Embryogenesis stages briefly. Embryonic period of development

Embryogenesis (Greek embryon - embryo, genesis - development) is the early period of individual development of the organism from the moment of fertilization (conception) to birth, is the initial stage of ontogenesis (Greek ontos - being, genesis - development), the process of individual development of the organism from conception to of death.

The development of any organism begins as a result of the fusion of two sex cells (gametes), male and female. All cells of the body, despite differences in structure and functions, are united by one thing - a single genetic information stored in the nucleus of each cell, a single double set of chromosomes (except for highly specialized blood cells - red blood cells, which do not have a nucleus). That is, all somatic (soma - body) cells are diploid and contain a double set of chromosomes - 2 n, and only sex cells (gametes) formed in specialized sex glands (testes and ovaries) contain a single set of chromosomes - 1 n.

When germ cells merge, a cell is formed - a zygote, in which a double set of chromosomes is restored. Recall that the nucleus of a human cell contains 46 chromosomes, respectively, sex cells have 23 chromosomes

The resulting zygote begins to divide. The first stage of zygote division is called cleavage, as a result of which the multicellular structure of the morula (mulberry) is formed. The cytoplasm is distributed unevenly between the cells; the cells of the lower half of the morula are larger than the upper half. The volume of the morula is comparable to that of the zygote.

At the second stage of division, as a result of cell redistribution, a single-layer embryo is formed - a blastula, consisting of one layer of cells and a cavity (blastocoel). Blastula cells vary in size.

At stage III, the cells of the lower pole seem to invaginate (invaginate) inward, and a two-layer embryo is formed - the gastrula, consisting of an outer layer of cells - ectoderm and an inner layer of cells - endoderm.

Very soon, between the I and II layers of cells, as a result of cell division, another layer of cells is formed, the middle one is the mesoderm, and the embryo becomes three-layered. This completes the gastrula stage.

From these three layers of cells (they are called germinal layers) the tissues and organs of the future organism are formed. The integumentary and nervous tissue develops from the ectoderm, the skeleton, muscles, circulatory system, genitals, excretory organs from the mesoderm, and the respiratory and nutritional organs, liver, and pancreas from the endoderm. Many organs are formed from several germ layers.

Embryogenesis includes the processes from fertilization to birth.

The development of the human body begins after fertilization of the female reproductive cell - the egg (ovium) of the male - by a spermatozoon (spermatozoon, spermium).

The detailed study of the development of the human embryo (embryo) is the subject of embryology. Here we will limit ourselves to only a general overview of the development of the embryo (embryogenesis), which is necessary for understanding the human physique.

The embryogenesis of all vertebrates, including humans, can be divided into three periods.

1. Crushing: a fertilized egg, spermovium, or zygote is successively divided into cells (2,4,8,16 and so on) as a result of which a dense multicellular ball, morula, is first formed, and then a single-layer vesicle - a blastula, which contains a primary cavity, blastocoel. The duration of this period is 7 days.

2. Gastrulation consists of the transformation of a single-layer embryo into a two-layer, and later three-layer – gastrula. The first two layers of cells are called germ layers: the outer ectoderm and the inner endoderm (up to two weeks after fertilization), and the third, middle layer that appears later between them is called the middle germ layer - mesoderm. The second important result of gastrulation in all chordates is the emergence of an axial complex of rudiments: on the dorsal (dorsal) side of the endoderm, the rudiment of the dorsal string, notochord, appears, and on its ventral (ventral) side - the rudiment of the intestinal endoderm; on the dorsal side of the embryo, along its midline, a neural plate stands out from the ectoderm - the rudiment of the nervous system, and the rest of the ectoderm goes to build the epidermis of the skin and is therefore called cutaneous ectoderm.

Subsequently, the embryo grows in length and turns into a cylindrical formation with a head (cranial) and caudal caudal ends. This period lasts until the end of the third week after fertilization.

3. Organogenesis and histogenesis: the neural plate sinks under the ectoderm and turns into a neural tube, which consists of separate segments - neurotomes - and gives rise to the development of the nervous system. The mesodermal primordia are detached from the endoderm of the primary intestine and form a paired row of metamerically located sacs, which, growing on the sides of the body of the embryo, are each divided into two sections: the dorsal, which lies on the sides of the notochord and neural tube, and the ventral, which lies on the sides of the embryo. intestines. The dorsal sections of the mesoderm form the primary segments of the body - somites, each of which in turn is divided into a sclerotome, which gives rise to the skeleton, and a myotome, from which muscles develop. A skin segment, the dermatome, is also distinguished from the somite (on its lateral side). The abdominal sections of the mesoderm, called splanchnotomes, form paired sacs that contain the secondary body cavity.

The intestinal endoderm, which remains after the separation of the notochord and mesoderm, forms the secondary gut - the basis for the development of internal organs. Subsequently, all the organs of the body are laid down, the material for the construction of which is the three germ layers.

1. From the outer germ layer, ectoderm, develop:

a) epidermis of the skin and its derivatives (hair, nails, skin glands);

b) epithelium of the mucous membrane of the nose, mouth and anus;

c) nervous system and epithelium of sensory organs.

2. From the internal germ layer, the endoderm, the mucosal epithelium of most of the digestive tract develops with all the glandular structures belonging here, most of the respiratory organs, as well as the epithelium of the thyroid and thymus glands.

3. From the middle germ layer, mesoderm, the musculature of the skeleton, the mesothelium of the membranes of the serous cavities with the rudiments of the gonads and kidneys develop.

In addition, from the dorsal segments of the mesoderm, embryonic connective tissue, mesenchyme, arises, which gives rise to all types of connective tissue, including cartilage and bone. Since at first the mesenchyme carries nutrients to different parts of the embryo, performing a trophic function, then later blood, lymph, blood vessels, lymph nodes, and the spleen develop from it.

In addition to the development of the embryo itself, it is also necessary to take into account the formation of extra-embryonic parts, with the help of which the embryo receives the nutrients necessary for its life.

In the multicellular dense ball, there is an internal embryonic nodule, the embryoblast, and an outer layer of cells, which plays an important role in the nutrition of the embryo and is therefore called the trophoblast. With the help of trophoblast, the embryo penetrates into the thickness of the uterine mucosa (implantation), and here the formation of a special organ begins, with the help of which the embryo is connected with the mother’s body and is nourished. This organ is called the baby's place, litter, or placenta. Mammals that have a placenta are called placentals. Along with the formation of the placenta, there is a process of separation of the developing embryo from the extra-embryonic parts as a result of the appearance of the so-called trunk fold, which, protruding with a ridge towards the middle, seems to lace the body of the embryo from the extra-embryonic parts with a ring. At the same time, however, the connection to the placenta is maintained through the umbilical stalk, which then turns into the umbilical cord. In the early stages of development, the vitelline duct passes through the latter, which connects the intestine with its protrusion into the extraembryonic area, the yolk sac. In vertebrates that do not have a placenta, the yolk sac contains the nutritional material of the egg - the yolk - and is an important organ through which the embryo is nourished.

In humans, although the yolk sac appears, it does not play a significant role in the development of the embryo and, after absorption of its contents, is gradually reduced. The umbilical cord also contains umbilical (placental) vessels, through which blood flows from the placenta to the body of the fetus and back. They develop from the mesoderm of the urinary sac, or allantois, which protrudes from the ventral wall of the intestine and exits the body of the embryo through the umbilical opening into the extraembryonic part. In humans, from the part of the allantois, which is contained in the middle of the body of the embryo, part of the bladder is formed, and from its vessels the umbilical blood vessels are formed. The developing embryo is covered with two germinal membranes. The inner membrane, the amnion, forms a voluminous sac, which is filled with protein fluid and forms a liquid environment for the embryo, through which the sac is called the aqueous membrane. The entire embryo, along with the amniotic and yolk sacs, is surrounded by an outer membrane (which also includes the trophoblast). This membrane, having villi, is called villous, or chorion. The chorion performs trophic, respiratory, excretory and barrier functions.

13. Unlike mosses, in ferns, horsetails and mosses, the development cycle is dominated by the sporophyte, a leafy plant. Representatives of these three groups of plants have leaves, stems and roots. Most of them have underground rhizomes with modified leaves and adventitious roots. Modern horsetails, club mosses and ferns are mostly herbaceous plants. Only in the tropics and subtropics are there tree ferns. However, in ancient times - 200-350 million years ago, these groups of plants were represented by tree-like forms and constituted dense forests, which gave rise to the largest stone

coal deposits of the world (Donbass, Kuzbass, etc.).

