A person is born when a sperm, a male reproductive cell, enters a woman’s body, merges with her egg and forms a single cell. A new cell develops by division. At some time, the embryo appears and then disappears the signs inherent in representatives of the animal world: gill arches are formed in the image and likeness of fish, the jaw joint that reptiles have, a tail and thin hair grow. These ancient forms do not exist for long and then either change or disappear.
Germ It seems to quickly pass through all stages of evolution. This process is called recapitulation(repetition).
German biologists Fritz Müller and Ernst Haeckel formulated in the 19th century. biogenetic law: “The individual development of each individual is a short and rapid repetition historical development the species to which this individual belongs."
Developing in the mother's womb, the human embryo goes through the entire evolution of the living. This four-week-old embryo (its length is only 4 mm) has clearly visible gill apparatus, like a fish, and a tail. They will disappear in a few weeks. Russian biologist A.N. Severtsov (1866 - 1936) established that in individual development the characteristics are repeated not of adult ancestors, but of their embryos.
A child develops in the mother's womb for approximately 266 days, or 38 weeks (the first eight weeks are called an embryo, then a fetus). During the embryonic period, an embryo gradually forms from a shapeless accumulation of cells, which in general terms already resembles a human being. By the end of these eight weeks, all the main internal and external human organs have been formed. True, according to appearance The sex of the embryo cannot yet be determined - this will only be possible after another two weeks.
At the ninth week, the fertile, or fetal, period begins - the time of growth and maturation of the body. From this time on, the tiny child, lying in a special water shell, begins to bend and move his arms and legs. His skin, initially transparent as glass, becomes cloudy and loses its transparency. By the end of the fourth month, the baby's heart becomes noticeably stronger. Every day it pumps more than 30 liters of blood through its blood vessels. Now the fetus reaches 16 cm in length and weighs 170 g. In the fifth month, the unborn child is already very noticeably pushing, dangling his arms and legs. He already feels and hears movement. Loud noises make his heart beat faster. And here’s something else that happens at this time: a pattern of thin twisted lines appears on the fingertips. This pattern “sticks” to your fingers forever. Having touched any object, a person leaves his fingerprints on it. They are unique: you won’t find two people on Earth with the same fingerprints.
By the beginning of the sixth month, the fetus weighs 600 g. If the child is born in the sixth month of pregnancy (that is, prematurely), then - with good care from doctors - he will survive. And if everything goes well, he will be born at the end of the ninth month. Such newborns weigh at least 3200 g, with an average height of 50 cm.
MAIN STAGES OF INDIVIDUAL DEVELOPMENT OF THE HUMAN ORGANISM - ONTOGENESIS
Depending on the environment in which the individual develops, the entire ontogenesis breaks up into 2 large periods, separated from each other by the moment of birth:
1. Intrauterine, when the newly born organism develops in the mother’s womb; this period lasts from conception to birth.
2. Extrauterine, or postnatal, when a new individual continues its development outside the mother’s body; this period lasts from birth to death.
The intrauterine period, in turn, is divided into 2 phases: 1) embryonic (the first 2 months), when the initial development of the embryo (embryo) occurs and when the main formation of organs occurs; 2) fetal (3-9 months), when further development of the fetus (fetus, lat. - fruit) occurs.
Human embryonic development is studied in the Course of General Embryology, but here we will limit ourselves to the briefest initial information necessary to understand the structure of the body of an adult.
The development of the human embryo in the oviduct and uterus is conventionally divided into five periods.
1. Fertilization, formation of a zygote. The male sperm cell (spermium, lat.) penetrates the female egg (ovium, lat.), and they, merging, form a new organism - the zygote.
2. Crushing. The zygote is split into cells - blastomeres (blastos, Greek - embryo, meros, Greek - part), of which some are grouped into a nodule - an embryoblast, while others grow over its surface, forming a trophoblast. The trophoblast villi grow into the mucous membrane of the uterus and together with it create a baby's place, or placenta (plax, Greek - flat body, pie). This organ is also called the afterbirth because it follows the birth of the child.
3. Gastrulation consists of transforming a single-layer embryo into a three-layer one - gastrula (gaster, Greek - stomach).
The outer layer is called ectoderm, the inner layer is called endoderm, and the middle layer between them is called mesoderm.
