Cleavage is the division of the embryo, but this division is somewhat different from the usual division by mitosis: during cleavage there is no presynthetic phase G1, therefore the daughter (fledgelings (blastomeres) after mitosis do not synthesize proteins and do not grow - with each division the size of the resulting blastomeres becomes smaller and smaller (hence the name “crushing”). As a result of cleavage, the normal nuclear-cytoplasmic ratio is restored (in a fertilized egg it is very low) and a blastula is formed. The type of cleavage depends on the amount and distribution of the yolk in the egg. Oligolecithal isolecithal human egg is crushed according to the type: complete uneven asynchronous Complete - all parts of the fertilized egg are involved in fragmentation; uneven - the resulting blastomeres are not the same, unequal: some are large and dark, located in the center of the embryo; their aggregate is called zembryoblast - the future body, others are small and light, surrounding the outside of the embryoblast blastomeres - their totality is called trophoblast - participates in the formation of the placenta; asynchronous = the number of blastomeres does not increase in a geometric progression (2-> 4-> 8, etc., i.e. a multiple increase in the number of blastomeres). At the same time, it should be noted that the first blastomeres (at least up to the first 8 blastomeres) are potentially not different. The proof of this statement is the formation of identical twins. Identical twins are formed from one egg when, for some reason, at the stage of a 2- or 3-blastomere embryo, they separate and an independent separate organism develops from each blastomere. On the other hand, if in an experiment you take a two-blastomere embryo and destroy one blastomere, then a completely normal organism can develop from the second blastomere. Or more; if you take two embryos at the stage up to the 8-blasgomeric stage and combine them into one morula, you can get a new organism that inherits the characteristics of not 2 but 4 parents - such experimentally obtained animals are called chimeras. These experiments proved that blastomeres of the early stages of cleavage (up to the 8-cell stage) are almost identical and have unlimited potency. Using the chimera method, it was proven that which blastomeres will give rise to a trophoblast and which ones will give rise to an embryoblast does not depend on the characteristics of the blastomeres themselves, but only on the place they accidentally occupied in the morula. Blastomers located outside the morula are exposed to certain conditions, while internal blastomeres are exposed to other conditions, which determines the direction of their further differentiation. The fragmentation of the human zygote begins at the end of the 1st day after fertilization in the distal part of the fallopian tube and ends on the 7th day in the uterine cavity. On the 2-3rd day, the embryo is in the fallopian tubes and has the appearance of a dense nodule - a morula, in the central part of which there are large dark blastomeres - embryoblasts, and along the periphery there are small light blastomeres = trophoblast. On the 4th day, the blastula is located in the proximal part of the fallopian tube, i.e. fits closely to the uterine cavity and has the appearance of a bubble. The blastomeres of the trophoblast absorb the secretion of the fallopian tube and secrete fluid themselves, so the trophoblast stretches and turns into a vesicle filled with fluid, and the embryolast is attached at one pole to the trophoblast inside. Such a blastula is called an epiblastula (or synonyms: blastocyst, steroblastula). On the 5th day, the blastocyst enters the uterine cavity and remains there until the 7th day, during which time it slightly increases in size (100 or more blastomeres).

4. Concept and basic mechanisms of gastrulation. Types of gastrulation. The structure of a two-week embryo. Concept of critical periods.

Gastrulation is a complex process where, as a result of reproduction, growth, differentiation and directional movement of cells, a 3-leaf embryo is formed. Gastrulation occurs on days 7-17 and is carried out by delamiacin or cleavage (days 7-14) and immigration or eviction (days 14-17). On the 7th day, the embryoblast splits into 2 layers: the upper layer - the epiblast or primary ectoderm (contains the material of the future ectoderm, mesoderm, notochord and parts of the endoderm) and the lower layer - the hypoblast (the future endoderm after the addition of the cellular material of the prechordal plate from the epiblast). Almost simultaneously with this, the eviction of cells from the epi- and hypoolast occurs - extra-embryonic mesenchyme, which lines the inner surface of the trophoblast. Then, during the 2nd week, the epiblast and hypoblast begin to bend in opposite directions and turn into vesicles: the amniotic sac is formed from the epiblast, and the yolk vesicle from the hypoblast. These 2 vesicles are surrounded by extraembryonic mesenchyme. The contacting surfaces of the amniotic and vitelline vesicles have the form of a disk (or scutellum) and are respectively called the germinal epiblast and the germinal hypoblast, and together - the germinal scutellum. The remaining areas of the amniotic and vitelline sacs are called the extraembryonic epi- and hypoblast.

At the beginning of the 3rd week (14-17 days), immigration (eviction) of cells from the epiblast occurs, and this occurs in 2 phases: in phase I, preparation for eviction occurs - the cellular material subject to immigration moves (slowly moving cells: from the future cranial end to the caudal end along the center of the epiblast, and fast-moving cells: first, also from the cranial pole to the caudal pole, but along the edge of the epiblast, and then turn and go along the center of the epiblast to the cranial end) and gather together and form 3 on the surface of the epiblast structures: prechordal plate, nodule I and stripe I; Phase II is the actual eviction of the material of these 3 structures. The first node moves out and between the existing two leaves forms the first axial organ - the notochord. The material of the prechordal plate moves out and joins the hypoblast, from now on the lower layer will be called endoderm. The cells of stripe 1 move out, occupy a position between the two existing leaves and form the middle layer - the mesoderm. The remaining part of the epiblast after the eviction of cells of 3 structures will be called ectoderm. In the next stage, the differentiation of germ layers into tissues (histogenesis) of organs (organogenesis) and the formation of organ systems from organs (systemogenesis) begin. In this case, it is necessary to highlight 20-21 days of embryogenesis - during these periods the following important processes occur: 1 From all three germ layers, but mainly from the mesoderm, cells are evicted, filling the spaces between the three germ layers, i.e. embryonic mesenchyme is formed. 2. The mesoderm differentiates into component parts (tomes) somites, segmental legs and splanchnotomes. 3 The three-leaf flat embryo folds “into a tube” - the body is formed (the endoderm, folding into a tube, turns into the first intestine, surrounded by a layer of mesenchyme and splaphnatoms; the ectoderm covers the body from the outside). 4. When the embryo is folded “into a tube,” the extra-embryonic parts of the body are separated from the body of the embryo and the provisional organs of the yolk sac and hypnotic membrane are formed. The material of the germ layers differentiates into tissues, organs and organ systems (for more details on the development of specific organs and systems, see lectures on private histology): I Ectoderm: - nervous tissue of the nervous system organs; - epidermis of the skin and its derivatives (sebaceous, sweat, mammary glands, nails, hair); - epithelium of the cornea and lens of the eye, epithelium of the vestibule of the oral cavity and anal rectum; II Mesoderm: -dermatomes -> skin dermis; - myotomes -> skeletal muscles; - sclerotomes -> axial skeleton (bones and cartilage of the spinal column; - segmental legs (nephrototomes) - epithelium of the genitourinary system; - splanchiotomes ->