What are the structural features of mosses, horsetails and ferns?

Let's consider the features of mosses, horsetails and ferns. Modern lycophytes are perennial, usually evergreen grasses. The most famous representative of the club mosses is the common club moss, common in central Russia in damp spruce and

pine forests. This is a plant with a flexible, branched stem that spreads along the ground. The leaves are small, arranged in a spiral on the stem. At the end of summer, two spore-bearing spikelets usually appear on the side branches. Each spikelet is formed by small thin modified leaves called sporophylls. At the base of the sporophylls there are sporangia, where spores are formed. Horsetails, or horsetails, are easily distinguished by the segmented structure of their stems: they have a pronounced alternation of nodes and internodes. The leaves on the stem are arranged in whorls (several pieces per node), surrounding the stem. At the tops of the stems, spore-bearing spikelets are formed, in which spores ripen. In some species, for example, horsetail, the stems are of two types: spore-bearing (brownish pink, develop in the spring and die after sporulation) and vegetative (appear in the summer from the same rhizome). Ferns are represented in nature by perennial grasses, vines, trees and epiphytes that settle on tree trunks. Ferns have large leaves; young ones are usually curled up like a snail. Our country's ferns have rhizomes. Their sporangia are located on the underside of the leaf and are collected in groups - they are called sori. In extinct ferns, the sporangia were solitary. On the territory of our country there are common bracken ferns, male shield ferns, female stomatid ferns, common millipedes and other species.

What is the peculiarity of the development of ferns, horsetails and mosses?

Reproduction in all three groups of higher spore plants occurs according to the same pattern. Let's look at it using the example of a fern. Sporangia with spores develop on the underside of the leaf of an adult plant. Finding itself in favorable conditions, the spore germinates and gives rise to a gametophyte. It looks like a small plate with rhizoids and is called a prothallus. Male and female gametangia with

sex cells - eggs and sperm. After fertilization, which occurs in the presence of water, the zygote first develops into an embryo, and then into an adult plant - a sporophyte. Thus, in mosses, horsetails and ferns, there is an alternation of sexual generation (thallus - gametophyte) and asexual generation (adult

leafy plant - sporophyte).

14. Post-embryonic development: direct and indirect. Reasons for the weakening of competition between parents and offspring during indirect development.

1. Individual development of an organism (ontogenesis) - a period of life that, during sexual reproduction, begins with the formation of a zygote, is characterized by irreversible changes (an increase in mass, size, the appearance of new tissues and organs) and ends with death.

2. Germinal (embryonic) and post-embryonic (postembryonic) periods of individual development of the organism.

3. Post-embryonic development (replaces embryonic) - the period from birth or the emergence of the embryo from the egg until death. Various ways of post-embryonic development of animals - direct and indirect:

1) direct development - the birth of offspring that are externally similar to an adult organism. Examples: the development of fish, reptiles, birds, mammals, and some types of insects. Thus, a young fish resembles an adult fish, a duckling resembles a duck, a kitten resembles a cat;

2) indirect development - the birth or emergence from an egg of offspring that differ from the adult organism in morphological characteristics, lifestyle (type of nutrition, nature of movement). Example: worm-like larvae emerge from the eggs of the May beetle, live in the soil and feed on roots, unlike an adult beetle (lives on a tree, feeds on leaves).

Stages of indirect development of insects: egg, larva, pupa, adult. Features of the life of animals at the egg and pupal stages - they are motionless. Active lifestyle of the larva and adult organism, different living conditions, use of different food.

4. The significance of indirect development is the weakening of competition between parents and offspring, since they eat different foods and have different habitats. Indirect development is an important adaptation that arose during the process of evolution. It helps to weaken the struggle for existence between parents and offspring, and the survival of animals in the early stages of post-embryonic development.

General characteristics. The first gymnosperms appeared at the end of the Devonian period about 350 million years ago; they probably descended from ancient pteridophytes that became extinct at the beginning of the Carboniferous period. In the Mesozoic era - the era of mountain building, continental uplift and climate drying - gymnosperms reached their peak, but from the middle of the Cretaceous period they lost their dominant position to angiosperms.

The department of modern gymnosperms includes more than 700 species. Despite the relatively small number of species, gymnosperms have conquered almost the entire globe. In the temperate latitudes of the Northern Hemisphere, they form coniferous forests called taiga over vast areas.

Modern gymnosperms are represented mainly by trees, much less often by shrubs and very rarely by lianas; There are no herbaceous plants among them. The leaves of gymnosperms differ significantly from other groups of plants not only in shape and size, but also in morphology and anatomy. In most species they are needle-shaped (needles) or scale-like; in some representatives they are large (for example, in the amazing Velvichia their length reaches 2-3 m), pinnately dissected, bilobed, etc. The leaves are arranged singly, two or several in bunches.

The vast majority of gymnosperms are evergreen, monoecious or dioecious plants with a well-developed stem and root system formed by main and lateral roots. They spread by seeds, which are formed from ovules. The ovules are glabrous (hence the name of the department), located on megasporophylls or on seed scales collected in female cones.

In the development cycle of gymnosperms, there is a successive change of two generations - the sporophyte and the gametophyte with the dominance of the sporophyte. Gametophytes are greatly reduced, and male gametophytes of holo- and angiosperms do not have antheridia, which sharply differs from all heterosporous seedless plants.

Gymnosperms include six classes, two of which have completely disappeared, and the rest are represented by living plants. The best preserved and most numerous group of gymnosperms is the class Conifers, numbering at least 560 species, forming forests over vast areas of Northern Eurasia and North America. The largest number of species of pine, spruce, and larch are found along the coasts of the Pacific Ocean.

Class Conifers. All conifers are evergreen, less often deciduous (for example, larch) trees or shrubs with needle-like or scale-like (for example, cypress) leaves. The needle-shaped leaves (needles) are dense, leathery and hard, covered with a thick layer of cuticle. The stomata are immersed in depressions filled with wax. All these structural features of the leaves ensure that conifers are well adapted to growing in both arid and cold habitats.

Conifers have erect trunks covered with scaly bark. In a cross section of the stem, developed wood and less developed bark and pith are clearly visible. The xylem of conifers is 90-95% formed by tracheids. Conifer cones are dioecious; plants are more often monoecious, less often dioecious.

The most widespread representatives of conifers in Belarus and Russia are Scots pine and Norway spruce, or Norway spruce. Their structure, reproduction, and alternation of generations in the development cycle reflect the characteristic features of all conifers.

Scots pine is a monoecious plant (Fig. 9.3). In May, bunches of greenish-yellow male cones, 4-6 mm long and 3-4 mm in diameter, form at the base of young pine shoots. On the axis of such a cone there are multilayer scaly leaves, or microsporophylls. On the lower surface of the microsporophylls there are two microsporangia - pollen sacs in which pollen is formed. Each pollen grain is equipped with two air sacs, which facilitates the transfer of pollen by wind. The pollen grain contains two cells, one of which subsequently, when it hits the ovule, forms a pollen tube, the other, after division, forms two sperm.

On other shoots of the same plant, female cones of a reddish color are formed. On their main axis there are small transparent covering scales, in the axils of which there are large, thick, subsequently lignified scales. On the upper side of these scales there are two ovules, in each of which a female gametophyte develops - an endosperm with two archegonia with a large egg in each of them. At the apex of the ovule, protected from the outside by the integument, there is an opening - the pollen passage, or micropyle.

In late spring or early summer, ripe pollen is carried by the wind and lands on the ovule. Through the micropyle, pollen is drawn into the ovule, where it grows into a pollen tube, which penetrates the archegonia. The two sperm cells formed by this time travel through the pollen tube to the archegonia. Then one of the sperm fuses with the egg, and the other dies. A seed embryo is formed from a fertilized egg (zygote), and the ovule turns into a seed. Pine seeds ripen in the second year, fall out of the cones and, picked up by animals or the wind, are transported over considerable distances.

In terms of their importance in the biosphere and role in human economic activity, conifers occupy second place after angiosperms, far surpassing all other groups of higher plants.

They help solve enormous water conservation and landscape problems, serve as an important source of wood, raw materials for the production of rosin, turpentine, alcohol, balms, essential oils for the perfume industry, medicinal and other valuable substances. Some conifers are cultivated as ornamental trees (fir, thuja, cypress, cedar, etc.). The seeds of a number of pine trees (Siberian, Korean, Italian) are used as food and oil is also obtained from them.

