O. in humans and animals follows the initial stages of embryo development - crushing of the fertilized egg (see Crushing of the egg), blastula (see) and gastrulation (see). During the process of gastrulation, a single-layer embryo is transformed into a three-layer one, consisting of an outer layer - ectoderm (see), an inner layer - endoderm (see) and an intermediate cellular layer - mesoderm (see). The general structure of the three-layer embryo and the subsequent processes of O. are basically similar in vertebrates and humans.
The starting point of O. should be considered the separation of the main rudiments of future organs and tissues; the process of neurulation occurs - thickening of the dorsal ectoderm, leading to the formation of the neural plate, the edges of the cut rise and close with each other along the midline, forming a hollow neural tube - the rudiment of the brain and spinal cord. During the process of neurulation (partially and gastrulation), the body of the embryo along the head-tail axis is divided into zones: the zone of the rudiments of the anterior cephalic structures (irresumptive brain, eye rudiments, nasal codes); the zone of primordia of posterior head structures (presumptive medulla oblongata, auditory vesicles) and the zone of primordia of trunk-caudal structures (kidneys, limbs and tail). Soon after the closure of the neural tube, its dorsal part loosens and cells migrate outside the neural tube, giving rise to the development of spinal and sympathetic ganglia, pigment cells, ectomesenchyme, adrenal medulla cells, neurolemma (Schwann's membranes), facial skeleton, etc. Mesoderm on the sides of the neural tube the tube, formed into metamerically located clusters, or somites, gives rise to the development of muscles, dermis and skeletal elements of the spine, and the eplanchnotome (part of the mesoderm located ventral to the somites on the sides of the intestinal tube) - the heart, blood vessels, kidneys and gonads, peritoneal linings and stroma organs of endodermal origin. The cavity between the outer (parietal) and inner (visceral) layers of the splanchnotome forms a coelom, corresponding to the abdominal, pleural and pericardial cavities of the adult organism. The interaction of accumulations of mesenchyme of the parietal layer with the integumentary ectoderm leads to the formation of limb primordia. The pharynx, stomach, intestines, liver, lungs, pancreas and thyroid glands develop from the endoderm.
Organs typically arise from several (two to four) different primordia, giving rise to various tissue components. For example, the epithelium lining the walls of the intestine develops from endoderm, connective tissue and smooth muscles - from mesenchyme (see), etc. Bones, vessels, lymph nodes are formed from mesenchyme, but here the ingrowth of derivatives of the neural rudiment occurs with the formation of nerve fibers, nerve endings, etc.
In the human embryo, the processes of gastrulation and neurulation occur between the middle of the 2nd and the end of the 3rd week after fertilization of the egg - the stage of formation of the so-called. primitive streak (see Germ). On the 9-14th day of pregnancy, the embryo is formed. arr. auxiliary structures - trophoblast (see), extraembryonic mesenchyme and yolk sac. The cellular material of the embryo itself is located at the point of contact between the bottom of the amniotic sac and the roof of the yolk sac (Fig. 1). The process of gastrulation in humans, birds and placental mammals has much in common. By the end of the 3rd week, an axial complex of primordia, characteristic of chordates, is formed. A feature of neurulation in the human embryo is the formation of a large anterior section of the neural plate, which is associated with the subsequent development of the brain. By the end of the 3rd week, the body of the embryo is separated from the extra-embryonic parts and the neural tube is closed. The closure begins in the future cervical region and continues in the trunk in the caudal direction, and in the head in the cranial direction. The head section of the neural tube closes most late. Simultaneously with the formation of the neural tube, the rudiments of the eyes are separated in the form of outgrowths of the forebrain vesicle, nasal placodes and, at the level of the rudiment of the medulla oblongata, auditory vesicles. Between the 3rd and 4th weeks, the initial stage of heart development is completed and a one-way blood flow is established, the rudiments of the respiratory system, liver, thyroid gland and mesonephros (the final kidney) appear. The oral bay is formed at the end of the 3rd-2nd week, and the anus - at the end of the 4th week. Limb buds begin to appear at the end of the 4th week, and their development proceeds throughout the 5-8th weeks. In the third month of pregnancy, the fetal period of development begins (see Fetus).
In the 20s 20th century the idea of development as a chain of causal dependencies in the differentiation of some parts of the body from other parts arose. * Indeed, the influence of the chordo-mesoderm anlage, which migrates deep into the embryo during gastrulation, causes the formation of a neural tube in the ectoderm. After separation, the notochord, together with the neural tube, causes condensation and cartilage of the somitic mesenchyme that forms the vertebrae. The ability of the rudiment to develop into one or another organ after transplantation to a foreign place (if it is taken at the stage after induction, but before the start of its differentiation) is called self-differentiation, and the ability for self-differentiation is called determination of the rudiment (see Tissue determination). Later studies showed that the initial induction effect ensures the implementation of only the first stage of O.; the subsequent dismemberment of the rudiments into parts and their differentiation are associated not only with new influences of the surrounding tissues, but also with the spatial arrangement of cells within the rudiment and in the general system of the body.