Types of gastrulation

During gastrulation, the changes that began at the blastula stage continue, and therefore different types of blastula correspond to different types of gastrulation. Transition from blastula to gastrulu can be carried out in 4 main ways: intussusception, immigration, delamination and epiboly. Intussusception or invagination is observed in the case of coeloblastula. This is the simplest method of gastrulation, in which the vegetative part invaginates into the blastocoel. Initially, a small depression appears in the vegetative pole of the blastula. Then the cells of the vegetative pole protrude more and more into the cavity of the blastocoel. Subsequently, these cells reach the inner side of the animal pole. The primary cavity, the blastocoel, is displaced and is visible only on both sides of the gastrula in places where the cells bend. The embryo takes on a dome-shaped shape and becomes two-layered. Its wall consists of an outer layer - ectoderm and an inner layer - endoderm. As a result of gastrulation, a new cavity is formed - the gastrocoel or the cavity of the primary intestine. It communicates with the external environment through a ring-shaped opening - the blastopore or primary mouth. The edges of the blastopore are called lips. There are dorsal, ventral and two lateral lips of the blastopore. According to the subsequent fate of the blastopore, all animals are divided into two large groups: protostomes and deuterostomes. Protostomes include animals in which the blastopore remains a permanent or definitive mouth in an adult (worms, mollusks, arthropods). In other animals (echinoderms, chordates), the blastopore either turns into an anal opening or becomes overgrown, and the oral opening appears anew at the anterior end of the body of the embryo. Such animals are called deuterostomes. Immigration or invasion is the most primitive form of gastrulation. With this method, individual cells or a group of cells move from the blastoderm to the blastocoel to form the endoderm. If the invasion of cells into the blastocoel occurs only from one pole of the blastula, then such immigration is called unipolar, and from different parts of the blastula - multipolar. Unipolar immigration is characteristic of some hydroid polyps, jellyfish and hydromedusae. While multipolar immigration is a rarer phenomenon and is observed in some hydromedusas. During immigration, the internal germ layer, the endoderm, can be formed immediately during the penetration of cells into the cavity of the blastocoel. In other cases, the cells may fill the cavity in a continuous mass and then arrange themselves in an orderly manner near the ectoderm to form the endoderm. In the latter case, the gastrocoel appears later. Delamination or delamination is reduced to splitting of the wall of the blastula. The cells that separate inward form the endoderm, and the outer cells form the ectoderm. This method of gastrulation is observed in many invertebrates and higher vertebrates. In some animals, due to an increase in the amount of yolk in the egg and a decrease in the blastocoel cavity, gastrulation only through intussusception becomes impossible. Gastrulation then occurs by epiboly or fouling. This method consists in the fact that small animal cells intensively divide and grow around larger vegetative ones. Small cells form the ectoderm, and cells of the vegetative pole form the endoderm. This way gastrulation observed in cyclostomes and amphibians. The periods of greatest sensitivity to the effects of environmental factors are called critical periods. In humans, there are 3 main critical periods in embryogenesis:

Implantation - implantation of an embryo into the uterine mucosa (6-7 days after fertilization)

Placentation - the beginning of placenta formation (14-15 days)

Childbirth - leaving the mother of the body, restructuring the functioning of all systems, changing the way of eating (39-40 weeks). Critical periods coincide with the transition from one period of development to another and changes in the conditions of existence of the embryo.

5. The concept of differentiation of germ layers. The idea of ​​induction as a factor causing differentiation. Differentiation of germ layers in the human embryo.

Germ layers consist of cellular materials that go into the development of various organs and tissues. The cells of different germ layers differ from each other in their structure; endoderm cells are always larger and less regular in shape than ectodermal cells. The endoderm is distinguished by the properties of the future anlage, which has trophic significance. The ectoderm remains on the surface and initially has a protective value. Unlike endoderm, it consists of regularly arranged cells of a more uniform shape. Gastrulation leads to a noticeable difference between the outer and inner layers and the germinal material becomes heterogeneous. The process that leads to the appearance of differences in an initially homogeneous material is called differentiation. Primary organizers or inducers play a major role in the differentiation of cellular material. Inducers are chemicals that are released by groups of cells and affect other groups of cells, changing their developmental path. As a result of differentiation of the germ layers, various organs and tissues are formed. When studying these processes in different animals, it was found that the fate of each germ layer in all multicellular organisms is, as a rule, the same. Induction (from the Latin inductio - motivation, guidance) in embryology is the effect of some parts of the developing embryo (inducers) on its other parts (the reacting system), which occurs upon their contact and determines the direction of development of the reacting system, similar to the direction of differentiation of the inductor (homotypic induction) or different from it (heterotypic induction). induction was discovered in 1901 by the German embryologist H. Spemann while studying the formation of the lens (lens) of the eye from the ectoderm in amphibian embryos. When the eye rudiment was removed, the lens did not appear. The eye rudiment, transplanted onto the side of the embryo, caused the formation of a lens from the ectoderm, which normally should have differentiated into the epidermis of the skin. Later, Spemann discovered the inducing effect of chordomesoderm on the formation of the rudiment of the central nervous system - the neural plate - from the gastrula ectoderm; he called this phenomenon primary embryonic induction, and the inducer - chordomesoderm - organizer. Further studies involving the removal of parts of the developing organism and their cultivation separately or in combination and transplantation into an alien place of the embryo showed that the phenomenon of induction is widespread in all chordates and many invertebrate animals. Induction is possible only if the cells of the reacting system competent to this influence, that is, they are able to perceive the inducing stimulus and respond to it by forming appropriate structures. The ability of cells differentiating under inductive influence to induce differentiation of a new group of cells themselves is called secondary induction.

In many cases, it has been established that in the process of induction, not only the inductor influences the differentiation of the reacting system, but also the reacting system exerts on the inductor the effect necessary both for its own differentiation and for its implementation of the inducing influence, i.e. that induction - interaction of groups of cells of the developing embryo with each other. For a number of organogenesis, it has been shown that during the process of induction, substances (inducing agents) are transferred from the cells of the inducer to the cells of the reacting system, which are involved in the activation of the synthesis of specific messenger RNAs necessary for the synthesis of the corresponding structural proteins in the nuclei of cells of the reacting system.

The action of inducers, as a rule, lacks species specificity. Organ-specific action of own. inducers can be experimentally replaced by the action of a number of organs and tissues of older embryos and adult animals (foreign, or heterogeneous, inducers) or by chemical substances isolated from them - inducing factors (for example, t n. vegetative factor - a protein with a molecular weight of about 30,000, which causes the formation of endoderm and, secondarily, notochord, muscles and other derivatives of mesoderm in the competent ectoderm of the gastrula of amphibians. The effect of inducers can be imitated by treating competent tissue cells with simpler chemical compounds, for example, sodium and lithium salts, sucrose, as well as some cell-damaging effects; Apparently, in this case, the cells release their own. inducing factors that were in a bound state. This induction is sometimes called evocation, and inducing stimuli - evocators induction.