Another important result of gastrulation is the emergence of the axial primordium complex, which consists of the following anlage:
1. The neural plate (neuroectoderm) or groove emerging from the ectoderm and lying along the midline of the dorsal side, which later turns into the neural tube - the rudiment of the nervous system.
2. The underlying chord (chorde, Greek - string).
3. Located laterally from it, to the right and left - the mesoderm (Fig. 1).
The location of the axial complex of primordia on the dorsal side and their mutual arrangement are very characteristic of all chordates, including humans, and are the most ancient and common feature for them. The appearance of this feature in the structure of the embryo ends the period of gastrulation.
4. Isolation of the body of the embryo. The embryo separates from the extraembryonic parts, grows in length and turns into a cylindrical formation with a head (cranial) and tail (caudal) ends; in this case, the germ layers are transformed.
The outer germ layer, or ectoderm, gives rise to the cutaneous ectoderm, from which develop: the epithelium (integumentary tissue) of the skin, or epidermis, and its derivatives - hair, nails, sebaceous, sweat and mammary glands; part of the integumentary epithelium of the mucous membrane and glands of the oral cavity; tooth enamel; stratified epithelium of the anus of the rectum; epithelium of the urinary and seminal ducts.
From the neuroectoderm develop all parts of the central and peripheral nervous system and various auxiliary ependymoglial elements that are part of the adult nervous system and sensory organs (for example, contractile elements of the iris, pigment epithelium, etc.).
The internal germ layer, or endoderm, is heterogeneous: its anterior part is represented by ectoderm material, which is secondary to the endoderm and forms the prechordal plate, and the rest is intestinal endoderm.
From the prechordal plates develop: the epithelium of the airways and lungs, a significant part of the mucous membrane of the oral cavity and pharynx, glandular tissues of the pituitary gland, thyroid and parathyroid glands, thymus, as well as the integumentary epithelium and glands of the esophagus.
From the intestinal endoderm, the integumentary epithelium and glands of the stomach, intestines and bile ducts, as well as the liver and glandular tissues of the pancreas are formed.
The middle germ layer, or mesoderm, is initially represented by metamerically located dorsal segments, or somites (soma, Greek - body), located metamerically to the right and left of the notochord, which, through segmental legs (nephrotomes), are connected to the ventral non-segmented sections of the mesoderm, called splanchnotomes (splanchna, Greek - insides) or side plates (see Fig. 1). The maximum number of somites is 43-44 pairs by the end of the fifth week of development, when the length of the embryo is 11 mm.
Each somite, with the exception of the first two, is differentiated into three sections: 1) the dorsolateral section, which represents the mesenchymal rudiment of the connective tissue of the skin. - dermatome; 2) the medioventral area, giving rise to the cartilaginous and bone tissues of the skeleton, the sclerotome (scleros, Greek - hard) and 3) the area located between the dermatome and the sclerotome and being the rudiment of skeletal muscles, the myotome (mys, Greek - mouse; myo , Greek - muscular).
Subsequently, the muscles of the body develop from the myotomes. The skin plate underlies the cutaneous ectoderm and develops into the connective tissue layer of the skin. From the sclerotomes, mesenchymal skeletogenic cells arise, accumulating around the neural tube and notochord and giving rise to vertebrae, ribs and intervertebral discs. The latter contain very phylogenetically instructive remains of the notochord in the form of so-called gelatinous nuclei. Sclerotomes are used to form other parts of the skeleton.
In the embryonic development of segmental legs, or nephrotomes (nephros, Greek - kidney), the historical path of development of excretory organs in vertebrates and humans is clearly reflected.
Nephrotomes are located from the head to the caudal end of the body of the embryo in the head, trunk and pelvic regions, giving rise to various formations.
Splanchnotomes, or lateral plates (unsegmented part of the mesoderm), form a secondary cavity of the body - or coelom (celom Greek - cavity), as a result of which each splanchnotome (right and left) is divided into two leaves: 1) parietal, or parietal, leaf (paries , lat. - wall), which lines the wall of the body and is adjacent to the ectoderm (from the abdominal cavity), and the visceral, or visceral, leaf (viscera, lat. - viscera), which forms the serous membrane of the viscera. The coelom gives rise to the pericardial, pleural and peritoneal cavities.
Process cells are evicted from the embryonic coelomic lining of both layers, which fill all the spaces between the germ layers and embryonic rudiments in the body of the embryo and in its extra-embryonic parts. Together, they form a special embryonic rudiment that spreads throughout the body of the embryo and beyond, called mesenchyme.