Ectoderm, like endoderm to a large extent, is not capable of self-differentiation, and the formation of their inherent derivatives is associated with their interaction with mesoderm. Numerous experiments combining ectoderm with inducing mesoderm have shown that despite the fact. that without the action of the mesoderm, such derivatives of the ectoderm as hair, feathers, scales, etc., do not develop; the specificity of the emerging structures is determined by the ectoderm, i.e., the reacting system. In the endoderm, on the contrary, the specificity of the mesoderm for the development of certain organs from it is extremely important. In a reactive system, the competence to form the anlage of a particular organ is distributed much wider than the place of normal localization of this rudiment, due to which the development of additional organs is possible when normal connections are disrupted (in experience or during abnormal development). With increasing age of the embryo, a narrowing of the competent areas of the reacting system occurs, and then a complete disappearance of competence. The properties of inductors also change with age. Thus, the normal development of organs depends on the timely combination of the reacting system and the inductor. Delay or loss of any link in this process leads to the absence or defective development of organs. The processes of cell motility are of great importance in the timely combination of various parts of the embryo during gastrulation and oxygenation.
O.'s process is always accompanied by the processes of cell differentiation and histogenesis (see). Spatial and temporal coordination of oxygenation processes is achieved through intercellular interactions caused by such properties of cells and their complexes as adhesiveness, contact inhibition, mobility, ability to deform, release of intercellular substances (matrix) and chemicals. regulators Many form-forming processes in O. are associated with movements of individual cells and their complexes, cell layers, and even entire organ primordia. Individual cells move with the help of long contractile phlopodia (lobopodia), and cell layers move with the help of undulating membranes of marginal cells (less often with the help of filopodia). The ability of cells to deform, which, along with their uneven growth, underlies the bending of cell layers, is associated with changes in the structure and orientation of microfibrils and microtubules.
Intercellular interactions are also characteristic of the period of histogenesis. Processes specialized for O. are the formation by the germ layers and their derivatives of folds, invaginations, thickenings and thinnings, zones of uneven growth (Fig. 2), fusions, as well as the separation of anlages, their mutual germination, etc. The spatio-temporal coordinated complex of these processes is essentially ontogenetic O.
O. occurs under the direct control of the genome (see). This is evidenced by a large number of malformations (see), etiologically associated with mutations of individual genes (see Mutation).
Experiments with transplantation of somatic cell nuclei at different stages of development into an unfertilized egg and DNA hybridization have provided indisputable evidence of the preservation of the entire genome in most somatic tissues of the body. The idea of the differential action of genes in development, based on the interaction of the nucleus and cytoplasm, was firmly established.
O.'s implementation is associated not only with cell differentiation, but also with the development of coordination mechanisms. The system of inductive dependencies is one of the earliest ways to integrate developmental processes. The possibility of their implementation is prepared by even earlier phenomena. The maturation of the competence of reacting systems, the properties of inducers, the synthesis and accumulation of molecules belonging to various classes of RNA and proteins, which subsequently control the differential activity of genes, are prepared by the synthetic activity of the nucleus in oogenesis (see). At later stages of development, hormones and neural connections are included as integrating factors.
Bibliography: B o d e m e r Ch. Modern embryology, trans. from English, M., 1971; Dyukar E. Cellular interactions in the development of animals, trans. from English, p. 28, 141, M., 1978; K a f i a n i K. A. and K o s t o m a r o v a A. A. Information macromolecules in the early development of animals, M., 1978; Kn about p p e A. G. Brief sketch of human embryology, p. 150, L., 1967.
O. G. Stroeva.
Organogenesis is the anatomical formation of organs. The acquisition of morphological, physiological and biochemical specific properties by developing cells and tissues is called histological differentiation, and the process of development of properties characteristic of the tissue of an adult organism is usually referred to as histogenesis.
In parallel with the differentiation (or differentiation) of the embryo, i.e., the emergence from relatively homogeneous cellular material of the germ layers of increasingly heterogeneous rudiments of organs and tissues, integration develops and intensifies, i.e., the unification of parts into one harmoniously developing whole.
At first, this interaction is carried out in primitive ways (biochemical action of cells), and later the integrating function is assumed by the nervous system and its subordinate endocrine glands.
The further development goes, the more and more, but in general very slowly, the changes occurring in the embryo bring the ratio of its parts closer to the definitive state. The tissues and organs of the embryo arising from the embryonic rudiments begin to function specifically with the onset of histological differentiation in them. This occurs at different times for different organs: in general, they are ahead of those organs whose functioning is currently necessary for the further development of the embryo (cardiovascular system, hematopoietic tissues, some endocrine glands, etc.).
Along with the organs that form in the embryo itself, auxiliary extraembryonic organs play a huge role in its development: 1) chorion, 2) amnion, 3) allantois 4) yolk sac.
The chorion forms the outer membrane of the fetus and surrounds it along with the amniotic and yolk sacs.
Amnion (amnion, Greek - cup) - the inner membrane of the fetus, is a bladder filled with fluid (amniotic) in which the embryo develops, which is why this membrane is called aqueous; the fetus remains in it until birth.