In the next stage, the differentiation of germ layers into tissues (histogenesis) of organs (organogenesis) and the formation of organ systems from organs (systemogenesis) begin. In this case, it is necessary to highlight 20-21 days of embryogenesis - during these periods the following important processes occur: 1 From all three germ layers, but mainly from the mesoderm, cells are evicted, filling the spaces between the three germ layers, i.e. embryonic mesenchyme is formed. 2. The mesoderm differentiates into component parts (tomes) somites, segmental legs and splanchnotomes. 3 The three-leaf flat embryo folds “into a tube” - the body is formed (the endoderm, folding into a tube, turns into the first intestine, surrounded by a layer of mesenchyme and splaphnatoms; the ectoderm covers the body from the outside). 4. When the embryo is folded “into a tube,” the extra-embryonic parts of the body are separated from the body of the embryo and the provisional organs of the yolk sac and hypnotic membrane are formed. The material of the germ layers differentiates into tissues, organs and organ systems (for more details on the development of specific organs and systems, see lectures on private histology): I Ectoderm: - nervous tissue of the nervous system organs; - epidermis of the skin and its derivatives (sebaceous, sweat, mammary glands, nails, hair); - epithelium of the cornea and lens of the eye, epithelium of the vestibule of the oral cavity and anal rectum; II Mesoderm: -dermatomes -> skin dermis; - myotomes -> skeletal muscles; - sclerotomes -> axial skeleton (bones and cartilage of the spinal column; - segmental legs (nephrototomes) - epithelium of the genitourinary system; - splanchiotomes -> mesothelium of the serous integument (peritoneum, pleura and pericardial sac), gonadal epithelium (Sertoli cells of the testicle and follicular cells of the ovaries ), cortical part of the adrenal glands, myocardium and epicardium; III Endoderm: - epithelium and glands (including the liver and pancreas) of the digestive and respiratory system; IV Mesenchyme: tissues of the internal environment (blood and lymph, all types of fibrous connective tissue - loose fibrous connective tissue , dense fibrous formed and unformed fibrous connective tissues, connective tissues with special properties, bone and cartilage tissues) and smooth muscle tissue.

During the formation of extraembryonic membranes, the organs and systems of the embryo continue to develop. At certain moments, one part of the cells of the germ layers begins to divide faster than the other, groups of cells migrate, and cell layers change their spatial configuration and location in the embryo. During certain periods, the growth of some types of cells is very active and they increase in size, while others grow slowly or stop growing altogether.

The nervous system is the first to develop after implantation. During the second week of development, the ectodermal cells of the posterior side of the germinal shield rapidly increase in number, causing the formation of a bulge above the shield - the primitive streak. Then a groove is formed on it, in the front of which a small pit appears. In front of this fossa, the cells quickly divide and form the head process, the precursor of the so-called. dorsal string, or chord. As the notochord elongates, it forms an axis in the embryo that provides the basis for the symmetrical structure of the human body. Above the notochord is the neural plate, from which the central nervous system is formed. Around the 18th day, the mesoderm along the edges of the notochord begins to form dorsal segments (somites), paired formations from which the deep layers of skin, skeletal muscles and vertebrae develop.

After three weeks of development, the average length of the embryo is only slightly more than 2 mm from crown to tail. Nevertheless, the rudiments of the notochord and nervous system, as well as eyes and ears, are already present. There is already an S-shaped heart, pulsating and pumping blood.

After the fourth week, the length of the embryo is approximately 5 mm, the body is C-shaped. The heart, which forms the largest bulge on the inside of the body's curve, begins to subdivide into chambers. Three primary areas of the brain (brain vesicles), as well as the visual, auditory and olfactory nerves are formed. The digestive system is formed, including the stomach, liver, pancreas and intestines. The structuring of the spinal cord begins, and small paired limb rudiments can be seen.

A four-week human embryo already has gill arches that resemble the gill arches of a fish embryo. They soon disappear, but their temporary appearance is one example of the similarity of the structure of the human embryo with other organisms

At five weeks of age, the embryo has a tail and the developing arms and legs resemble stumps. Muscles and ossification centers begin to develop. The head is the largest part: the brain is already represented by five brain vesicles (cavities with fluid); there are also bulging eyes with lenses and pigmented retinas.

In the period from the fifth to the eighth week, the actual embryonic period of intrauterine development ends. During this time, the embryo grows from 5 mm to approximately 30 mm and begins to resemble a person. His appearance changes as follows:

1) the curvature of the back decreases, the tail becomes less noticeable, partly due to reduction, partly because it is hidden by the developing buttocks;

2) the head straightens, the outer parts of the eyes, ears and nose appear on the developing face;

3) the arms are different from the legs, you can already see the fingers and toes;

4) the umbilical cord is fully defined, the area of ​​its attachment on the abdomen of the embryo becomes smaller;

5) in the abdominal area, the liver grows greatly, becoming as convex as the heart, and both of these organs form a lumpy profile of the middle part of the body until the eighth week; at the same time, the intestines become noticeable in the abdominal cavity, which makes the stomach more rounded;

6) the neck becomes more recognizable mainly due to the fact that the heart moves lower, as well as due to the disappearance of the gill arches;

7) external genitalia appear, although they have not yet fully acquired their final appearance.

By the end of the eighth week, almost all internal organs are well formed, and the nerves and muscles are so developed that the embryo can produce spontaneous movements. From this time until birth, the main changes in the fetus are associated with growth and further specialization.

Stages of development

Once the egg meets the sperm in the fallopian tube, they unite and fertilization occurs. The fertilized egg moves through the fallopian tube and then attaches to the wall of the uterus. This takes 7-10 days. From this moment you can count down your pregnancy.

First month

During the first month, a tiny embryo grows from a microscopic cell - 1.25 cm in length. This is the "fish-like" stage - the embryo has a tail, gill slits and a tadpole-like appearance. By the way, based on this stage of embryo development, scientists have put forward the theory that life on our planet began in water.

By the end of the first month, rudimentary tubercles are formed, from which the head and limbs will form, a thin tube is formed in the fetus, which will then turn into a heart, and its own blood circulation begins. Moreover, the rudiments of the central nervous system already exist.

And the expectant mother begins to suffer from attacks of nausea in the morning and complain about increased urination. However, this does not always happen and can be corrected. Very often at this time the breasts enlarge and become more sensitive.

Second month

During the second month, the embryo grows, its length is already about 3 cm. During this time, eyes, tiny limbs, the spine, the digestive system, kidneys, and the rudiments of the genital organs are formed. The gill slits and tail disappear, and the umbilical cord, which is initially attached near the tail, moves to the center of the abdominal region.