Since at first the mesenchyme carries nutrients to various parts of the embryo, performing a trophic function, subsequently blood and hematopoietic tissues, lymph, blood vessels, lymph nodes, and the spleen develop from it.
In addition to the previously noted derivatives of sclerotomes and skin plates, mesenchyme also produces: a) fibrous connective tissues, differing in the nature and quantity of intercellular substance and cells (ligaments, joint capsules, tendons, fascia, etc.); b) cartilage and bones, smooth muscles.
5. Organ development(organogenesis) and tissues (histogenesis). Organogenesis is the anatomical formation of organs. It will be described when presenting the anatomy of individual systems. 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 embryo at the end of the second month of intrauterine development has a disproportionately large head (due to the powerful development of the brain): its pelvis and short lower limbs are disproportionately small. At the 5th month of development, the head makes up 1/3, and at the 10th month, 1/4 of the total body length of the fetus.
Growth rates in the prenatal period are incomparably greater than after birth. If we compare the masses of the zygote, the body of a newborn and an adult, it turns out that a newborn child is 32,000,000 times larger than the zygote, and the body of an adult is only 20-25 times the weight of a newborn. And at the same time, it should also be taken into account that 9 months pass from conception to birth, and about 20 years, if not more, from birth to maturity.
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 necessary at the moment for the further development of the embryo ( the 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 (Fig. 2, 3): 1) chorion, 2) amnion, 3) allantois and 4) yolk sac.
The chorion forms the outer membrane of the fetus and surrounds it along with the amniotic and yolk sacs.
In the human placenta, chorionic villi grow into wide blood vessels - lacunae, located in the mucous membrane of the uterus. Such a placenta is called hemochorial (haima, Greek - blood), which emphasizes the hemotrophic nature of the human placenta. The placenta is connected to the fetus by the umbilical cord, which contains the umbilical (placental) vessels through which blood flows from the placenta to the fetal body and back.
Humans and mammals that have a placenta are united on this basis into the subclass placentalia, in contrast to lower viviparous animals (marsupials, monotremes) that do not have a placenta and make up the group aplacentalia.
Amnion (amnion, Greek - bowl) - 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 amnion is present in all higher vertebrates. On this basis they are united in the group amniota; accordingly, lower vertebrates constitute the anamnia group (i.e., animals that do not form an amnion).
Amniotic fluid is involved in metabolism, protects the fetus from adverse mechanical influences and contributes to the correct course of the birth act.
Allantois, or urinary sac, resembling a sausage in shape, hence the name (alias, will give birth, allantos, Greek - sausage), plays a role in higher vertebrates and in humans. important role. It is associated with the function of excretion; it “accumulates metabolic products - uric acid salts (from which it gets its name, the urinary sac).
In humans, the endodermal anlage of this extraembryonic organ is reduced, but in the extraembryonic mesenchyme surrounding the reduced anlage, blood vessels develop powerfully, which then turn into the vessels of the umbilical cord. The allantoic circulatory system, which is later in phylogenetic origin, provides the embryo with the ability to metabolize substances, and this is the new meaning acquired by the allantois.
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. Thanks 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 intrauterine period lasts from the very moment of conception until birth. It is divided into two phases: the embryonic period (the first two months) and the fetal period (from 3 to 9 months). In humans, the entire intrauterine period is about 280 days, in which the developing organism in the first 2 months of intrauterine life is called a fetus (embryo). From the 3rd month it is called a fetus.
Beginning of embryo development
Fertilization - the fusion of an egg with a sperm - occurs within 12 hours after ovulation. Only one sperm in a million penetrates the egg, after which a strong membrane immediately forms on the surface of the egg, preventing other sperm from entering. As a result of the close fusion of two nuclei with the correct set of chromosomes, a biploid zygote is formed - a cell that is a single-celled organism of a new generation of daughters.
Already on the first day after fertilization, the very first period of embryonic development begins - fragmentation. This process occurs inside the oviduct and ends on the 4th day. All this time, the embryo is nourished by yolk reserves in the egg itself. After crushing, a microscopic multicellular embryo with an open cavity inside is formed, which after 5 days reaches the uterus and is fixed in it.