The allantois, or urinary sac, which is shaped like a sausage, hence the name (allas, rodit, allantos, Greek - sausage), plays an important role in higher vertebrates and in humans. It is associated with the function of excretion; metabolic products accumulate in it - uric acid salts (from which it gets its name, the urinary sac).
The yolk sac in all animals whose eggs do not have a supply of nutritional materials in the form of yolk loses its significance as a source of nutritional resources for the embryo. The first blood vessels appear in the mesenchyme of the yolk sac wall, but the yolk circulation in placental animals and humans is significantly reduced.
The appearance of the yolk sac in humans has phylogenetic significance. As already indicated, a characteristic feature for humans and apes is the very early and powerful development of extraembryonic parts - the amnion, yolk sac, and trophoblast. In humans, unlike all animals, the extraembryonic mesoderm develops most intensively. Due to this, even before the formation of the embryo itself, extraembryonic adaptations arise that create conditions for the development of the embryo as such.
The development of an embryo from a fertilized egg occurs in higher animals as a result of repeated cell divisions (cleavage); The resulting cells are gradually distributed to their places in different parts of the future embryo. Initially, embryonic cells are similar to each other, but as their number increases, they begin to change, acquiring characteristic features and the ability to perform certain specific functions. This process, called differentiation, ultimately leads to the formation of different tissues. All tissues of any animal come from three original germ layers: 1) the outer layer, or ectoderm; 2) the innermost layer, or endoderm; and 3) the middle layer, or mesoderm. For example, muscles and blood are derivatives of mesoderm, the lining of the intestinal tract develops from endoderm, and ectoderm forms integumentary tissues and the nervous system. There are four main tissues in humans and higher animals: epithelial, muscle, connective (including blood) and nervous. In some tissues, the cells have approximately the same shape and size and fit one another so tightly that there is no or almost no intercellular space left between them; such tissues cover the outer surface of the body and line its internal cavities. In other tissues (bone, cartilage), the cells are not so densely located and are surrounded by the intercellular substance (matrix) that they produce. The cells of the nervous tissue (neurons) that form the brain and spinal cord have long processes that end very far from the cell body, for example, at points of contact with muscle cells. Thus, each tissue can be distinguished from others by the nature of the arrangement of cells. Some tissues have a syncytial structure, in which the cytoplasmic processes of one cell transform into similar processes of neighboring cells; this structure is observed in embryonic mesenchyme, loose connective tissue, reticular tissue, and can also occur in some diseases. Many organs are composed of several types of tissue, which can be recognized by their characteristic microscopic structure.
When tissues are damaged, there may be some loss of their typical structure as a reaction to the disturbance.
The first type of violation is due to the fact that the bookmark either does not form or is severely deformed.
The second type of violation is associated with the sequence of organ formation.
The third type is underdevelopment of organs as a result of suppression of its anlage. (dwarfism)
The fourth type is the opposite phenomenon - excessive growth of the organ. (for example, the formation of a full-fledged kidney must precede the formation of the ureters. If for some reason the formation of the ureters does not occur, then the kidneys will not form).
Fifth type – change in the number of parts of an organ (for example, fingers)
The sixth type is irreducible embryonic structures (for example, the lack of development of the skeletal formations of the posterior wall of the sacrum leads to the fact that the spinal cord is covered only with soft tissues).
On 3rd week of development in the villous chorion, more precisely, at the site of formation of the placenta, tertiary villi are formed. A capillary grows into each villus, and from that time on, the histotrophic type of nutrition of the embryo is replaced by hematotrophic (more complex and effective).
Not only embryonic but also maternal tissues are involved in the construction of the placenta. The chorionic villi are in direct contact with maternal blood. Thanks to this, the embryo (embryo, fetus) throughout intrauterine development receives from the mother the nutrients it needs, oxygen, and releases metabolic products and carbon dioxide.
From the 3rd week of development, the placenta performs the following functions:
Food;
Breathing;
Discharge;
Synthesis of hormones necessary for fetal development;
Immunosuppression (suppression of cellular immunity);
Regulation of hemostasis in the intervillous space and the fetal circulatory system, providing low-resistance blood flow.
The early placenta lacks a protective function, therefore physical, chemical, medicinal, and radiation exposures easily damage the process of differentiation and specialization of cells, which can stop the vital activity of the embryo and the development of the placenta or cause gross malformations.
A primary stripe appears on the surface of the two-layer germinal disc, which determines the axis of symmetry, the location of the head and tail ends of the embryo, its dorsal and ventral surfaces. Determination of the polarity of organ anlage precedes the process of embryogenesis and is ensured by a number of organs.
At the 3rd week of development, two important structures appear on the surface of the embryonic disc on either side of the midline: the neural plate and somites.
Inside the two-layer embryo, the third (mesoderm) layer develops.
During the entire 3rd week of development, the primary yolk sac appears, an extraembryonic organ that provides nutrition and respiration between the mother and the embryo until the chorionic villi begin to vascularize.
By the end of the 6th week of the embryo's life, the yolk sac undergoes reverse development. Simultaneously with the yolk sac, another extraembryonic organ develops - amnion. After some time, a large amniotic cavity will form into which the embryo will be immersed.