A woman expecting a child does not necessarily have to suffer from toxicosis and any unpleasant sensations. But a decrease in blood pressure and mild ailments in the morning are not excluded. It's time to go to the doctor and get tested.

Third month

During this month, the fetus grows up to 9 cm, and its weight reaches 14 g. Nails grow on the fingers, and the external genitalia form. The baby may grimace and clench its fists, swallow amniotic fluid and emit drops of urine.

The uterus is already beginning to protrude above the pelvic plane, the placenta is enlarging, and the volume of blood flow is increasing. Often by this time early toxicosis passes and health improves significantly. During the third month, it is advisable to do the first ultrasound - check the condition of the fetus, uterus and kidneys to identify gross pathologies.

The placenta is an extra-embryonic organ through which a connection between the embryo and the mother’s body is established. The human placenta belongs to the type of discoidal hemochorial villous placenta.

This is an important temporary organ with multiple functions, providing communication between the fetus and the mother’s body. The placenta performs trophic, excretory (for the fetus), endocrine (produces chorionic gonadotropin, progesterone, placental lactogen, estrogens, etc.), protective (including immunological protection). However, through the placenta (via blood-placental barrier) Alcohol, narcotic and medicinal substances, nicotine, as well as many hormones easily penetrate from the mother’s blood into the fetus’s blood.

In the placenta there are germinal or fetal part And maternal or uterine. The fetal part is represented by a branched chorion and the amniotic membrane attached to it, and the maternal part is represented by a modified basal part of the endometrium.

The development of the placenta begins in the 3rd week, when vessels begin to grow into the secondary (epitheliomesenchymal villi) and form tertiary villi. At 6-8 weeks, macrophages, fibroblasts, and collagen fibers differentiate around the vessels. Vitamins C and A play an important role in the differentiation of fibroblasts and collagen synthesis, without sufficient supply of which into the body of a pregnant woman, the strength of the bond between the embryo and the maternal body is disrupted and the threat of spontaneous abortion is created.

At the same time, the activity of hyaluronidase increases, due to which the breakdown of hyaluronic acid molecules occurs.

Reducing the viscosity of the main substance creates the most favorable conditions for the exchange of substances between the tissues of the mother and fetus. 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.

The formation of collagen fibers in the villi coincides in time with an increase in the proteolytic activity of the trophoblastic epithelium ( cytotrophoblast) and its derivative (syncytiotrophoblast).

With the development of the placenta, the uterine mucosa is destroyed and the histiotrophic nutrition changes to hematotrophic. This means that the chorionic villi are washed with the mother’s blood, which flows from the destroyed endometrial vessels into the lacunae.

The embryonic, or fetal, part of the placenta by the end of the 3rd month is represented by branching chorionic plate, consisting of fibrous (collagen) connective tissue covered with cyto- and syncytiotrophoblast. Branching chorionic villi (stem, or anchor, villi) well developed only on the side facing the myometrium. Here they pass through the entire thickness of the placenta and with their apices are immersed in 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 mitotically. From them the syncytiotrophoblast develops - a multinuclear structure covering the reducing cytotrophoblast. The syncytiotrophoblast contains a large number of various proteolytic and oxidative enzymes, which is associated with its role in metabolic processes between the mother and fetus. In the cytotrophoblast and in the syncytium, pinocytosis vesicles, lysosomes and other organelles are detected. Starting from the 2nd month, the chorionic epithelium becomes thinner and is gradually replaced by syncytiotrophoblast. During this period, the syncytiotrophoblast is thicker than the cytotrophoblast; at the 9-10th week, the syncytium becomes thinner, and the number of nuclei in it increases. Numerous microvilli appear in the form of a brush border on the surface of the syncytium, facing into lacunae.

Between the syncytium and the cellular trophoblast there are slit-like submicroscopic spaces, in some places reaching the basement membrane of the trophoblast, which creates conditions for the bilateral penetration of trophic substances, hormones, etc. between the syncytium and the stroma of the villi.

In the second half of pregnancy, and especially at the end of it, the trophoblast becomes very thin in places and the villi become covered with a fibrin-like oxyphilic mass, which is apparently a product of plasma coagulation and trophoblast decay (“Langhans fibrinoid”).

With increasing gestational age, the number of macrophages and collagen-producing differentiated fibroblasts decreases, and fibrocytes appear. The amount of collagen fibers, although increasing, remains small in most villi until the end of pregnancy.

The structural and functional unit of the formed placenta is cotyledon, formed by the stem villi and its secondary and tertiary (terminal) branches. The total number of cotyledons in the placenta reaches 200.

The maternal part of the placenta is represented basal plate and connective tissue septa separating the cotyledons from each other, as well as gaps, filled with maternal blood. Trophoblastic cells are also found at the points of contact between the stem villi and the sheath. (peripheral trophoblast).

Already in the early stages of pregnancy, the chorionic villi destroy the outer, i.e., those closest to the fetus, layers of the main falling membrane, and in their place are formed filled with maternal blood gaps, into which the chorionic villi hang freely. The deep, undestroyed parts of the falling membrane, together with the trophoblast, form the basal plate.

Basal layer of endometrium- connective tissue of the uterine mucosa containing decidual cells. These large, glycogen-rich connective tissue cells are located in the deep layers of the uterine lining. They have clear boundaries, rounded nuclei and oxyphilic cytoplasm. In the basal lamina, often at the site of attachment of the villi to the maternal part of the placenta, there are clusters of peripheral cytotrophoblast cells. They resemble decidual cells, but are distinguished by more intense basophilia of the cytoplasm. Amorphous substance (Rohr fibrinoid) located on the surface of the basal plate facing the chorionic villi. Trophoblastic cells of the basal lamina, together with fibrinoid, play a significant role in ensuring immunological homeostasis in the mother-fetus system.

Part of the main falling membrane, 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. Growing tightly to the chorion, it forms a closing plate that prevents the flow of blood from the lacunae of the placenta.

The blood in the lacunae is continuously renewed. It comes from the uterine arteries, which enter here from the muscular lining 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 blood of the mother and the blood of the fetus circulate through independent vascular systems and do not mix with each other. hemochorionic barrier, separating both blood flows, consists of the endothelium of the fetal vessels, surrounding the connective tissue vessels, the epithelium of the chorionic villi (cytotrophoblast, syncytiotrophoblast), and, in addition, of fibrinoid, which in some places covers the villi from the outside.

The formation of the placenta ends at the end of the 3rd month of pregnancy.

The placenta formed by this time ensures the final differentiation and rapid growth of the rudiments of the fetal organs formed in the previous period.

Fourth month

By the end of this month, the baby reaches a length of 18-20 cm and weighs 120 g. Its heartbeat can be heard using a regular stethoscope, the skin becomes multi-layered, and muscles develop. At this time, it is already possible to determine the sex of the fetus.

Around this time, intensive growth of the placenta ends.