Representatives of other classes of gymnosperms (cycads, cycads, ginkgos) are much less common and less known than conifers. However, almost all types of cycads are decorative and are widely popular among gardeners in many countries. Evergreen leafless low shrubs of ephedra (Gnetaceae class) serve as a source of raw materials for the production of the alkaloid ephedrine, which is used as a central nervous system stimulant, as well as in the treatment of allergic diseases.

16. Radial symmetry- a form of symmetry in which a body (or figure) coincides with itself when the object rotates around a certain point or line. Often this point coincides with the center of symmetry of the object, that is, the point at which an infinite number of axes of bilateral symmetry intersect. Geometric objects such as a circle, ball, cylinder or cone have radial symmetry.

In biology, radial symmetry is said to occur when one or more axes of symmetry pass through a three-dimensional being. Moreover, radially symmetrical animals may not have planes of symmetry. Thus, the Velella siphonophore has a second-order symmetry axis and no planes of symmetry

Usually two or more planes of symmetry pass through the axis of symmetry. These planes intersect along a straight line - the axis of symmetry. If the animal rotates around this axis by a certain degree, then it will be displayed on itself (coincide with itself).

There can be several such axes of symmetry (polyaxon symmetry) or one (monaxon symmetry). Polyaxonal symmetry is common among protists (e.g. radiolarians).

As a rule, in multicellular animals, the two ends (poles) of a single axis of symmetry are unequal (for example, in jellyfish, the mouth is located on one pole (oral), and the tip of the bell is on the opposite (aboral) pole. Such symmetry (a variant of radial symmetry) in comparative anatomy is called uniaxial-heteropole. In a two-dimensional projection, radial symmetry can be preserved if the axis of symmetry is directed perpendicular to the projection plane. In other words, the preservation of radial symmetry depends on the viewing angle.

Radial symmetry is characteristic of many cnidarians, as well as most echinoderms. Among them there is the so-called pentasymmetry, based on five planes of symmetry. In echinoderms, radial symmetry is secondary: their larvae are bilaterally symmetrical, and in adult animals, external radial symmetry is broken by the presence of a madrepore plate.

In addition to typical radial symmetry, there is biradial radial symmetry (two planes of symmetry, for example, in ctenophores). If there is only one plane of symmetry, then the symmetry is bilateral (animals from the Bilateria group have such symmetry).

In flowering plants, radially symmetrical flowers are often found: 3 planes of symmetry (frogwort), 4 planes of symmetry (cinquefoil erect), 5 planes of symmetry (bellflower), 6 planes of symmetry (colchicum). Flowers with radial symmetry are called actinomorphic, flowers with bilateral symmetry are called zygomorphic.

Bilateral symmetry(bilateral symmetry) - mirror reflection symmetry, in which an object has one plane of symmetry, relative to which its two halves are mirror symmetrical. If we lower a perpendicular from point A to the plane of symmetry and then extend it from point O on the plane of symmetry to the length AO, then it will end up at point A1, which is similar in all respects to point A. There is no axis of symmetry for bilaterally symmetrical objects. In animals, bilateral symmetry is manifested in the similarity or almost complete identity of the left and right halves of the body. At the same time, there are always random deviations from symmetry (for example, differences in papillary lines, branching of blood vessels and the location of moles on the right and left hands of a person). There are often small but natural differences in the external structure (for example, more developed muscles of the right arm in right-handed people) and more significant differences between the right and left half of the body in the location of the internal organs. For example, the heart in mammals is usually placed asymmetrically, with a shift to the left.

In animals, the appearance of bilateral symmetry in evolution is associated with crawling along the substrate (along the bottom of a reservoir), due to which the dorsal and ventral, as well as the right and left halves of the body appear. In general, among animals, bilateral symmetry is more pronounced in actively mobile forms than in sessile ones. Bilateral symmetry is characteristic of all fairly highly organized animals, except echinoderms. In other kingdoms of living organisms, bilateral symmetry is characteristic of a smaller number of forms. Among protists, it is characteristic of diplomonads (for example, Giardia), some forms of trypanosomes, bodonids, and the shells of many foraminifera. In plants, it is usually not the entire organism that has bilateral symmetry, but its individual parts - leaves or flowers. Botanists call bilaterally symmetrical flowers zygomorphic.

17. Angiosperms (flowering, pistillate) In terms of time of appearance on Earth, they are the youngest and at the same time the most highly organized group of plants. In the process of evolution, representatives of this department appeared later than others, but they very quickly took a dominant position on the globe.

The most characteristic distinctive feature of angiosperms is the presence of a peculiar organ - a flower, which is absent in representatives of other plant divisions. This is why angiosperms are often called flowering plants. Their ovule is hidden, it develops inside the pistil, in its ovary, which is why angiosperms are also called pistillates. Pollen in angiosperms is captured not by ovules, as in gymnosperms, but by a special formation - the stigma, which ends at the pistil.

After fertilization of the egg, a seed is formed from the ovule, and the ovary grows into a fruit. Consequently, the seeds of angiosperms develop in fruits, which is why this division of plants is called angiosperms.

Angiosperms (Angiospermae), or flowering plants (Magnoliophyta) are a division of the most advanced higher plants that have flowers. Previously included in the department of seed plants along with gymnosperms. Unlike the latter, the ovules of flowering plants are enclosed in an ovary formed by fused carpels.

The flower is the generative organ of angiosperms. It consists of a peduncle and a receptacle. The latter contains the perianth (simple or double), androecium (collection of stamens) and gynoecium (collection of carpels). Each stamen consists of a thin filament and an expanded anther in which sperm mature. The carpel of flowering plants is represented by a pistil, which consists of a massive ovary and a long style, the apical expanded part of which is called the stigma.

Angiosperms have vegetative organs that provide mechanical support, transport, photosynthesis, gas exchange, and the storage of nutrients, and generative organs involved in sexual reproduction. The internal structure of tissues is the most complex of all plants; phloem sieve elements are surrounded by companion cells; Almost all representatives of angiosperms have xylem vessels.

The male gametes contained inside the pollen grains land on the stigma and germinate. Flowering gametophytes are extremely simplified and miniature, which significantly reduces the duration of the reproduction cycle. They are formed as a result of a minimum number of mitoses (three in the female gametophyte and two in the male). One of the features of sexual reproduction is double fertilization, when one of the sperm fuses with the egg, forming a zygote, and the second fuses with the polar nuclei, forming the endosperm, which serves as a supply of nutrients. The seeds of flowering plants are enclosed in the fruit (hence their second name - angiosperms).

18. DNA This phenomenon was discovered in experiments with pneumococci, that is, with bacteria that cause pneumonia. Two forms of pneumococci are known: A-form with a polysaccharide capsule and B-form without a capsule. Both of these traits are hereditary.

A-form pneumococci, when they infect mice, cause pneumonia, which kills the mice. The B-form is harmless to them.

In 1928, the English bacteriologist F. Griffiths infected mice with a mixture consisting of heat-killed A-form pneumococci and live B-form pneumococci. The scientist assumed that the mice would not get sick. But contrary to expectations, the experimental animals died. F. Griffiths managed to isolate pneumococci from the tissues of dead mice. All of them turned out to be encapsulated, that is, A-form. Consequently, the killed form somehow transferred its properties to the living cells of the B-form. But how? With the help of what substance: the polysaccharide that makes up the capsule, protein or DNA?

Much depended on the solution to this question, since by identifying the substance that transmits the hereditary trait - the formation of a capsule, it was possible to obtain the desired answer. However, this could not be done for quite some time. Only 16 years after the experiments of F. Griffiths, in 1944, the American scientist A. Avery and his colleagues, having carried out a series of clear experiments, were able to prove with full justification that the polysaccharide and protein have nothing to do with the transmission of the hereditary properties of A-form pneumococcus.

During these experiments, using a special enzyme, they dissolved the polysaccharide capsule of killed A-form pneumococci and checked whether the remains of form A cells continued to transmit hereditary information to cells of form B. It turned out that they did. It became clear that the polysaccharide as a source of genetic information was no longer needed.

Thus, by the method of exclusion it was established that the hereditary information in the cell is stored and transmitted by the DNA molecule. And indeed, when the DNA was destroyed, the formation of capsule forms A from non-capsule forms B stopped.

The phenomenon of transformation, that is, a hereditary change in the properties of one form of bacteria under the influence of substances of another form, was called transformation. The substance that causes transformation is called a transforming agent. They were found to be DNA.