With the beginning of the 3rd week of pregnancy, differentiation of cells into specialized organs and tissues begins - the formation of all organs. The first to develop are the neural tube, the heart, and the reproductive gonads. On the 21st day of pregnancy, ultrasound can record the heartbeat at a frequency of 110-130 beats/min. The formation of the neural tube (separation of its head section), the heart and the first vessels are a signal for the simultaneous formation of the liver, trachea, lungs, primary intestine, pancreas, and primary kidney.
The beginning of the embryonic period (3rd week of development) coincides with the beginning of the first wave of interstitial cytotrophoblast invasion and the formation of a new circulation - the utero-placental-fetal.
The period of organogenesis, which is characterized by high rates of proliferation, mitotic division, cell differentiation, protein synthesis, growth factors, requires optimal blood flow, good blood supply, low vascular resistance, which helps improve the fluidity of the rheological properties of blood.
At the stage of histo- and organogenesis, genes-regulators of differentiation and growth of organs, spatial morphogenesis are turned on, since during this period directed processes of induction, migration (movement) of cell layers, specialization of some, and programmed death of other cells occur. Some of the cells and capillaries that were unclaimed disappear; the tail of the embryo is eliminated. The gills transform into jaw appendages; the development of the genital organs according to the male type reduces the Müllerian ducts.
The process of embryogenesis is strictly sequential, complex, and integrative. Therefore, the cessation of pregnancy is explained by the general term “embryoplacental insufficiency,” which depends on many factors, but the main thing remains the genetic plan of human development.
Organogenesis - This is the most dangerous period of development.
Its calm natural course without exposure to damaging factors is ensured by the synchronicity of the development of the placenta and fetus.
Violation of the integrated system mother - placenta - fetal organs can lead to severe developmental defects that are incompatible or (even worse!) compatible with the life of the fetus. A child may be born with severe external and internal malformations and die either immediately or after a long time.
The development of the gonads in a male embryo begins early - from the 3rd week, simultaneously with the heart and neural tube.
The first stage of gonad formation is the migration of undifferentiated germ cells from the yolk sac to the genital ridges. There they turn into gonadoblasts, and the coelomic epithelium covering the genital ridges transforms into germinal epithelium. Gonadoblasts, plunging into the primary germinal epithelium, form into sex cords.
Histologically, the gonads are already clearly distinguishable, but for now they represent bipotent cells capable of becoming a testicle or ovary. Their structural organization is entirely determined by signals from the region SRY , which is located on Y -chromosome. In this area Y -chromosome, a gene called “male sex determination factor” (MSDF) is induced. In its presence, sustentocytes (Sertoli cells) are formed, secreting anti-Müllerian factor, which suppresses the development of Müllerian ducts. The fetal testicles immediately produce the male sex hormone - testosterone (the second stage of development of the fetal genital organs).
Further differentiation of the reproductive organs depends on testosterone. If the testicular hormone is absent, the phenotype will develop exclusively in the female type.
At 4 weeks the embryonic disc “rolls up” into a cylinder, inside which a intestinal tube.
IN In the middle segment of the intestinal tube, a connection is formed with the secondary yolk sac.
Organogenesis begins from this stage.
The first organ of the fetus is the heart. Its contractions can be observed using ultrasound from the 22nd day after fertilization.
Happens in the 4th week neurulation - formation of the nervous system, and by the end of this week the embryo has segments of the brain and spinal cord.
The brain is divided into cerebral vesicles (anterior, middle and posterior). At the same time, the respiratory system is formed (2 lung buds), the primary kidney is differentiated ( mes - onephros ) and mesonephric (Wolffian) duct.
In addition to the heart, neural tube, and reproductive gonads, at 4 weeks of gestation the embryo clearly shows the rudiments of the upper and lower extremities and a bulging area of the pulsating heart. There are 5 pairs of gill arches. Of course, the human embryo does not need gills, but this fact is attributed to the biological law of development: “Ontogenesis repeats the main stages of phylogenesis.” The repetition is, of course, not complete. The openings of the gill slits soon become overgrown. The middle ear develops from the first pair of gill pouches, and the thyroid and parathyroid glands develop from the rest. Eyes are formed (there are no eyelids yet, and the eyes are wide open), a nose, and nasal passages.
The embryo grows and develops quickly. From 4 weeks the first flexion movements in lateral directions appear. The movements coincide with the enlargement of the head end of the neural tube. At this stage of development, the future brain occupies almost half of the neural tube. The beginning of the formation of spinal nerves and nodes can be traced. In a two-chamber heart, an interventricular septum and thickenings arise, from which atrioventricular (atrioventricular) valves are formed.
At 4 weeks, the rudiments of the adenohypophysis and then the hypothalamus appear in the brain.
Fifth week of development - the fetal brain is most intensively formed. Nerve fibers are formed that run from the organs to the brain. The rectum and bladder, trachea and esophagus are isolated from each other. The urogenital sinus is differentiated. The spine grows in length, forming the first curve. The structure of the pancreas becomes more complex. The upper and lower extremities grow intensively, with the upper ones much faster. The genital ridges are differentiated and migration of germ cells to the gonad primordia is observed.