Fifth month

By the end of the first half of pregnancy, the baby recovers up to 300 g. At this time, his muscles are already developed enough for him to be able to express himself with pushes. Hair begins to grow, the skin loses its transparency.

At this time, the woman usually feels the first movements of the child - this is a very pleasant event. The uterus reaches the umbilical cavity. Half the way is over. And your health usually improves.

Sixth month

By the end of this month, all the main organs and vital systems have been formed, and in the future they will only improve. Lines appear on the palms of the hands, and the soles of the feet are fully formed. Weight reaches 700 g, and length - 35 cm.

From now on, the baby will begin to accumulate fat, and the mother’s tummy will grow faster.

Seventh month

By this time, the baby can already open and close his eyes, he begins to have visual sensations, as far as possible inside the tummy. The skin becomes covered with a wax-like coating, which will subsequently facilitate passage through the birth canal. The baby becomes more beautiful, as a subcutaneous fat layer appears and small folds on the body disappear. The length of the baby reaches 38-40 cm, and it weighs 1200 g.

The baby becomes heavier and the woman may experience back pain. The expectant mother gets tired more often and sleeps more. Due to the fact that the enlarged uterus is pushing against the internal organs, some women complain of heartburn and indigestion. From this moment on, women periodically feel painless contractions of the uterus (tone).

Eighth month

By this time, the child weighs about two kilograms, and the length reaches 43-45 cm. He already hears perfectly. What’s interesting is that while the baby is in the tummy, he reacts more actively to low sounds (that is, dad’s voice), and after birth - to high ones (that is, mom’s voice). The lungs and other organ systems are almost completely formed, the baby is almost ready for independent life. He can even distinguish the taste. When he swallows amniotic fluid, he sometimes winces if he doesn’t like the taste.

During this period, the main hormone affecting the female body is progesterone. Its main function is to reduce pain during uterine contractions and general relaxation and increase muscle elasticity. During this period, women often experience increased drowsiness and apathy.

Ninth month

By this time, the skin is almost completely formed, and the layer of subcutaneous fat becomes thicker, and the baby becomes more and more beautiful. Weight reaches 2.5 kg. All other organs are prepared for life in the air. At this time, the baby takes its final position, since it is already difficult to change it in the future. The most favorable position is considered to be head down, facing the mother's back.



Stages of development of the human embryo

Before entering this complex and, at the same time, interesting world, a person needs to go a long way. In this article we will look at the stages of human development in the womb.
So, as you know, a woman can become pregnant only if she has a menstrual cycle, and, accordingly, ovulation. Ovulation is the process of the release of a mature egg into the fallopian tube from the ovary. It usually begins 14 days before the start of a new menstrual cycle and lasts several days. These days are considered especially progressive and successful for conception.
Having “caught” the right moment and having had sexual intercourse, partners should prepare for the fact that they will soon become happy parents. So what are the stages of development of a human embryo in the mother’s body?
First of all, after the logical end of sexual intercourse, while the partners are doing their household chores, work begins to “boil” in the female body: for several hours, sperm “fight” for a place “under the sun”, and the most agile ones penetrate the female egg (one would not have managed it), after which other sperm cannot penetrate there due to a complex process of chemical reactions.
After fertilization of the egg, it contains 2 nuclei that carry their own set of chromosomes - 23 male and 23 female chromosomes, which are connected together. In other words, a zygote is formed (a cell resulting from the fusion of chromosomes), which, in turn, divides into two more. The result is an embryo that divides into four, six, etc. cells connected together. When the required number of cells is reached, the embryo slows down on its way to the uterus. This stage is called morula. The cells, in the process of crushing into several hundred pieces, form a kind of cavity, the blastocoel, and from this moment the stage passes into the blastula. At this point, the blastula can divide into two (or sometimes more) cavities to produce identical twins.
Later, the process of gastrulation occurs, that is, the formation of germinal “leaves” or layers of cells. The formation of layers of ectoderm, endoderm and mesoderm begins. Ectoderm is the outer layer, endoderm is the inner layer, mesoderm is the “middle” layer. They allow the immediate development of the future human body to begin. This stage is called neurula. From the ectoderm comes the process of development of the neural plate, later the neural tube, which in turn will give rise to the development of the brain and spinal cord in the embryo, as well as the organs of hearing and vision, and the skin. From the endoderm is the process of development of internal organs. And the mesoderm is responsible for the development of the bone skeleton, muscles and blood vessels of the future person. At the final stage of neurulation in the embryo, it is already possible to distinguish the posterior and anterior parts. Organogenesis begins, the embryo can already be called an embryo...

Stages of animal embryo development

Just like a person, before being born, future creatures go through exactly the same path. The stages of development of an animal embryo are not very different from human ones.
At the first stage, a zygote is formed, and during its fragmentation 2, 4, 8... etc. are formed. cells, division occurs very quickly, many times faster than in humans; for example, in a frog, division occurs at intervals of 30 minutes, and in a cat, almost 10 hours.
A blastula is formed, which can have many cavities, unlike the blastula of a human embryo. But the function of such cavities is the same.
Then the speed of the blastula on the way to the uterus decreases, the process of gastrulation begins - with the help of the germ layers of the ectoderm, endoderm and mesoderm (except for coelenterates and sponges), the organism of a living creature is formed. During the gastrulation process, no cell growth occurs, so the embryo remains the same size as it was at the first stage.

Stages of embryonic development of the fetus

After passing the neurula stage, the process of primary organogenesis begins (that is, the formation of organs). Late organogenesis, that is, prenatal development of the fetus, is also included in the stages of embryonic development of the embryo. We need to understand what primary organogenesis is.
The fertilized egg, resulting from complex chemical reactions, attaches to the wall of the uterus. Already in the fourth week, the rudiments of the formation of many organs appear, from the rudiments of the eyes and ears to the rudiments of striated muscles. On the fifth stage, legs are formed, and the length of the embryo is, on average, 7.5 mm. At the end of 8-9 weeks, the embryo is about 3-4 cm long. The gonads are already formed and the body knows who will be born - a girl or a boy. The expectant mother, in turn, learns about this only at 15-17 weeks during an ultrasound, when the genital organs are clearly and finally formed. At the same time, the first fetal movements appear. By the end of the 5th month, the mother herself can use her palm to “feel” the baby’s heartbeat, and this is an exciting moment, because before this the baby’s heart could only be heard through an ultrasound machine. At this time, the baby's skin is covered with the finest hairs. And at 6-7 months, the fetus is covered with birth lubricant, and its movements become more and more intense, but the fetus lies head down. By week 40 (or earlier), the fetus is ready to leave its “home” and meet the one who carried it under her heart for 9 months.
Actually, this is how complex and interesting it is, from a huge number of small cells to a small man 50-60 cm tall, and the development of the future person occurs.

To understand the individual characteristics of the structure of the human body, it is necessary to become familiar with the development of the human body in the prenatal period. Each person has individual characteristics of external appearance and internal structure, the presence of which is determined by two factors. First of all, this is heredity - traits inherited from parents, as well as the result of the influence of the external environment in which a person grows, develops, learns, and works.