Each protein is represented by one or more polypeptide chains. A section of DNA that carries information about one polypeptide chain is called a gene. Each DNA molecule contains many different genes. The totality of DNA molecules in a cell acts as a carrier of genetic information. Thanks to a unique property - the ability to duplicate, which no other known molecule has, DNA can be copied. When dividing, “copies” of DNA are dispersed into two daughter cells, each of which will therefore have the same information that was contained in the mother cell. Since genes are sections of DNA molecules, two cells formed during division have the same sets of genes. During sexual reproduction, each cell of a multicellular organism arises from a single fertilized egg as a result of multiple divisions. This means that a random error in the gene of one cell will be reproduced in the genes of millions of its descendants. This is why all the red blood cells of a patient with sickle cell anemia have equally degraded hemoglobin. The error occurred in the gene that carries information about the beta chain of the protein. A copy of the gene is mRNA. According to it, like a matrix, the wrong protein is “printed” thousands of times in each red blood cell. Children receive damaged genes from their parents through their reproductive cells. Genetic information is transmitted both from one cell to daughter cells and from parents to children. A gene is a unit of genetic, or hereditary, information.

Animal ontogeny

Comparison of vertebrate embryos at different stages of embryonic development. An infamous illustration from the work of Ernst Haeckel, in which the differences between embryos are artificially reduced in order to be more consistent with the theory of recapitulation (repetition of phylogeny in ontogeny). It should be noted that the falsification of this illustration does not negate the fact that embryos usually do appear to be more similar to each other than adult organisms, which was noted by embryologists even before the theory of evolution.

Ontogenesis is divided into two periods:

embryonic - from the formation of the zygote to birth or exit from the egg membranes;
postembryonic - from exit from the egg membranes or birth to the death of the organism.

Embryonic period

There are three main stages in the embryonic period: cleavage, gastrulation and primary organogenesis. Embryonic, or embryonic, the period of ontogenesis begins from the moment of fertilization and continues until the embryo emerges from the egg membranes. In most vertebrates it includes stages (phases) fragmentation, gastrulation, histo- and organogenesis.

Splitting up

Cleavage is a series of successive mitotic divisions of a fertilized or initiated egg. Cleavage represents the first period of embryonic development, which is present in the ontogenesis of all multicellular animals and leads to the formation of an embryo called a blastula (single-layer embryo). At the same time, the mass of the embryo and its volume do not change, that is, they remain the same as that of the zygote, and the egg is divided into smaller and smaller cells - blastomeres. After each cleavage division, the cells of the embryo become smaller and smaller, that is, the nuclear-plasma relationship changes: the nucleus remains the same, but the volume of the cytoplasm decreases. The process continues until these indicators reach values characteristic of somatic cells. The type of crushing depends on the amount of yolk and its location in the egg. If there is little yolk and it is evenly distributed in the cytoplasm (isolecithal eggs: echinoderms, flatworms, mammals), then crushing proceeds according to the type full uniform: blastomeres are identical in size, the entire egg is crushed. If the yolk is distributed unevenly (telolecithal eggs: amphibians), then crushing proceeds according to the type completely uneven: blastomeres are of different sizes, those containing the yolk are larger, the egg is crushed entirely. With incomplete crushing, there is so much yolk in the eggs that the crushing furrows cannot separate it entirely. The crushing of an egg, in which only the “cap” of cytoplasm concentrated at the animal pole, where the zygote nucleus is located, is crushed, is called incomplete discoidal(telolecithal eggs: reptiles, birds). At incomplete surface crushing in the depths of the yolk, the first synchronous nuclear divisions occur, not accompanied by the formation of intercellular boundaries. The nuclei, surrounded by a small amount of cytoplasm, are evenly distributed in the yolk. When there are enough of them, they migrate into the cytoplasm, where then, after the formation of intercellular boundaries, the blastoderm (centrolecithal eggs: insects) appears.

Gastrulation

One of the mechanisms of gastrulation is invagination (invagination of part of the blastula wall into the embryo) 1 - blastula, 2 - gastrula.

Primary organogenesis

Primary organogenesis is the process of formation of a complex of axial organs. In different groups of animals this process is characterized by its own characteristics. For example, in chordates, at this stage the formation of the neural tube, notochord and intestinal tube occurs.

During further development, the formation of the embryo is carried out through the processes of growth, differentiation and morphogenesis. Growth ensures the accumulation of cell mass of the embryo. During the process of differentiation, variously specialized cells arise that form various tissues and organs. The process of morphogenesis ensures that the embryo acquires a specific shape.

Postembryonic development

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Embryogenesis
Biology Embryology Developmental biology
Stages, processes/ result	Egg cleavage/Zygote, Morula Blastulation/Blastula, Blastocyst Gastrulation (intussusception, epiboly)/Gastrula Neurulation/Neurula Organogenesis/Embryo
Germinal leaflets	Ectoderm Endoderm Mesoderm
Differentiation cells	Blastomer Embryoblast Trophoblast Epiblast Hypoblast

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See what “Embryogenesis” is in other dictionaries:

Embryogenesis... Spelling dictionary-reference book

- (from embryo and...genesis) the same as embryonic development... Big Encyclopedic Dictionary

Noun, number of synonyms: 1 embryogeny (2) ASIS Dictionary of Synonyms. V.N. Trishin. 2013… Synonym dictionary

embryogenesis- development of the embryo. zygote fertilized egg; diploid cell formed by the fusion of gametes of different sexes; in this case, the double set of chromosomes inherent in a given organism is restored. embryo embryonic, germinal. embryo... Ideographic Dictionary of the Russian Language

embryogenesis- — EN embryogenesis The formation and development of an embryo from an egg. (Source: MGH) Topics: environmental protection... ... Technical Translator's Guide

- (gr. embryon embryo + ...genesis) biol. the process of germinal (embryonic) development of organisms. New dictionary of foreign words. by EdwART, 2009. embryogenesis biol. process of embryonic development Large dictionary of foreign words. Publishing house... ... Dictionary of foreign words of the Russian language

- [ne], a; m. [from Greek. embryon embryo and genesis origin, occurrence] Biol. The process of embryonic development of organisms. * * * embryogenesis (from embryo and...genesis), the same as embryonic development. * * * EMBRYOGENESIS EMBRYOGENESIS (from embryo... ... encyclopedic Dictionary

Human embryogenesis

Human embryogenesis is part of his individual development, ontogenesis. It is closely related to progenesis (the formation of germ cells and early postembryonic development. Human embryology studies the process of human development, from fertilization to birth. Human embryogenesis, which lasts an average of 280 days (10 lunar months), is divided into three periods: initial (first week of development), embryonic (second to eighth weeks), and fetal (from the ninth week until the birth of the child).In the course of human embryology at the Department of Histology, the early stages of development are studied in more detail.

In the process of embryogenesis, the following main stages can be distinguished:

1. Fertilization ~ fusion of female and male reproductive cells. As a result, a new unicellular zygote organism is formed.

2. Crushing. A series of rapidly successive divisions of a zygote. This stage ends with the formation of a multicellular embryo, which in humans has the form of a vesicle-blastocyst, corresponding to the blastula of other vertebrates.

3. Gastrulation. As a result of division, differentiation, interaction and movement of cells, the embryo becomes multilayered. The germ layers ectoderm, endoderm and mesoderm appear, bearing linings of various tissues and organs.

4. Histogenesis, organogenesis, systemogenesis. During the differentiation of the germ layers, tissue rudiments are formed that form the organs and systems of the human body.

Sex cells. Mature germ cells, gametes, unlike somatic ones, contain a haploid set of chromosomes (23 chromosomes in humans). Male reproductive cells are called spermatozoa or sperm, female reproductive cells are called eggs. All chromosomes of gametes are called autosomes with the exception of one - the sex chromosome. Female germ cells contain X chromosomes. There are two types of male reproductive cells - some sperm contain an X chromosome, and others contain a Y chromosome. Human male reproductive cells are 70 microns in size. They develop and mature in large quantities in the testicles of a man. On average, 3 ml of ejaculate contains 350 million sperm. Male germ cells are very motile, especially those with the Y chromosome. In 1.5-2 hours they can reach the fallopian tube, where maturation of the female reproductive cell and fertilization occurs. Sperm retains fertilizing ability in a woman’s genital tract for two days. Male reproductive cells consist of a head and a tail, in which there are a connecting (or neck), an intermediate (body), a main and terminal parts. The head contains a dense nucleus surrounded by a small rim of cytoplasm. In front, the core is covered with a flat sac called a “case”. in which at the anterior pole

the acrosome is located. The case with the chromosome is a derivative of the Golgi complex. The acrosome contains a set of enzymes, including hyaluronidase and proteases that can dissolve the membranes of the egg. In the connecting part of the sperm in the cytoplasm there is a proximal centriole and a distal one, from which the axial thread begins, the axoneme. In the intermediate section (body), the axial filament (2 central and 9 pairs of peripheral tubes) is surrounded by mitochondria arranged in a spiral, which provide the energy of the sperm. The main part of the tail resembles an eyelash in structure, surrounded by a fine fibrillar sheath. The terminal part of the tail contains single contractile fibrils.