The structure of the blood vessels of the placenta becomes more complex. At 5-6 weeks of development, the peak of the first wave of cytotrophoblast invasion into the walls of the spiral arteries of the endomyometrial segments is observed, due to which the elastomuscular components are destroyed. The endothelium of blood vessels, placenta and subplacental zone is lined with fibrinoid. This process is very complex and is regulated by endometrial decidual cells, which simultaneously produce regulatory proteins (PAPP-A), which enhance the processes of cytotrophoblast invasion, and TGF, which limits the proliferation and invasion of the cytotrophoblast. The regulatory role of two opposing processes is played by fibronectin, laminin and type 4 collagen, which are synthesized by the extracellular (extracellular) matrix.
As a result of the first wave of cytotrophoblast invasion, blood flow increases and the blood supply to the embryo increases. It has been proven that the invasion process is, as it were, duplicated from the side of the internal cytotrophoblast, which penetrates through the endothelium deep into the muscle wall (intravascular invasion) and from the side of the anchor villi, which not only tightly fix the villous tree of the placenta, but are also stem cells for the formation of interstitial cytotrophoblast.
In the first 5-12 weeks and only II trimester of development, invasion of interstitial and internal cytotrophoblast adapts the vascular system of the uterus (in the area of the placental bed) to optimal blood flow in the placenta and blood supply to the rapidly developing fetus.
Sixth week of development - rapid structural separation of the brain and spinal cord continues, the structure of neurons becomes more complex, and the cerebellum differentiates. Brain development is accompanied by activation of DAP. At this stage of growth, the embryo bends and straightens its head and makes sideways movements. The size of the head prevails over the body. A man's face appears. The upper and lower limbs become distinctly different. The elbow and wrist areas are formed, the fingers and toes are clearly distinguishable. The eyes are still wide open, pigment has appeared in the retinal cells. The ears are formed, the thymus gland is formed. Immediately after its formation, it is populated by fetal lymphocytes.
If there is no chromosome set Y -chromosomes, the gonad develops into the ovary. Primary germ cells from the yolk sac move to the gonadal cortex (the gonadal medulla degenerates). Unlike male germ cells, female ones undergo mitosis and meiosis, oogonia are formed, then oocytes, which by the 20th week of development are covered with granulosa cells and turn into primordial follicles. By the 7th week of development, up to 7 million stem cells are present in the ovary, most of which undergo reverse development.
The reproductive organs of the embryo develop from different ductal systems. Male ones are from Wolffian ducts, female ones are from Müllerian ducts.
Male sex determination factor located at the locus SRY Y -chromosomes, suppresses the formation of Müllerian ducts and stimulates the development of Wolffian ducts. Under the influence of fetal testosterone, the epididymis, vas deferens and seminal vesicles are formed from the Wolffian ducts.
The synthesis of testosterone by embryonic testes is not controlled by the cells of the hypothalamus and pituitary gland that are formed at the same time. It is induced by hCG of placental origin.
In the absence of anti-Müllerian factor, the Müllerian ducts form the uterus, fallopian tubes and the upper third of the vagina. It is interesting to emphasize that the neck of the mark and the inner layer of the myometrium are initially formed. And much later - by 20 weeks of gestation, the middle and outer layers of the myometrium are formed.
The formation of the female reproductive gonad and internal genital organs of the female fetus occurs against the background of a high content of estrogens of maternal origin. And although it is believed that hormones are not as necessary for the intrauterine development of a female fetus as testosterone is for the formation of male genital organs, nevertheless, hormonal disorders during 6-12 weeks of pregnancy can cause deviations in the formation of the fetal uterus.
It is known that the use of diethylstilbestrol, prescribed for the threat of miscarriage in I trimester of pregnancy, caused cervical and vaginal cancer in a number of patients exposed in utero. Diethylstilbestrol does not affect the development of male fetuses. The consequences of damaging factors, including hormonal disorders, may appear only after 20-30 years.
Prenatal exposure to diethylstilbestrol affected individuals born between 1940 and 1980 whose mothers took this synthetic estrogen during pregnancy to prevent miscarriage. Subsequently, it was revealed that diethylstilbestrol causes uterine malformations, cervical hypoplasia, and disruption of the shape and structure of the uterus.
The mechanism of action of synthetic estrogens is the activation of estrogen-dependent genes.
Testosterone is the main androgen synthesized by the fetal testicle (as in the adult male). Testosterone secretion begins in the 5th week of gestation. Testosterone has a direct stimulating effect on the Wolffian ducts, inducing the development of the epididymis and vas deferens.
By acting on the urogenital sinus, testosterone determines the formation of the male urethra and prostate gland, and its effect on the urogenital tubercle leads to the formation of the external male genitalia. During these developmental periods, dehydrotestosterone is produced, which influences the formation of the external genitalia according to the male type. A fetus exposed to dehydrotestosterone during this period will masculinize regardless of its genotypic or gonadal sex. On the contrary, the absence of androgens will lead to the development of a female phenotype.