Individual development, or development in ontogenesis, occurs during all periods of life - from conception to death. In human ontogenesis, two periods are distinguished: before birth (intrauterine, prenatal; from the Greek natos - born) and after birth (extrauterine, postnatal). During the prenatal period, from conception to birth, the embryo (embryo) is located in the mother's body. During the first 8 weeks, the main processes of formation of organs and body parts occur. This period is called embryonic, and the body of the future person is an embryo (fetus). Starting from the 9th week, when the main external human features have already begun to appear, the organism is called a fetus, and the period is called fertile.

After fertilization (fusion of sperm and egg), which usually occurs in the fallopian tube, the fused germ cells form a single-celled embryo - a zygote, which has all the properties of both sex cells. From this moment the development of a new (daughter) organism begins.

First week of embryo development

This is the period of fragmentation (division) of the zygote into daughter cells. During the first 3-4 days, the zygote divides and simultaneously moves along the fallopian tube towards the uterine cavity. As a result of the division of the zygote, a multicellular vesicle is formed - a blastula with a cavity inside (from the Greek blastos - sprout). The walls of this vesicle consist of two types of cells: large and small. The walls of the vesicle, the trophoblast, are formed from the outer layer of small light cells. Subsequently, trophoblast cells form the outer layer of the membranes of the embryo. Larger dark cells (blastomeres) form a cluster - the embryoblast (germinal nodule, embryonic rudiment), which is located medially from the trophoblast. From this accumulation of cells (embryoblast) the embryo and adjacent extra-embryonic structures (except the trophoblast) develop. A small amount of fluid accumulates between the surface layer (trophoblast) and the germinal nodule.

By the end of the 1st week of development (6-7th day of pregnancy), the embryo is introduced (implanted) into the uterine mucosa. The surface cells of the embryo, forming a vesicle - trophoblast (from the Greek trophe - nutrition, trophicus - trophic, nourishing), release an enzyme that loosens the surface layer of the uterine mucosa. The latter is already prepared for the implantation of the embryo into it. By the time of ovulation (the release of an egg from the ovary), the uterine mucosa becomes thicker (up to 8 mm). The uterine glands and blood vessels grow in it. Numerous outgrowths - villi - appear on the trophoblast, which increases the surface of its contact with the tissues of the uterine mucosa. The trophoblast turns into the nutritious membrane of the embryo, which is called the villous membrane, or chorion. At first, the chorion has villi on all sides, then these villi are retained only on the side facing the wall of the uterus. In this place, a new organ develops from the chorion and the adjacent mucous membrane of the uterus - the placenta (baby place). The placenta is an organ that connects the mother’s body with the embryo and provides its nutrition.

Second week of embryo development

This is the stage when the embryoblast cells are divided into two layers (two plates), from which two vesicles are formed. From the outer layer of cells adjacent to the trophoblast, an ectoblastic (amniotic) vesicle filled with amniotic fluid is formed. An endoblastic (yolk) vesicle is formed from the inner layer of cells of the embryonic nodule of the embryoblast. The anlage (“body”) of the embryo is located where the amniotic sac comes into contact with the yolk sac. During this period, the embryo is a two-layer shield, consisting of two germ layers: the outer - ectoderm (from the Greek ektos - outside, derma - skin) and the inner - endoderm (from the Greek ёntos - inside). The ectoderm faces the amniotic sac, and the endoderm is adjacent to the yolk sac. At this stage, the surfaces of the embryo can be determined. The dorsal surface is adjacent to the amniotic sac, and the ventral surface is adjacent to the yolk sac. The trophoblast cavity around the amniotic and vitelline vesicles is loosely filled with strands of extraembryonic mesenchyme cells. By the end of the 2nd week, the length of the embryo is only 1.5 mm. During this period, the embryonic shield thickens in its posterior (caudal) part. Here, the axial organs (notochord, neural tube) subsequently begin to develop.

Third week of embryo development

The period of formation of a three-layer shield. The cells of the outer, ectodermal layer of the embryonic shield are displaced towards its posterior end, resulting in the formation of a roller elongated in the direction of the axis of the embryo. This cellular strand is called the primitive streak. In the head (front) part of the primary streak, cells grow and multiply faster, resulting in the formation of a small elevation - the primary nodule (Hensen's node). The primary streak determines the bilateral symmetry of the embryo body, i.e. its right and left sides. The location of the primary node indicates the cranial (head) end of the embryo's body.

As a result of the rapid growth of the primary streak and primary node, the cells of which grow laterally between the ectoderm and endoderm, the middle germ layer, the mesoderm, is formed. The mesoderm cells located between the sheets of the scutellum are called intraembryonic mesoderm, and those that migrate beyond its boundaries are called extraembryonic mesoderm.

Part of the mesoderm cells within the primary node grows especially actively forward, forming the head (chordal) process. This process penetrates between the outer and inner layers from the head to the tail end of the embryo and forms a cellular cord - the dorsal string (chord). The head (cranial) part of the embryo grows faster than the tail (caudal), which, together with the region of the primary tubercle, seems to retreat back. At the end of the 3rd week, anterior to the primary tubercle in the outer germ layer, a longitudinal strip of actively growing cells stands out - the neural plate. This plate soon bends, forming a longitudinal groove - the neural groove. As the groove deepens, its edges thicken, move closer and grow together, closing the neural groove into the neural tube. Subsequently, the entire nervous system develops from the neural tube. The ectoderm closes over the formed neural tube and loses connection with it.

During the same period, a finger-like outgrowth, the allantois, penetrates into the extraembryonic mesenchyme (the so-called amniotic leg) from the posterior part of the internal (endodermal) layer of the embryonic shield, which does not perform certain functions in humans. Along the allantois, blood umbilical (placental) vessels grow from the embryo through the amniotic pedicle to the chorionic villi. A cord containing blood vessels that connects the embryo with the extraembryonic membranes (placenta) forms the abdominal stalk. Thus, by the end of the 3rd week, the human embryo looks like a three-layer shield. In the region of the outer germ layer the neural tube is visible, and deeper - the dorsal chord, i.e. the axial organs of the human embryo appear.

Fourth week of embryo development

This is the period when the embryo, which looks like a three-layer shield, begins to bend in the transverse and longitudinal directions. The embryonic shield becomes convex, and its edges are delimited from the amnion by a deep groove - the trunk fold. The body of the embryo turns from a flat shield into a three-dimensional one; the exodermis covers the body of the embryo from all sides.

The endoderm, once inside the body of the embryo, curls into a tube and forms the embryonic rudiment of the future intestine. The narrow opening through which the embryonic intestine communicates with the yolk sac later turns into the umbilical ring. The epithelium and glands of the digestive tract and respiratory tract are formed from the endoderm. The ectoderm forms the nervous system, the epidermis of the skin and its derivatives, the epithelial lining of the oral cavity, anal rectum, and vagina. The mesoderm gives rise to internal organs (except for endoderm derivatives), the cardiovascular system, organs of the musculoskeletal system (bones, joints, muscles), and the skin itself.