Female reproductive cells, eggs, are classified according to the number and location of the yolk found in their cytoplasm. The amount of yolk depends on the conditions and duration of embryo development,

TYPES OF EGGS

1. Alecital (yolkless).

2. Oligolecithal (low yolk), in them the yolk is evenly distributed throughout the cytoplasm, which is why they are called isolecithal. Among them, there are primary isolecithal (in the lancelet) and secondary isolecithal (in mammals and humans),

3. Polylecithal (multi-yolk)

The yolk in these eggs can be concentrated in the center - these are centrolecithal cells. Among telolecithal eggs, in turn, there are moderately telolecithal or mesolecital with an average yolk content (in amphibians) and sharply telolecithal, overloaded with yolk from which only a small part of the animal pole is free (in birds )

The maturation of the egg and its fertilization occurs in the fallopian tubes. The human egg cannot move independently. It has a diameter of up to 130 microns, surrounded by a transparent (transparent shell) and a layer of follicular cells. The egg contains a large amount of RNA and a well-developed endoplasmic reticulum. A small amount of yolk grains is enough for the egg to feed for 12-24 hours after ovulation, then it dies, or fertilization occurs and the source of nutrition changes.

There are three phases in fertilization.

1. Distant interaction, in which the chemicals gynogamones 1 and 2 of the egg and androgomones 1 and 2 of the sperm play an important role. Gynogamones 1 activate the motor activity of snermia, and androgamones 1, on the contrary, suppress. Gynogamones II (fertilisins) cause sperm adhesion when interacting with androgamon II, built into the cytolemma of the sperm and prevent the penetration of many sperm into the egg.

2. Contact interaction of germ cells. Under the influence of spermatolysins, the acrosomes of sperm cells undergo fusion of plasma membranes and plasmogamy - the union of the cytoplasm of contacting gametes,

3. The third phase is the penetration of sperm into the ooplasm (cytoplasm of the egg), followed by a cortical reaction - compaction of the peripheral part of the ooplasm and the formation of the fertilization membrane.

Fertilization is distinguished between external (for example, in amphibians) and internal (in birds, mammals, humans), as well as polyspermic, when several sperm penetrate the egg (for example, in birds) and monospermic (in mammals, humans).

Fertilization in humans is internal, monospermic. It occurs in the ampullary part of the fallopian tube. The egg is surrounded by numerous sperm. which, by beating their flagella, cause the egg to rotate. Capacitation occurs - activation of sperm under the influence of the mucous secretion of glandular cells of the oviduct and an acrosomal reaction - the release of hyaluronidase and trypsin from the sperm acrosome. They split the zona pellucida and the contacts between follicular cells, and the sperm penetrates the egg. The nuclei - the pronuclei of the egg and sperm - come together, and a synkaryon is formed. Next, the pronuclei merge and a zygote is formed - a new single-celled organism into which maternal and paternal heredity are combined. The sex of the child is determined by the combination of sex chromosomes in the zygote and depends on the sex chromosomes of the father. An abnormal karyotype leads to developmental pathology.

The fragmentation of the zygote begins towards the end of the first day in the oviducts as the fertilized egg moves towards the uterus and ends in the uterus. Cleavage depends on the type of egg, the amount of yolk and its distribution. The following types of crushing are distinguished:

1. Complete, uniform (in primary isolecithal lancelet eggs, the zygote is completely split into equal parts - blastomeres.

2. Complete, uneven (in mesolecithal eggs of amphibians). The zygote is completely fragmented, but unequal blastomeres are formed (small on the animal pole and large on the vegetative pole, where the yolk is concentrated).

3. Partial or meroblastic (in polylecithal avian eggs). Only part of the animal pole of the egg, free from yolk, is crushed.

4. Complete, uneven, asynchronous (in secondary isolecithal eggs of placental mammals and humans).

Crushing is characterized by the appearance of crushing furrows: meridian latitudinal and tangential, parallel to the crushing surface. The more yolk the egg contains, the less completely and evenly the fragmentation occurs. As a result of fragmentation, the embryo becomes multicellular - a blastula. The blastula has a wall - blastoderm, consisting of cells - blastomeres and a cavity - blastocoel, filled with fluid, the product of blastomere secretion. In the blastoderm, a roof is distinguished, formed by the animal pole, a bottom - from the material of the vegetative pole, and a marginal zone located between them. In the lancelet, with complete uniform fragmentation, a spherical blastula is formed - with a single-layer blastoderm (only meridian and latitudinal grooves) and with a centrally located blastocoel - coeloblastula. In frogs, as a result of complete uneven cleavage (all three types of cleavage furrows), a blastula is formed with a multilayer blastoderm and an eccentrically located blastocoel - this is an amphiblastula. In birds and reptiles with sharply telolecithal eggs, only a part of the animal pole, free from the yolk, is crushed, and a discoblastula with a slit-like blastocoel is formed between the blastomeres in the area of the animal pole and the uncrushed yolk. In mammals and humans with secondary isolecithal eggs, the fragmentation is complete (the entire zygote is fragmented without a remainder), asynchronous (the number of blastomeres increases in an irregular and special order in different animals (in humans 2, 3, 4, 5, 7), uneven (two types are formed blastomeres). Some blastomeres are dark, large, slowly fragmenting - this is the embryoblast. The body of the embryo and all extra-embryonic organs, except the trophoblast, are formed from it. The second type of blastomeres is represented by small, light, rapidly dividing cells - this is the trophoblast, which connects the embryo with the mother's body and provides its trophism. Light blastomeres overgrow a bunch of dark blastomeres and the crushed embryo takes on the appearance of a dense ball - a morula after 50-60 hours. On the third day, the formation of a blastocyst begins - a hollow vesicle formed from the outside by a trophoblast and filled with liquid, with an embryoblast in the form of a nodule of cells attached from the inside to the trophoblast at one pole of the blastocyst.The blastocyst enters the uterus on the 5th day and is freely located in it. Preparations for implantation are underway. There are more lysosomes in the trophoblast, and outgrowths appear in the trophoblast. The germinal node, flattening, transforms into a germinal shield, preparing for the first phase of gastrulation

From the seventh day, implantation begins - the implantation of the blastocyst into the wall of the uterus, in which the embryo is completely immersed in the mucous membrane of the uterus, and the mucous membrane fuses over the embryo (interstitial implantation). There are two stages in implantation: adhesion (sticking) and invasion (penetration). On the resulting villi-outgrowths of the trophoblast, two layers are formed: cytotrophoblast - internal and external - symplastotrophoblast, which produces proteolytic enzymes that melt the uterine mucosa. This is how an implantation fossa appears in the uterus, into which the blastocyst penetrates. The histiotrophic type of nutrition due to the consumption of decay products of maternal tissues in the first two weeks is replaced by the hematrophic type - directly from the maternal blood. Implantation is a critical period in human embryogenesis.

Gastrulation is also a critical period in development. It leads to the formation of a multilayer embryo (gastrula). The methods of gastrula formation are different:

1. Invagination-invagination (in lancelet).

2. Epiboly-fouling (in amphibians, epiboly occurs together with partial invagination).

3. Delamination - splitting (in birds, mammals, humans).

4. Immigration - eviction, movement (in birds, mammals, humans).

In humans, gastrulation occurs in two phases: the first (7th day) - by delamination of the embryoblast, two layers are formed: the outer one - the epiblast and the inner one - the hypoblast. The second stage (14-15 days) occurs as in birds with the formation of the primary streak and primary nodule through movement and immigration of cell masses, which ultimately leads to the formation of mesoderm and notochord. Between the two stages of gastrulation, extraembryonic organs are formed: amniotic, vitelline vesicles and chorion, which provide conditions for the development of the embryo and constitute one of the features of human development. In a seven-day embryo, process cells (extraembryonic mesoderm) are evicted from the embryonic shield, which participates in the formation of the amnion along with the ectoderm, the yolk sac along with the endoderm, and the chorion along with the trophoblast in the second week of human development. By day 2, the extraembryonic mesoderm fills the cavity of the blastocyst and grows towards the trophoblast, forming the chorion. The extraembryonic mesoderm grows into the outgrowths of the trophoblast, and later blood vessels sprout - this is how chorionic villi are formed. The latter, upon contact with the endometrium of the uterus, will form the placenta. On days 13-14, the human embryo has two layers: epiblast (primary ectoderm) and hypoblast (primary endoderm), and two vesicles - amniotic and vitelline. The bottom of the amniotic sac (epiblast) and the roof of the vitelline sac (hypoblast) together form the embryonic shield. The strand of extraembryonic mesoderm, the amniotic or embryonic stalk, attaches two vesicles to the chorion: amniotic and vitelline.