Dehydrotestosterone is formed from testosterone using the enzyme 5?-reductase.
Under the influence of unfavorable factors in early pregnancy (hormonal disorders), the FDMP gene may switch to X -chromosome, and then a male fetus with a female karyotype 46XX or a female fetus with a male karyotype develops XY.
The FDMP gene encodes the formation of a protein called the “zinc finger” protein ( ZFY ) and is capable of producing sex reversal not only in the fetus, but also in adolescence and even adulthood. A gene mutation can cause gonadal dysgenesis, and sometimes gonadal dysgenesis develops in the absence of a gene mutation. The causes of this pathology are not known; hormonal disorders and viral infections that easily penetrate the early placenta are possible. As a rule, the offspring of such women are infertile.
The causes of gene mutation and their movement onto chromosomes, including “point mutations,” are still unknown. Gene mutations lead to structural and functional disorders in the hypothalamus, pituitary gland, adrenal glands, ovaries, causing deviations in the sexual differentiation of the brain (which differs in male and female fetuses), gender reversal, and changes in sexual orientation. But all this can happen many years after birth, when neither the mother nor the obstetrician remembers what factors could have caused the deviation.
The sixth week of development includes the peak of cytotrophoblast invasion into the walls of the spiral arteries of the endometrial segments of the uterus and the formation of the utero-embryonic circulation.
On seventh week of development The limbs of the embryo change greatly. Most often, the embryo holds the upper limbs on the chest, the lower limbs are bent at the knee joints, and the embryo periodically straightens the legs or places them along the body.
The vessels of the placental bed stop responding to vasoconstrictor factors, their lumen expands, blood flow increases, and the intensity of the BMD increases significantly.
Cytotrophoblast cells and multinucleated giant cells periodically accumulate in the lumen of the spiral arteries, preventing the penetration of maternal red blood cells into the fetal bloodstream. By this time, instead of erythroblasts, erythrocytes circulate in the blood of the embryo. Cytotrophoblast cells sometimes move against the blood flow, indicating their extreme activity.
The embryo (with the formation of placental-embryonic blood circulation) grows even more intensively. In one week (from the 7th to the 8th), the embryo completely loses the somitone, turning into a fetus with the species-specific characteristics of the human body. The final kidney, adrenal glands, and ureters are formed. Fingers and toes separated. The fetus periodically brings its hands to its face, its thumb touches its mouth, and sucking movements appear. The eyes are still wide open, the brow ridges are strongly developed. Sleep phases are replaced by short periods of active movements. For the first time, isolated movements of individual hands are observed.
Eighth week of development - the last week of the embryogenesis period, during which the embryo has everything to be considered a fetus.
After 8 weeks, the embryo is called a fetus.
The fetus now has its own blood type and has (or does not have) a Rh factor. In the brain zones, differentiation of the first layer of the cerebral cortex occurs, although their processes are still short and the cells do not contact each other. The boundaries of the forebrain, hindbrain and midbrain deepen, the boundaries of the medulla oblongata are clearly visible. All brain structures are intensively supplied with blood.
The head has a rounded shape, its size is still disproportionately large. It occupies almost half the length of the body.
The end of the embryonic period is characterized by complete differentiation of the brain and spinal cord, central and peripheral nervous systems.
The behavioral reactions of the fetus become more complicated. The fetus covers its face with its hands and tries to suck its thumb. In case of danger (artificial termination of pregnancy), he tries to evade the inserted instruments, while movements of the fetus away from the medical curette are recorded. The fetus swallows amniotic fluid, the kidneys function, and urine accumulates in the bladder.
At 8 weeks of pregnancy, the first wave of cytotrophoblast invasion ends. All walls of the spiral arteries are lined with fibrinoid. The spiral arteries of the uterus essentially turn into typical uteroplacental arteries, providing a constant flow of arterial blood to the intervillous space.
Each supporting villi is divided into 20 new villi. Their number at 8 weeks is 3 times higher than the number of villi in a 5-week placenta.
Stromal channels appear, oriented along the course of some villi; multinucleated Kashchenko-Hoffbauer cells, which have the function of placental macrophages, circulate through them.
Increase in placental mass I trimester advances the growth of the embryo/fetus.
At 6-8 weeks of pregnancy, the most active synthesis of hCG takes place, which coincides with the laying of the nuclei of the hypothalamic-pituitary region and the formation of the sex gonads. After 10 weeks of pregnancy, the level of hCG in the blood and urine decreases and remains constantly low until the end of pregnancy, increasing by 5% at 32-34 weeks of pregnancy. At the same time, the permeability of the microchannels of the placenta increases. In multiple pregnancies, the hormone content is higher, proportional to the number of fetuses.
HCG has the property of immunosuppression, which is important for pregnancy. An embryo that has foreign paternal genes, in the absence of a decrease in cellular immunity, should be rejected from the mother's body as a foreign transplant. However, most often this does not happen precisely due to the suppression of the activity of the immune system. CG provides immunological tolerance, reducing the risk of immune rejection of the fetus in the first 12 weeks of pregnancy.