The embryonic (primary) gut is initially closed in front and behind. At the anterior and posterior ends of the body of the embryo, invaginations of the ectoderm appear - the oral fossa (future oral cavity) and the anal (anal) fossa. Between the cavity of the primary intestine and the oral fossa there is a two-layer (ectoderm and endoderm) anterior (oropharyngeal) plate (membrane), between the intestine and the anal fossa there is a cloacal (anal) plate (membrane), also two-layer. The anterior (oropharyngeal) membrane breaks through in the 4th week of development. At the 3rd month, the posterior (anal) membrane breaks through.

As a result of bending, the body of the embryo is surrounded by the contents of the amnion - amniotic fluid, which acts as a protective environment that protects the embryo from damage, primarily mechanical (concussion). The yolk sac lags in growth and in the 2nd month of intrauterine development it looks like a small sac, and then is completely reduced. The abdominal stalk lengthens, becomes relatively thin and later receives the name umbilical cord.

During the 4th week, the differentiation of its mesoderm, which began at the end of the 3rd week of embryo development, continues. The dorsal part of the mesoderm, located on the sides of the notochord, forms paired projections - somites. Somites are segmented, i.e. are divided into metamerically located areas. Therefore, the dorsal part of the mesoderm is called segmented. Segmentation of somites occurs gradually in the direction from front to back. On the 20th day, the 3rd pair of somites is formed, by the 30th day there are already 30 of them, and on the 35th day - 43-44 pairs. The ventral part of the mesoderm is not divided into segments, but is represented on each side by two plates (the non-segmented part of the mesoderm). The medial (visceral) plate is adjacent to the endoderm (primary gut) and is called splanchnopleura, the lateral (outer) plate is adjacent to the wall of the body of the embryo, to the ectoderm, and is called somatopleura. From the splanchno- and somatopleura the epithelial cover of the serous membranes (mesothelium), as well as the lamina propria of the serous membranes and the subserosal base develop. The mesenchyme of the splanchnopleura also goes to the construction of all layers of the digestive tube, except for the epithelium and glands, which are formed from the endoderm. The endoderm gives rise to the glands of the esophagus, stomach, liver with bile ducts, glandular tissue of the pancreas, epithelial lining and glands of the respiratory organs. The space between the plates of the unsegmented part of the mesoderm turns into the body cavity of the embryo, which is divided into the abdominal, pleural and pericardial cavities.

The mesoderm at the border between the somites and the splanchnopleura forms nephrotomes (segmental legs), from which the tubules of the primary kidney develop. Three primordia are formed from the dorsal part of the mesoderm - somites. The ventromedial portion of the somites - the sclerotome - is used to build skeletogenic tissue, which gives rise to the bones and cartilages of the axial skeleton - the spine. Lateral to it lies the myotome, from which striated skeletal muscles develop. In the dorsolateral part of the somite there is a dermatome, from its tissue the connective tissue base of the skin is formed - the dermis.

At the 4th week, in the head section on each side of the embryo, the rudiments of the inner ear (first the auditory pits, then the auditory vesicles) and the future lens of the eye, which is located above the lateral protrusion of the brain - the optic vesicle, are formed from the ectoderm. At the same time, the visceral parts of the head are transformed, grouping around the oral bay in the form of the frontal and maxillary processes. Caudal to these processes, the contours of the mandibular and sublingual (hyoid) visceral arches are visible.

On the anterior surface of the embryo's body, the cardiac tubercles are distinguished, followed by the hepatic tubercles. The depression between these tubercles indicates the place of formation of the transverse septum - one of the rudiments of the diaphragm.

Caudal to the hepatic tubercle is the abdominal stalk, which includes large blood vessels and connects the embryo to the placenta (umbilical cord).

The period from the 5th to the 8th week of embryo development

The period of development of organs (organogenesis) and tissues (histogenesis). This is the period of early development of the heart, lungs, complication of the structure of the intestinal tube, the formation of visceral and gill arches, and the formation of capsules of the sensory organs. The neural tube closes completely and expands in the cerebrum (the future brain). At the age of about 31-32 days (5th week, embryo length 7.5 cm), fin-like rudiments (buds) of hands appear at the level of the lower cervical and first thoracic segments of the body. By the 40th day, the rudiments of the legs are formed (at the level of the lower lumbar and upper sacral segments).

At the 6th week, the outer ear buds are noticeable, from the end of the 6-7th week - the fingers, and then the toes.

By the end of the 7th week, the eyelids begin to form. Thanks to this, the eyes are outlined more clearly. At the 8th week, the formation of embryonic organs ends. From the 9th week, i.e. from the beginning of the third month, the embryo takes on the appearance of a person and is called a fetus.

The period of embryo development is from 3 to 9 months

Starting from the third month and throughout the entire fetal period, growth and further development of the formed organs and body parts occur. At the same time, differentiation of the external genitalia begins. The nails on the fingers are laid. From the end of the 5th month, eyebrows and eyelashes become noticeable. In the 7th month, the eyelids open and fat begins to accumulate in the subcutaneous tissue. At 9 months the fetus is born. Age-related features of the development of individual organs and organ systems are presented in the relevant sections of the textbook.

The individual development of each organism is a continuous process that begins from the formation of the zygote and continues until the death of the organism.

The concept of ontogenesis

Ontogenesis is a cycle of individual development of each organism; it is based on the implementation of hereditary information at all stages of existence. In this case, the influence of environmental factors plays an important role.

Ontogenesis is determined by the long historical development of each specific species. The biogenetic law, which was formulated by scientists Müller and Haeckel, reflects the relationship between individual and historical development.

Stages of ontogeny

When viewed from a biological perspective, the most significant event in all individual development is the ability to reproduce. It is this quality that ensures the existence of species in nature.

Based on the ability to reproduce, the entire ontogenesis can be divided into several periods.

1. Pre-reproductive.
2. Reproductive.
3. Post-reproductive.

During the first period, the implementation of hereditary information occurs, which manifests itself in structural and functional transformations of the body. At this stage, the individual is quite sensitive to all influences.

The reproductive period realizes the most important purpose of each organism - procreation.

The last stage is inevitable in the individual development of each individual; it is manifested by aging and extinction of all functions. It always ends in the death of the organism.

The pre-reproductive period can still be divided into several stages:

• larval;
• metamorphosis;
• juvenile

All periods have their own characteristics, which manifest themselves depending on the organism’s belonging to a particular species.

Stages of the embryonic period

Taking into account the developmental features and responses of the embryo to damaging factors, all intrauterine development can be divided into the following stages:

The first stage begins with the fertilization of the egg and ends with the implantation of the blastocyst into the lining of the uterus. This occurs approximately 5-6 days after the formation of the zygote.