After the second stage of gastrulation, on days 15-17, a finger-like outgrowth from the posterior section of the intestinal tube grows into the amniotic pedicle - the allantois, along which vessels grow to the chorion. In a 17-day embryo, three germ layers and extraembryonic organs have already been formed, and differentiation of the germ layers and the laying of the axial main rudiments of organs are occurring.

DIFFERENTIATION OF GERM LAYERS.

Differentiation is changes in the structure of cells associated with the specialization of their functions and determined by the activity of certain genes. There are 4 stages of differentiation:

1. Ootypic differentiation at the zygote stage is represented by presumptive primordia - sections of the fertilized egg.

2. Blastomer differentiation at the blastula stage consists in the appearance of unequal blastomeres (for example, roof blastomeres, bottom marginal zones in some animals).

3. Rudimentary differentiation at the early gastrula stage. Isolated areas - germ layers - appear.

4. Histogenetic differentiation at the late gastrula stage. Within one leaf, the rudiments of various tissues appear (for example, in the somites of the mesoderm). The rudiments of organs and systems are formed from tissues. During the process of gastrulation and differentiation of the germ layers, an axial complex of organ primordia appears.

The germ layers differentiate in the same way in most vertebrates, with each leaf differentiating in a certain direction. From the primary ectoderm, the neural tube, ganglion plates, placodes, cutaneous ectoderm, prechordal plate and extraembryonic ectoderm are formed. Primary endoderm is the source of embryonic intestinal endoderm and extraembryonic (yolk) endoderm. When the mesoderm differentiates, three parts arise: in the dorsal part (1) somites appear, followed by (2) segmental legs (nephrotomes), from which the epithelium of the kidneys and gonads is formed. The ventral mesoderm is not segmented and forms (3) a splanchnotome, splitting into two layers: the parietal, accompanying the ectoderm, and the visceral, adjacent to the endoderm. A coelomic cavity appears between the sheets, and the epithelium of the serous membranes, the peritoneum, is formed from the sheets of the splanchnotome. pleura, pericardium. Further in the body of the somite, it is differentiated from its outer part by the dermatome (the source of the dermis of the skin), from the central part by the myotome (the rudiment of skeletal muscle tissue) and from the internal sclerotome (the rudiment of skeletal connective tissues - bones and cartilage). During the process of differentiation of the germ layers of the mesoderm, mesenchyme appears in the embryo.

On days 20-21, the human embryo develops trunk folds, separating the body of the human embryo from the extraembryonic organs, and the axial rudiments of the organs are finally formed: the notochord, from the ectoderm - the neural tube, which closes by the 25th day. The intestinal tube is formed. The mesoderm of the embryo differentiates into somites (somite period), nephrotome and splanchnotome with parietal and visceral layers. The body of a somite is divided into: dermatome, myotome and sclerotome. During the period of mesoderm differentiation, from all three germ layers, but mainly from the mesoderm, the mesenchyme of the embryo appears - process cells, the embryonic rudiment of many tissues and organs of all types of connective tissue (hence it is often called embryonic connective tissue), as well as smooth muscle tissue, vascular microglia, blood, lymph, hematopoietic organs. By the second month, the human embryo has undergone initial histo- and organogenesis and there are anlages of almost all organs. By the end of the 8th week of embryogenesis, the embryonic period of development ends and the fetal period begins.

The early stages of human development have a number of features: 1. Asynchronous type of complete uneven fragmentation with the formation of “dark” and “light” blastomeres; 2. Interstitial type of implantation. 3. The presence of two phases of gastrulation - delamination and immigration, between which extraembryonic organs rapidly develop; 4. Early separation and formation of extraembryonic organs; 5. Early formation of the amniotic sac without amniotic folds; 6. Strong development of the amnion, chorion and weak yolk mark and allantois.

Extraembryonic organs (provisional, temporary or embryonic membranes) that ensure the development of the embryo. In evolution, they appear for the first time in fish (yolk sac). Birds have the following extraembryonic organs: amnion, serosa, yolk sac and allantois. The amnion is a water membrane, the serosa is a respiratory organ. These two membranes are formed in birds by the closure of the amniotic folds. The yolk sac performs trophic and hematopoietic functions in birds, and the allantois is an organ of excretion and gas exchange in birds.

During human embryogenesis, five extraembryonic organs are formed: the amnion, the yolk sac, the chorion, which forms the placenta, and the allantois. The amnion, which creates an aquatic environment in humans, is formed without amniotic folds. The yolk sac in humans practically loses its trophic function and performs mainly the hematopoietic function and the formation of primary germ cells. Allantois. reducing in the second month is a conductor of blood vessels to the chorion. A well-developed chorion in humans forms the placenta, through which a connection between the embryo and mother is established.

The placenta, which provides a connection between the embryo and the mother’s body, performs numerous functions: trophic, respiratory, excretory, endocrine, protective, and depository. Based on morphological characteristics, there are four types of placenta: epitheliochorial, desmochorial, endotheliochorial and hemochorial. Epitheliochorionic diffuse placentas (in dolphins, pigs, horses) are characterized by ingrowth of chorionic villi into the uterine glands. In desmochorionic multiple placentas (in bark, sheep), chorionic villi, destroying the epithelium of the uterine glands, grow into the underlying connective tissue of the endometrium of the uterus. The endotheliochorial cingulate type of placenta is characteristic of predators (cats, wolves, martens, foxes). Chorionic villi in this type of placenta destroy the epithelium, connective tissue and come into contact with the endothelium of the endometrial vessels of the uterus. The hemochorionic type of placenta (for example, in bats, primates, humans) is characterized by the destruction of the walls of the endometrial vessels of the uterus by chorionic villi and their direct contact with maternal blood. At birth, newborns who have placentas of the first two types are capable of independent feeding and movement. whereas newborns with the last two types of placentas after birth are not able to feed independently for a long time.

The human placenta, the hemochorial discoidal villous placenta, performs numerous functions that ensure the growth and development of the embryo at the expense of the mother's body. The placenta has two parts: embryonic or fetal (children's) and maternal or uterine. The fetal part is formed by a branched chorion covered with the amniotic membrane, and the maternal basal lamina is a modified basal part of the endometrium. The development of the placenta occurs parallel to the beginning of the formation of organ primordia: from 3 to 6 weeks (the critical period in human embryogenesis) and ends at the end of the 3rd month of pregnancy. By this time, the fetal part of the placenta consists of a dense connective tissue chorionic plate with branching chorionic villi extending from it, immersed in lacunae with maternal blood. The chorionic plate is covered on top by part of the amniotic membrane.

After fertilization, the mucous membrane of the uterus is called decidual, decidual, and there are 3 parts in it: the main deciduous one, where implantation took place between the embryo and the muscular lining of the uterus: the second part is the bursa culcumsa. separating the embryo from the uterine cavity and the third part - the parietal part, the remaining part of the decidua. The chorion villi, facing the main one, grow strongly and branch - this is a branched (lush chorion). It is in this area that the placenta is formed: due to the branched chorion - its fetal part, and due to the main falling away - its maternal part. In the area of the parietal and bursa, the chorionic villi subsequently disappear altogether (smooth chorion). Chorionic villi consist of embryonic fibrous connective tissue stroma with vessels. The cellular and fibrous composition of this connective tissue, the viscosity of the main substance (the content of hyaluronic and chondroitin-sulfuric acid, which regulate the permeability of the placental villi) changes with the duration of pregnancy. On the surface, the connective tissue stroma of the villi in the early stages of pregnancy is covered with trophoblastic epithelium, which has a cellular structure. It is represented by a single-layer epithelium - cytotrophoblast, which gradually reduces from the second month of embryogenesis. An outer layer appears on the surface of the cytotrophoblast - syncytiotrophoblast - a multinuclear structure with a large number of proteolytic and oxidative enzymes. At the end of pregnancy, the syncytiotrophoblast also undergoes decay and in places a fibrin-like oxyphilic mass (Langhans fibrinoid) appears on the surface of the villi.