In subsequent trimesters of pregnancy, placental proteins are immunosuppressants: trophoblastic? 1-glycoprotein (TBG), placental? 1-microglobulin and? 2-microglo-bulin fertility.
At 6 weeks of pregnancy (at the peak of cytotrophoblast invasion and intensification of utero-embryonic circulation), the synthesis of all hormones that ensure the growth and development of the fetus moves from the ovary to the placenta.
It should be noted that from the 6th to the 8th week of pregnancy, the synthesis of PGE 2, which has a vasodilator, antiplatelet and anticoagulant effect, increases significantly. Their effect after the 8th week of gestation is so significant that blood pressure decreases by 8-12 mmHg. Art. in the general hemodynamic system of the mother.
Thus, the period of pregnancy from the 3rd to the 8th week is the most significant and responsible.
Main events:
Embryogenesis and construction of the structure of the early placenta;
Structural organization of all organs including their functional activity;
Formation of the phenotype in accordance with the genotype of the fetus.
The sex of the fetus is determined by the set of chromosomes: XX - female, XY - male gender. However, gonads and germ cells initially have the same organization. For the formation of the male reproductive gonad it is necessary not only Y -chromosome, but also FDMP, which suppresses the formation of female genital organs. If Y -chromosome is absent, only the female sex is formed.
The genital organs of the male fetus are determined by exposure to testosterone and dehydrotestosterone. Violation of hormonal ratios in the mother's body can lead to genetic errors in the development of the fetus.
Organogenesis is the formation of organs during the embryonic development of an organism. The process of organ formation during ontogenesis (see), i.e., ontogenetic organogenesis, is studied (see), and during the historical development of the species (phylogenetic organogenesis) - comparative anatomy.
Organogenesis (from the Greek organon - organ, genesis - development, formation) is the process of development, or formation, of organs in the embryo of humans and animals.
Organogenesis follows earlier periods of embryonic development (see Embryo) - egg fragmentation, gastrulation and occurs after the main rudiments (anlage) of organs and tissues have separated. Organogenesis proceeds in parallel with histogenesis (see), or tissue development. Unlike tissues, each of which has its source in one of the embryonic rudiments, organs, as a rule, arise with the participation of several (from two to four) different rudiments (see Germ layers), giving rise to different tissue components of the organ. For example, as part of the intestinal wall, the epithelium lining the organ cavity and glands develop from the internal germ layer - endoderm (see), connective tissue with blood vessels and smooth muscle tissue - from mesenchyme (see), mesothelium covering the serous membrane of the intestine - from the visceral layer of the splanchnotome, i.e., the middle germ layer - mesoderm, and the nerves and ganglia of the organ - from the neural rudiment. The skin is formed with the participation of the outer germ layer - ectoderm (see), from which the epidermis and its derivatives (hair, sebaceous and sweat glands, nails, etc.) develop, and dermatomes, from which mesenchyme arises, differentiating into the connective tissue basis of the skin (dermis ). Nerves and nerve endings in the skin, as elsewhere, are derivatives of the neural rudiment. Some organs are formed from one primordium, for example, bone, blood vessels, lymph nodes - from mesenchyme; however, here too, derivatives of the rudiment of the nervous system—nerve fibers—grow into the anlage, and nerve endings are formed.
If histogenesis consists mainly in the reproduction and specialization of cells, as well as in the formation of intercellular substances and other non-cellular structures by them, then the main processes underlying organogenesis are the formation of folds, invaginations, protrusions, thickenings, uneven growth, fusion or division by the germ layers (separation), as well as mutual germination of various bookmarks.
In humans, organogenesis begins at the end of the 3rd week and is generally completed by the 4th month of intrauterine development. However, the development of a number of provisional (temporary) organs of the embryo - chorion, amnion, yolk sac - begins already from the end of the 1st week, and some definitive (final) organs form later than others (for example, lymph nodes - from the last months of intrauterine development to onset of puberty). See also Morphogenesis, Ontogenesis.
The initial organogenesis is neurulation.
During the process of neurulation, mesoderm is formed.
Method 1: Enterocoelous - protrusions - pockets - are formed on both sides of the primary intestine. They are completely detached from the primary gut, grow between the ectoderm and endoderm and turn into mesoderm (in chordates)
Method 2: Teloblastic - one large cell, a teloblast, is formed near the blastopore on both sides of the primary gut. As a result of the reproduction of teloblasts, mesoderm is formed. (in invertebrates)
Formation of axial organs in chordate embryos
The ectoderm on the dorsal side of the embryo bends, forming a longitudinal groove, the edges of which close together. The resulting neural tube plunges into the ectoderm
The dorsal part of the endoderm, located under the nerve rudiment, gradually separates and forms a notochord.
The intestinal tube is formed from the ectoderm and endoderm.
Ectoderm - epidermis, skin glands, hair, enamel, conjunctiva, lens, retina, ears, epithelial lining of the nasal cavity and oral cavity, anus and vagina, anterior and posterior lobes of the pituitary gland, central nervous system, adrenal medulla, jaws.
Mesoderm - skeletal muscles, diaphragm, vertebrae, dentin, renal tubules, ureters, oviducts, uterus, part of the ovaries and testicle, adrenal cortex, heart, blood, lymphatic system, lungs, sclera, choroid and cornea.