Development of the embryo in the uterine cavity
On the 5-7th day, the embryo implants into the uterine mucosa, thanks to special enzymes that destroy it. This process lasts 48 hours. The hormone chorionic gonadotropin begins to be produced in the outer layer of the embryo. It sends a signal to the mother's body that pregnancy has occurred. At the same time, on the 7th day, germ layers are formed (the process of gastrulation), and germinal membranes are also formed, providing the necessary conditions for the further development of the embryo.
On the 14-15th day, contact is established between the outer villi of the developing membrane of the embryo and the blood vessels of the mother. In this case, nutrition and oxygen supply to the embryo is carried out directly from the maternal blood (at this point the egg’s own supply of nutrition is depleted). The umbilical cord and placenta begin to form (3rd week), which for the next 9 months will provide the child with oxygen, nutrition and remove by-products that are unnecessary for his body. This is followed by differentiation of the embryonic layers and the process of organogenesis - the spinal notochord and primary blood vessels begin to form. The 21st day is formed and the heart even begins to beat! The formation of the brain and spinal cord begins.
At the 4th week, the eye sockets become visible, and the rudiments of future arms and legs appear. Externally, the embryo resembles a small auricle, surrounded by a small volume of amniotic fluid. The formation of internal organs begins: liver, intestines, kidneys and urinary tract. The heart and brain improve their development. The growth of the embryo by the end of the initial month is 4 mm. By the 35th day, the nose and upper lip are formed. If normal development during this period is disrupted, then the rudiments may not grow together properly, and the child will be born with the so-called “cleft lip.”
At the 6th week, the arms and legs lengthen, but there are no fingers on them yet. The most important organ of the child’s immune system, the thymus gland or thymus, has already been formed. It has the largest size of all the endocrine glands combined. Its role has not yet been thoroughly clarified, but we can confidently note the extreme importance of this thymus for the further development of the fetus. Most likely, the thymus gland independently carries out immunological surveillance of the development of the child’s cells or takes an active part in this process.
At the 7th week, the structure of the small heart continues to improve: cardiac septa and main large vessels are formed, the heart already becomes four-chambered. Bile ducts appear inside the liver, and endocrine glands are rapidly developing. The brain develops, the ears take shape, and fingers appear at the ends of the limbs.
At the 8th week, the genital organs of the embryo are formed. Now, due to the influence of genes on the Y chromosome, boys begin to form male gonads or testes and they begin to produce testosterone. In girls, the external genitalia have not yet changed. The growth of the embryo by the end of the second month is about 3 cm.
During the first week of human embryonic development, fertilization, division of the zygote, formation of the morula and blastula, the first stage of gastrulation (delamination), formation of the epiblast and hypoblast, and implantation begin.
Fertilization
Fertilization is the fusion of male and female reproductive cells to form a single-celled embryo - a zygote. In humans – monospermic type fertilization: only one sperm can penetrate the egg (more precisely, oocyte II). Optimal time for fertilization - first 24 hours after ovulation(although the egg may retain the ability to fertilize for some time). Fertilization occurs normally Vampullary part of the fallopian tube.
There are several phases in the fertilization process:
1. Remote interaction
2. Contact interaction
3. Penetration of the head and neck of the sperm into the ooplasm.
Splitting up
During the first four days, fragmentation occurs.
Cleavage is the sequential division of the zygote without the growth of the resulting cells - blastomeres.
Crushing occurs in lumen of the oviduct, and towards the end the embryo reaches (moving along the oviduct) uterine cavity.
IN HUMANS, CRUSHING IS COMPLETE, UNEVEN, ASYNCHRONOUS.
During the cleavage process, small cells divide faster than large ones. As a result, small cells become overgrown with large ones on the outside. Therefore, the resulting cell mass, the morula, consists of two groups of cells. There are large cells inside. Their totality is called embryoblast. On the outside are small cells called trophoblast.
This blastula is called a blastocyst. It consists of:
1) trophoblast, which forms the wall of the blastula; consists of small light cells (subsequently, an extraembryonic organ, the chorion, develops from the trophoblast).
2) embryoblast cells located inside;
3) the cavity of the blastula (blastocoel), filled with fluid.
As free blastocyst the embryo is located in the uterine cavity for about 2 days – from the 5th to the 7th day. Due to the absorption of fluid from the uterine cavity by the trophoblast, the volume of the vesicle increases slightly. In the blastomeres themselves, synthetic processes are increasingly activated.