The maternal part of the placenta is represented by the basal lamina (deep, undestroyed parts of the falling membrane along with the trophoblast), connective tissue septa extending from the basal lamina and merging with the chorionic villi. These so-called anchor or stem villi divide the placenta into lobules-cotyledons. Also in the maternal part of the placenta there are lacunae with maternal blood and chorionic villi (the terminal branches of the stem villi). The basal layer of the endometrium - the deep layer of the uterine mucosa contains in its connective tissue large decidual cells with oxyphilic cytoplasm, rich in glycogen inclusions, rounded nuclei, and clear cell boundaries. In the basal lamina in the area of attachment of the anchor villi there are often accumulations of basophilic cells of peripheral cytotrophoblast. On the surface of the basal lamina facing the villi, an amorphous oxyphilic substance (Rohr fibrinoid) is sometimes formed, which, together with the trophoblastic cells of the basal lamina, ensures the immunological homeostasis of the mother-fetus system. Part of the main falling membrane along the edge of the placental disc at the border of the smooth and branched chorion tightly grows to the chorion and is not destroyed, forming an endplate that prevents the flow of blood from the lacunae.

The blood of the mother and fetus, circulating through independent systems, never mixes due to the presence of a hemoplacental (homochorial) barrier that separates the blood flow of the fetus from the blood flow of the mother. The hemoplacental barrier consists of endothelium with the basement membrane of the fetal vessels surrounding these vessels of the connective tissue stroma of chorionic villi and their epithelium (cytotrophoblast, syncytiotrophoblast) and fibrinoid. The embryo releases carbon dioxide and metabolic products into the mother's blood and receives oxygen, water, nutrients, vitamins, hormones, immunoglobulins, as well as drugs, alcohol, nicotine, and viruses from the mother's blood.

The umbilical cord develops mainly from the mesenchyme of the amniotic stalk and is an elastic connective tissue formation with vessels, as well as with the remains of the vitelline stalk and allantois, externally covered with the amniotic membrane. In its gelatinous, mucous connective tissue base (Wartoni's jelly) pass the umbilical arteries and umbilical vein, which ensure the metabolic processes of the embryo.

The mother-fetus system, which develops during pregnancy, consists of the mother and fetus, connected by the placenta. The main mechanisms that ensure interaction in the mother-fetus system are the neurohumoral mechanisms of the mother and the fetus: receptor, regulatory, executive. These mechanisms are aimed at creating optimal conditions for fetal development. In this case, a particularly important role belongs to the placenta, which accumulates and synthesizes substances and hormones necessary for the development of the fetus, and carries out humoral and nervous connections between the fetus and the mother. Humoral connections are carried out not only through the placenta, but also through the fetal membranes and amniotic fluid. Through the humoral communication channel, not only gas exchange, the supply of hormones, vitamins, and nutrients occurs, but also immunological homeostasis is maintained in the mother-fetus system. Nerve connections also include placental (in the fetus - interoceptive, caused by irritation of receptors in the vessels of the placenta and umbilical cord) and extraplacental channels (in the fetus - exteroceptive, associated with fetal growth).

In the human body - in progenesis, embryogenesis, in the process of formation of the mother-fetus system and the postnatal period - there are critical periods. These include oogenesis and spermatogenesis (see guidelines for the reproductive system), fertilization, implantation (7-8 days of embryogenesis), development of axial organs, and formation of the placenta (3-8 weeks of embryogenesis), a period of enhanced brain development ( 15-20 weeks) and the formation of the main body systems, including the reproductive system (20-24 weeks of development), birth, the neonatal period up to 1 year and puberty from 11 to 16 years.

Developmental biology– a new direction of modern biology. This is the science of the patterns and mechanisms of ontogenesis.

Ontogenesis(Greek Ontos - being, genesis - development) - individual development of the organism.

It includes a set of successive morphological, physiological and biochemical transformations from birth to death.

Ontogenesis multicellular organisms are divided into two periods: embryonic (embryonic, gr. embruоn - embryo) and postembryonic (post-embryonic). In higher animals and humans, ontogenesis is divided into prenatal(before birth), and postnatal(after birth).

Embryonic, or prenatal embryogenesis includes the development of an organism from fertilization of the egg to the release of the individual from the egg membranes or from the uterine cavity of the maternal organism.

The animal world has three most common types of ontogenesis: larval; non-larval; intrauterine.

Larval type of ontogenesis characterized by the development of the organism occurring with metamorphosis.

Non-larval type of ontogenesis characterized by the formation of the organism carried out in the egg.

Intrauterine Ontogenesis is determined by development within the maternal organism.

In humans, the body is up to 8 weeks old by the time the rudiments of organs are formed and is called an embryo, or fetus.

a fetus is an organism after the formation of the rudiments of organs and the body shape that a person has (8 weeks after the formation of the zygote).

Embryogenesis includes the following main stages (Fig. 5):

1. Fertilization and crushing of the egg.

Gastrulation and formation of germ layers.

3. Histogenesis and organogenesis. This is the formation of organs and tissues.

Fertilization represents the penetration of a sperm into an egg. in humans and mammals this occurs in the upper third of the fallopian tube.

After fertilization, a zygote is formed. She has genetic information from two parents and a diploid set of chromosomes (2 n). A fertilized egg (zygote) reproduces mitotically.

The early period of embryogenesis, i.e.

e. the development of a fertilized egg (zygote) is called crushing. The resulting cells are called blastomeres. Their development occurs through

successive mitotic divisions.

Fragmentation has a number of features: the mitotic cycle is characterized by a short duration, there are no pre- and postsynthetic phases, protein synthesis is repressed to a certain stage.

Since there is no postmitotic growth of germ cells, blastomeres decrease in size and, although their total number increases rapidly, the volume of the embryo does not change significantly in the early stages of development.

The nature of crushing depends on the type of egg cells and the amount of yolk in the oocyte. The following are distinguished: types of crushing :

1) Complete crushing (holoblastic) – uniform and uneven;

2) Incomplete cleavage (meroblastic) – discoidal and superficial.

With complete (holoblastic) fragmentation the zygote divides entirely.

Isolecithal and telolecithal eggs develop in this way.

With incomplete (meroblastic) fragmentation Only the part of the cytoplasm of the egg that does not have yolk inclusions divides.

incomplete crushing is discoidal and superficial.

In discoidal cleavage, segmentation occurs at the animal pole, while the vegetal pole of the egg remains intact. This method is typical for strongly telolecithal cells (for example, in birds).

Centrolecithal cells have superficial fragmentation. In this case, the entire peripheral zone of ovoplasm free from yolk is divided (for example, in insects).

The fragmentation of the zygote in humans and mammals is holoblastic and uniform.

the number of blastomeres increases in the wrong order, asynchronously. Fragmentation ends with the formation blastulas.

Blastula– it is a multicellular single-layer embryo. It has blastoderm.

This is the body wall that is formed by blastomeres. The blastocoel is the cavity of the blastula. There are different types of blastulas. With superficial crushing, the cavity is filled with yolk. This is periblastula. With discoidal cleavage, the germ cells are spread out in the form of a disk on the yolk. This is discoblastula.

In humans and mammals, crushing results in the formation of a blastocyst (germinal vesicle).

its walls are formed by trophoblast, a single layer of sharply flattened cells. The cavity of the blastocyst is filled with fluid. The blastula turns into gastrulu.

Gastrulation this is the directed movement of large groups of embryonic cells to the sites of formation of future organ systems.

As a result, three germ layers are formed. They consist of cells that differ in size, shape and other characteristics. In lower animals such as sponges and coelenterates, the gastrula consists of two layers of cells - the ectoderm (outer germ layer) and endoderm (inner germ layer).

All other higher phyla of animals have a three-layered gastrula. Then the third (middle) germ layer, the mesoderm, is formed.

From ectoderm The tissues of the nervous system develop, the outer covering of the skin - the epidermis and its derivatives (nails, hair, sebaceous and sweat glands), as well as tooth enamel, sensory cells of the organs of vision, hearing and smell, etc.

From endoderm epithelial tissue develops, lining the respiratory organs, partly the genitourinary and digestive systems, including the liver and pancreas.

Most numerous mesoderm derivatives– skeletal muscles, excretory organs and gonads; cartilage, bone and connective tissue.

Gastrula formation in various animals it is carried out in four ways: intussusception, immigration, delamination, epiboly .

A classic example of gastrulation by intussusception is the embryonic development of the lancelet.

In the blastula of the lancelet, a group of blastomeres begins to invaginate into the blastocoel. As a result, ectoderm and endoderm are formed. They form the cavity of the primary intestine - the gastrocoel. This cavity communicates with the external environment through an opening (blastopore). Then the mesoderm is formed in the form of paired outgrowths of the wall of the primary intestine (mesoderm pockets).