Endoderm- notochord, most of the digestive tract, lining of the intestines, bladder, lungs, pancreas, thymus, thyroid gland, parathyroid gland.
39. The concept of provisional organs of chordates. Features of the development of these organs in the group Anamnia and Amniota. Types of placentas. Disruption of the processes of development and reduction of embryonic membranes in humans.
Provisional organs are temporary organs necessary for the life of the embryo. The time of their formation depends on the egg and environmental conditions.
The presence or absence of provisional organs underlies the division of vertebrates into groups: Amniota and Anamnia.
The anamnia group includes evolutionarily more ancient animals that develop in an aquatic environment and do not require additional aquatic and other membranes of the embryo. (Cyclostomes, fish, amphibians)
The group of amniotes includes proto-terrestrial vertebrates, the embryonic development of which takes place in terrestrial conditions. (Reptiles, birds, mammals)
The structure and functions of the provisional organs of amniotes have much in common. The provisional organs of higher vertebrates are called embryonic membranes. They develop from the cellular material of already formed germ layers.
Provisional authorities.
Amnion is a sac filled with amniotic fluid, which creates an aqueous environment and protects the embryos from drying out and damage.
Chorion is the outer embryonic membrane adjacent to the shell or maternal tissues. Serves for exchange with the environment, participates in respiration, nutrition and excretion.
Yolk sac - it is involved in the nutrition of the embryo and is a hematopoietic organ.
Alantois - an outgrowth of the hindgut is involved in gas exchange and is a receptacle for urea and uric acid. In mammals, it forms the placenta together with the chorion. Vessels grow from the allantois to the chorion, with the help of which the placenta performs excretory, respiratory and nutritional functions.
Types of placentas.
1. Epitheliochorionic – (hemiplacenta) has the simplest structure. When it is formed, villi appear on the surface of the chorion in the form of small tubercles. They sink into the corresponding depressions of the uterine mucosa without disturbing it. (the chorion is in contact with the epithelium of the uterine glands) Pig horses
2. Desmochorionic – characterized by the establishment of the closest connection between the chorion of the embryo and the wall of the uterus. At the point of contact with the chorionic villi, the epithelium is destroyed. The branched plates are immersed in the connective tissue. (The chorion is in contact with the connective tissue.)
3. Endothelial chorionic - not only the epithelium is destroyed, but also the connective tissue. The villi are in contact with the vessels and are separated from the maternal blood only by their thin endothelial wall. (predators)
4. Hemochorionic - profound changes occur in the uterus. The villi are washed with blood and absorb nutrients from it.
By appearance:
1 Diffuse - The villi are distributed evenly over the entire surface of the chorion.
2 Cotyledonous - villi are collected in groups in the form of bushes
3 Girdle - villi form a belt encircling the water bladder.
4Discoid - The villi are located within the discoid region on the surface of the chorion.
41. Postembryonic period of ontogenesis, its periodization in humans. Basic processes: growth, formation of definitive structures, puberty, reproduction. The role of endocrine regulation in the postnatal period.
The postembryonic period begins from the moment the organism emerges from the egg membranes until the moment of death.
The postnatal period can be direct or indirect.
With direct development, a newborn organism is similar to an adult and differs only in size and incomplete development of organs. Direct development is typical for humans and other mammals, birds, reptiles and some insects.
Non-direct development occurs with metamorphosis.
With incomplete metamorphosis, the organism goes through three stages of development. Egg, larva and imango.
With a complete one there are 4 stages (pupa).
Periods of postembryonic human development.
1. Newborn – from birth to 4 weeks. The structure is not proportional; the bones of the skull and pelvis are not fused. The spine is without bends.
2. Infant - from 4 weeks to 12 months - the child moves with movements and milk teeth appear.
3. Nursery up to 3 years old. The proportions of the body change, the brain develops.
4. Preschool up to 7 years old. Changing teeth.
5. School children up to 17 years of age have body proportions similar to those of adults.
6. Youth - 16-20 girls, 17-21 boys. The processes of growth and formation of the organism are completed.
7. Mature from 21 years old.
8. Elderly 55-60 years old.
9. Starchisky – 75 years old
Growth - it manifests itself in a progressive increase in the mass and size of the body.
In invertebrates, growth is determined by an increase in cell size.
Proliferative growth is more common - it is based on cell division. cells increases exponentially. N n =2 n Where N is the number of cells, n is the order of division.
In the process of individual development, growth indicators change. In many animals, growth is confined to certain stages of ontogenesis. This type of growth is called limited.
There are organisms that grow throughout their lives (fish), but upon reaching puberty, the growth rate slows down. This type of growth is called unrestricted.
Growth indicators, on the one hand, are limited genetically, and on the other hand, depend on the environment.
The role of endocrine glands in postembryonic development is great.
E. zh. produce hormones that affect the growth of the body and puberty. Particularly important are the hormones produced by the pituitary gland, thyroid gland and sex glands. Issues of influence e. and. Zavodskoy considered the growth and development of the body.