Implantation
Implantation is the introduction of an embryo into the thickness of the endometrium (uterine mucosa). It begins on the 7th day and lasts 40 hours. The secretory phase takes place in the uterus at this time. menstrual cycle. The usual site of implantation is the upper part of the uterus, the anterior or posterior wall.
There are 2 stages in implantation:
- adhesion (sticking)– the embryo attaches to the endometrium with the help of trophoblast
- invasion (penetration)– is the main one in duration.
First phase of gastrulation
Gastrulation is the process of formation of germ layers. Gastrulation in humans occurs in two stages (Table 3). The first stage occurs through delamination (splitting), and the second through migration.
- First phase is being done on the 7th day– simultaneously with implantation. During the first stage, two germ layers (ecto- and endoderm), two provisional organs (amnion and yolk sac) are formed. In addition, immediately before the start of the first stage, the formation of such a provisional organ as the chorion occurs. The formation of the chorion is the second stage in the formation of the placenta.
There are 4 periods in human embryogenesis.
1) Initial (1 week of development, until the implantation of the embryo into the uterine mucosa).
2) Embryonic (2-8 weeks).
3) Pre-fetal (9-12 weeks).
4) Fetal (13th week – birth).
During the embryonic period, blastulation, gastrulation, and neurulation occur. Intense organogenesis and anatomical formation of organs take place in the prefetal stage. The fetal period is characterized by the creation of a fetus under the protection of the membranes.
The fragmentation of the zygote is characterized by the following features. The plane of the first division passes through the poles. In this case, one of the blastomeres turns out to be larger than the other, which indicates uneven division. The first two blastomeres enter the next division asynchronously. The furrow runs along the meridian and perpendicular to the first furrow. Thus, the stage of three blastomeres arises. During division of the smaller blastomere, the pair of resulting smaller blastomeres rotates by 90° so that the plane of the division furrow is perpendicular to the first two furrows. Thanks to asynchronous cleavage, there can be stages with an odd number of blastomeres - 5, 7, 9.
As a result of fragmentation, an accumulation of blastomeres is formed - morula. At approximately the stage of 58 blastomeres, fluid appears inside the morula, a cavity (blastocoel) is formed and the embryo turns into a blastocyst.
IN blastocyst distinguish between the outer layer of cells (trophoblast) and the inner cell mass (germinal nodule, or embryoblast). Later from trophoblast the outer fruit membrane, the chorion, will develop, and from embryoblast– the embryo itself and some extra-embryonic organs.
Approximately on the 6th – 7th day after fertilization, the embryo is ready for implantation, that is, for immersion into its mucous membrane. The radiant shell is destroyed. Having come into contact with maternal tissues, trophoblast cells quickly multiply and destroy the uterine mucosa. They form two layers: the inner - cytotrophoblast and the outer - syncytotrophoblast.
At 2 weeks extraembryonic parts grow, i.e. those parts that are formed by the embryo, but first play an auxiliary role - amnion, chorion, yolk sac. These are provisional organs - coenogenetic structures that do not take part in the formation of an adult organism. The cellular material from which the embryo develops is the embryonic shield. In the early stages, preparatory work is underway; it is not the embryo itself that develops, but parts that create the necessary conditions for the existence of the embryo and provide the functions of respiration, nutrition, excretion of metabolic products, creating a liquid environment around the embryo to protect it.
3 week– the placenta is formed. Consists of 2 parts - embryonic and maternal. Germinal – trophoblast and some other tissues (chorion – Greek “shell, afterbirth”). Maternal - highly modified uterine mucosa. In it, blood vessels are destroyed, connective tissue is loosened, and the epithelium is destroyed. The chorionic villi “bathe” in maternal blood. The area of the placental plexus is 5 square meters, and the total length of the chorionic villi is 5 km. The maternal and embryonic organisms do not have a common blood flow, the blood does not mix. Nutrients pass through the walls of the chorion. In a 3-week-old embryo, umbilical vessels appear, growing into the walls of the chorion and performing functions. Food.
4 week. The dimensions of the embryo together with the chorion are 5-7 mm. A new stage begins. The body of the embryo is separated from the extraembryonic parts. The embryo rises above the amniotic fluid, with which it is then connected only by the umbilical vessels. During embryonic development, a yolk sac appears early in humans - the first hematopoietic organ that stores and processes yolk, the first organ of respiration and nutrition. Primary germ cells begin to form in the yolk sac. There is an intestine that is blindly closed on both sides. The liver is a hematopoietic organ. The heart is beating. By the end of 4 weeks there is a rudiment of the respiratory system. Sizes up to 30mm.