Crushing period

Immediately after the fusion of the egg with the sperm, the embryonic period of ontogenesis begins. A zygote is formed and begins to fragment. In this case, blastomeres are formed, the more they become in number, the smaller they are in size.

The crushing process does not proceed in the same way among representatives of different species. This depends on the amount of nutrients and their distribution in the cytoplasm of the cell. The larger the yolk, the slower the division.

Crushing can be uniform or uneven, as well as complete or incomplete. Humans and all mammals are characterized by complete uneven fragmentation.

As a result of this process, a multicellular single-layer embryo with a small cavity inside is formed; it is called a blastula.

Blastula

This stage ends the first period of embryonic development of the organism. In blastula cells, one can already observe the ratio of nucleus and cytoplasm typical for a particular species.

From this moment on, the cells of the embryo are already called embryonic. This stage is characteristic of absolutely all organisms of any species. In mammals and humans, crushing is uneven due to the small amount of yolk.

In different blastomeres, division occurs at different rates, and one can observe the formation of light cells, which are located along the periphery, and dark cells, which line up in the center.

The trophoblast is formed from light cells; its cells are capable of:

• dissolve tissue, so the embryo has the opportunity to penetrate the wall of the uterus;
• peel off from the embryonic cells and form a vesicle filled with liquid.

The embryo itself is located on the inner wall of the trophoblast.

Gastrulation

After the blastula, the next embryonic period begins in all multicellular organisms - the formation of the gastrula. There are two stages in the gastrulation process:

• the formation of a two-layer embryo consisting of ectoderm and endoderm;
• the appearance of a three-layer embryo, the third germ layer is formed - the mesoderm.

Gastrulation occurs by intussusception, when blastula cells from one pole begin to invaginate. The outer layer of cells is called ectoderm, and the inner layer is called endoderm. The cavity that appears is called the gastrocoel.

The third germ layer, the mesoderm, is formed between the ectoderm and endoderm.

Formation of tissues and organs

The three germ layers formed at the end of the stage will give rise to all the organs and tissues of the future organism. The next embryonic period of development begins.

From the ectoderm develop:

• nervous system;
• leather;
• nails and hair;
• sebaceous and sweat glands;
• sense organs.

The endoderm gives rise to the following systems:

• digestive;
• respiratory;
• parts of the urinary system;
• liver and pancreas.

The third germ layer, the mesoderm, produces the most derivatives; from it the following is formed:

• skeletal muscles;
• gonads and most of the excretory system;
• cartilage tissue;
• circulatory system;
• adrenal glands and gonads.

After the formation of tissues, the next embryonic period of ontogenesis begins - the formation of organs.

Two phases can be distinguished here.

1. Neurulation. A complex of axial organs is formed, which includes the neural tube, notochord and intestine.
2. Construction of other organs. Individual areas of the body acquire their characteristic shapes and outlines.

Organogenesis ends completely when the embryonic period comes to an end. It is worth noting that development and differentiation continue after birth.

Control of embryonic development

All stages of the embryonic period are based on the implementation of hereditary information received from parents. The success and quality of implementation depends on the influence of external and internal factors.

The scheme of ontogenetic processes consists of several stages.

1. Genes receive all the information from neighboring cells, hormones and other factors in order to come into an active state.
2. Information from genes for protein synthesis at the stages of transcription and translation.
3. Information from protein molecules to stimulate the formation of organs and tissues.

Immediately after the fusion of the egg with the sperm, the first period of embryonic development of the organism begins - fragmentation, which is completely regulated by the information that is in the egg.

At the blastula stage, activation occurs by sperm genes, and gastrulation is controlled by the genetic information of the germ cells.

The formation of tissues and organs occurs due to the information contained in the cells of the embryo. The separation of stem cells begins, which give rise to various tissues and organs.

The formation of external characteristics of the body during the human embryonic period depends not only on hereditary information, but also on the influence of external factors.

Factors influencing embryonic development

All influences that can negatively affect the development of a child can be divided into two groups:

• environmental factors;
• mother's illnesses and lifestyle.

The first group of factors includes the following.

1. Radioactive radiation. If such an effect occurred at the first stage of the embryonic period, when implantation has not yet occurred, then most often a spontaneous miscarriage occurs.
2. Electromagnetic radiation. Such exposure is possible when you are near operating electrical appliances.
3. Exposure to chemicals This includes benzene, fertilizers, dyes, chemotherapy.

The expectant mother can also cause disruption of embryonic development; the following dangerous factors can be mentioned:

• chromosomal and genetic diseases;
• use of drugs, alcoholic beverages, any stages of the embryonic period are considered vulnerable;
• infectious diseases of the mother during pregnancy, for example rubella, syphilis, influenza, herpes;
• heart failure, bronchial asthma, obesity - with these diseases, the supply of oxygen to the tissues of the fetus may be impaired;
• taking medications; the features of the embryonic period are such that the most dangerous in this regard are the first 12 weeks of development;
• excessive addiction to synthetic vitamin preparations.

If you look at the following table, you can see that not only a lack of vitamins is harmful, but also their excess.

Vitamin name	Dangerous dose of the drug	Developmental disorders
A	1 million IU	Disturbances in brain development, hydrocephalus, miscarriage.
E	1 g	Anomalies in the development of the brain, visual organs, and skeleton.
D	50,000 IU	Skull deformation.
K	1.5 g	Reduced blood clotting.
C	3 g	Miscarriage, stillbirth.
B2	1 g	Fusion of fingers, shortening of limbs.
PP	2.5 g	Chromosomal mutation.
B5	50 g	Disturbance in the development of the nervous system.
B6	10 g	Stillbirth.

Fetal diseases in the last stages of embryonic development

In the last weeks of development, the child’s vital organs mature and prepare to endure all sorts of disorders that may arise during childbirth.

Before birth, a high level of passive immunization is created in the fetus's body. At this stage, various diseases that the fetus can get are also possible.

Thus, despite the child’s practically formed body, some negative factors are quite capable of causing serious disorders and congenital diseases.

Dangerous periods of embryonic development

Throughout embryonic development, periods can be identified that are considered the most dangerous and vulnerable, since at this time the formation of vital organs occurs.

1. 2-11 weeks, as the formation of the brain occurs.
2. 3-7 weeks - the formation of the organs of vision and heart begins.
3. 3-8 weeks - the formation of limbs occurs.
4. Week 9 - the belly is filled.
5. 4-12 weeks - the formation of the genital organs begins.
6. 10-12 weeks - laying of the sky.

The considered characteristics of the embryonic period once again confirm that for fetal development the most dangerous periods are considered to be from 10 days to 12 weeks. It is at this time that all the main organs of the future organism are formed.

Lead a healthy lifestyle, try to protect yourself from the harmful effects of external factors, avoid communicating with sick people, and then you can be almost sure that your baby will be born healthy.

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