The intestines grow in length, do not fit in the straightened state and begin to bend. By the end of 4 weeks, shoulder blades appear on the sides. Nerves and muscles grow into them - future arms and legs. By the end of the week, there is differentiation into parts; by the 5th week, sections of the embryo protrude on the sides of the back of the head and neck - 4 pairs of gill slits are formed, parts of the foregut protrude from the inside, forming 4 gill pouches. There is no connection between the gill slits and the gill pouches. The middle ear is formed from 1 pair of gill slits. The rest are the thyroid and thymus glands.
From 4 weeks the nervous system begins to form. Formation of the neural tube (neural plate - neural groove - neural tube). At the anterior end of the neural plate, 3 brain vesicles appear; at week 6, there are already 5 brain vesicles that correspond to parts of the brain; auditory vesicles, optic cups, and olfactory pits appear. Mesoderm differentiation occurs. A tail is formed (day 34) up to 10 mm.
At 2 months The primary sex glands are formed, where the primary sex cells migrate from the yolk sac.
week 7– formation of dental plates.
At 8 weeks rapid development of the amniotic membrane and accumulation of fluid occurs.
9-10week– kidney formation, nephrons are formed throughout embryogenesis and another 20 days after birth.
Start 3 months. The fruit is formed. Within a month, the tail disappears (cell death under the influence of lysosomal enzymes), leaving rudimentary vertebrae. The head is ahead of the body in development, then the proportions are restored.
Start of 4 months. Sizes 20-22cm. the muscular system is formed and begins to move.
5 month. The entire body is covered with hair. The upper limbs grow faster than the lower ones and appear earlier.
6. Periods of fragmentation, implantation, formation of extra-embryonic parts (provisional organs) and their functions.
In the initial period, there is a zygote - 1 cell of the embryo, in which individual sections of the cytoplasm are determined, DNA and proteins are synthesized. The zygote has a bisimitric structure. Gradually, the relationship between the nucleus and the cytoplasm is disrupted, resulting in stimulation of the process of division - crushing
The cleavage stage is a period of intense cell division. The size of the embryo does not increase, and synthetic processes are active. Intensive synthesis of DNA, RNA, histone and other proteins occurs. Crushing performs the following functions:
A sufficient number of cells necessary for the formation of tissues and organs are formed.
Redistribution of yolk and cytoplasm between daughter cells. The 1st and 2nd fission furrows run along the meridian, and the 3rd along the equator. Closer to the animal pole.
The plan of the embryo is determined - the dorsal-ventral axis, the anterior-posterior axis.
Nuclear-cytoplasmic relations are normalized. The number of nuclei increases, but the volume and mass remain the same. Gradually, division slows down.
Provisional authorities
Chorion arises from the trophoblast, which has already divided into cytotrophoblast and syncytotrophoblast. By the end of the 2nd week, primary chorionic villi are formed in the form of an accumulation of epithelial cytotrophoblast cells. At the beginning of the 3rd week, mesodermal mesenchyme grows into them, and secondary villi appear, and when, by the end of the 3rd week, blood vessels appear inside the connective tissue core, they are called tertiary villi. The area where the tissues of the chorion and the uterine mucosa are closely adjacent is called placenta. From the very beginning to the end, the fetal blood is isolated from the maternal blood by the placental barrier.
Placental barrier consists of trophoblast, connective tissue and fetal vascular endothelium. It is permeable to water, electrolytes, nutrients and dissimilation products.
Amnion occurs through the divergence of epiblast cells of the inner cell mass. The human amnion is called schizamnion in contrast to the pleuramnion of birds. The amniotic cavity is lined with epiblastic cells. Outside, the amniotic ectoderm is surrounded by extraembryonic mesodermal cells.
Yolk sac appears when a thin layer of hypoblast separates from the inner cell mass and its extraembryonic endodermal cells, moving, line the surface of the trophoblast from the inside.
The allantois arises in the human embryo, as in other amniotes, in the form of a pocket in the ventral wall of the hindgut, but its endodermal cavity remains a vestigial structure. Nevertheless, an abundant network of vessels develops in its walls, connecting with the main blood vessels of the embryo.