Further differentiation of the germ layers leads to the formation of the organs of the axial complex.

These are the neural tube, notochord and intestinal tube.

In humans, gastrulation occurs in two phases. First, a two-layer gastrula is formed by delamination of the embryoblast.

The second phase is the emergence of the middle germ layer and the appearance of the axial complex of primordia.

Histogenesis and organogenesis. Germ layers are the material from which the rudiments of certain tissues and organs are newly formed in all multicellular organisms . The embryonic development of organisms is carried out with the participation of provisional (extraembryonic) - temporarily functioning organs that provide the necessary vital functions and connect the embryo with the environment.

in animals with a non-larval type of development (fish, reptiles, birds), eggs have a lot of yolk.

Their provisional body is yolk sac. He is the organ of nutrition and hematopoiesis of the embryo. The reduced yolk sac of mammals is part of placenta. In terrestrial animals (reptiles, birds, mammals) provisional authorities(Fig. 6) this is a water shell (amnion), allantois And serous membrane (chorion). In placental mammals, the chorion, together with the uterine mucosa, forms the placenta.

In human embryonic development there are 3 main critical periods:

Implantation (b – 7th day after conception) – implantation of the zygote into the wall of the uterus.

2. Placentation (end of the 2nd week of pregnancy) – the formation of a placenta in the embryo.

3. Perinatal period (childbirth) - the transition of the fetus from the aquatic to the air environment 9 months after conception.

Critical periods in a newborn’s body are associated with a sharp change in living conditions and a restructuring of the activities of all body systems (the nature of blood circulation, gas exchange, and nutrition changes).

Embryogenesis stages

All cells of the body, despite differences in structure and functions, are united by one thing - a single genetic information stored in the nucleus of each cell, a single double set of chromosomes (except for highly specialized blood cells - red blood cells, which do not have a nucleus).

That is, all somatic (soma - body) cells are diploid and contain a double set of chromosomes - 2 n, and only sex cells (gametes) formed in specialized gonads (testes and ovaries) contain a single set of chromosomes - 1 n.

When germ cells fuse, a cell is formed - a zygote, in which a double set of chromosomes is restored.

Recall that the nucleus of a human cell contains 46 chromosomes, respectively, sex cells have 23 chromosomes

The resulting zygote begins to divide. The first stage of zygote division is called cleavage, as a result of which the multicellular structure of the morula (mulberry) is formed.

The cytoplasm is distributed unevenly between the cells; the cells of the lower half of the morula are larger than the upper half. The volume of the morula is comparable to that of the zygote.

At the second stage of division, as a result of cell redistribution, a single-layer embryo is formed - a blastula, consisting of one layer of cells and a cavity (blastocoel).

Blastula cells vary in size.

From these three layers of cells (they are called germinal layers) the tissues and organs of the future organism are formed.

The integumentary and nervous tissue develops from the ectoderm, the skeleton, muscles, circulatory system, genitals, excretory organs from the mesoderm, and the respiratory and nutritional organs, liver, and pancreas from the endoderm. Many organs are formed from several germ layers.
Embryogenesis includes the processes from fertilization to birth.

The development of the human body begins after fertilization of the female reproductive cell - the egg (ovium) of the male - by a spermatozoon (spermatozoon, spermium).
The detailed study of the development of the human embryo (embryo) is the subject of embryology.

Here we will limit ourselves to only a general overview of the development of the embryo (embryogenesis), which is necessary for understanding the human physique.

The embryogenesis of all vertebrates, including humans, can be divided into three periods.
1. Crushing: a fertilized egg, spermovium, or zygote is sequentially divided into cells (2,4,8,16 and so on) as a result of which a dense multicellular ball, morula, and then a single-layer vesicle - blastula, which contains a primary cavity in the middle, is formed. blastocoel.

The duration of this period is 7 days.
2. Gastrulation consists of the transformation of a single-layer embryo into a two-, and later three-layer - gastrula. The first two layers of cells are called germ layers: the outer ectoderm and the inner endoderm (up to two weeks after fertilization), and the third, middle layer that appears later between them is called the middle germ layer - mesoderm.

The second important result of gastrulation in all chordates is the emergence of an axial complex of rudiments: on the dorsal (dorsal) side of the endoderm, the rudiment of the dorsal string, notochord, appears, and on its ventral (ventral) side - the rudiment of the intestinal endoderm; on the dorsal side of the embryo, along its midline, a neural plate stands out from the ectoderm - the rudiment of the nervous system, and the rest of the ectoderm goes to build the epidermis of the skin and is therefore called cutaneous ectoderm.
Subsequently, the embryo grows in length and turns into a cylindrical formation with a head (cranial) and caudal caudal ends.

This period lasts until the end of the third week after fertilization.

The dorsal sections of the mesoderm form the primary segments of the body - somites, each of which in turn is divided into a sclerotome, which gives rise to the skeleton, and a myotome, from which muscles develop. A skin segment, the dermatome, is also distinguished from the somite (on its lateral side). The abdominal sections of the mesoderm, called splanchnotomes, form paired sacs that contain the secondary body cavity.
The intestinal endoderm, which remains after the separation of the notochord and mesoderm, forms the secondary gut - the basis for the development of internal organs.

Subsequently, all the organs of the body are laid down, the material for the construction of which is the three germ layers.

1. From the outer germ layer, ectoderm, develop:

A) epidermis of the skin and its derivatives (hair, nails, skin glands);
b) epithelium of the mucous membrane of the nose, mouth and anus;
V) nervous system and epithelium of sensory organs.

2. From the inner germ layer, the endoderm, the mucosal epithelium of most of the digestive tract with all the glandular structures belonging here, most of the respiratory organs, as well as the epithelium of the thyroid and thymus glands develops.

3. From the middle germ layer, mesoderm, the musculature of the skeleton, the mesothelium of the membranes of the serous cavities with the rudiments of the gonads and kidneys develop.
In addition, from the dorsal segments of the mesoderm, embryonic connective tissue, mesenchyme, arises, which gives rise to all types of connective tissue, including cartilage and bone.

Since at first the mesenchyme carries nutrients to different parts of the embryo, performing a trophic function, then later blood, lymph, blood vessels, lymph nodes, and the spleen develop from it.
In addition to the development of the embryo itself, it is also necessary to take into account the formation of extra-embryonic parts, with the help of which the embryo receives the nutrients necessary for its life.

With the help of trophoblast, the embryo penetrates into the thickness of the uterine mucosa (implantation), and here the formation of a special organ begins, with the help of which the embryo is connected with the mother’s body and is nourished.

This organ is called the baby's place, litter, or placenta. Mammals that have a placenta are called placentals. Along with the formation of the placenta, there is a process of separation of the developing embryo from the extra-embryonic parts as a result of the appearance of the so-called trunk fold, which, protruding with a ridge towards the middle, seems to lace the body of the embryo from the extra-embryonic parts with a ring.

At the same time, however, the connection to the placenta is maintained through the umbilical stalk, which then turns into the umbilical cord. In the early stages of development, the vitelline duct passes through the latter, which connects the intestine with its protrusion into the extraembryonic area, the yolk sac. In vertebrates that do not have a placenta, the yolk sac contains the nutritional material of the egg - the yolk - and is an important organ through which the embryo is nourished.

In humans, although the yolk sac appears, it does not play a significant role in the development of the embryo and, after absorption of its contents, is gradually reduced.

The umbilical cord also contains umbilical (placental) vessels, through which blood flows from the placenta to the body of the fetus and back. They develop from the mesoderm of the urinary sac, or allantois, which protrudes from the ventral wall of the intestine and exits the body of the embryo through the umbilical opening into the extraembryonic part. In humans, from the part of the allantois, which is contained in the middle of the body of the embryo, part of the bladder is formed, and from its vessels the umbilical blood vessels are formed.

The developing embryo is covered with two germinal membranes. The inner membrane, the amnion, forms a voluminous sac, which is filled with protein fluid and forms a liquid environment for the embryo, through which the sac is called the aqueous membrane.

The entire embryo, along with the amniotic and yolk sacs, is surrounded by an outer membrane (which also includes the trophoblast). This membrane, having villi, is called villous, or chorion.

The chorion performs trophic, respiratory, excretory and barrier functions.

Embryogenesis, according to the nature of the processes occurring in the embryo, is divided into three periods:

1) crushing period;

2) period of gastrulation;

3) the period of histogenesis (tissue formation), organogenesis (organ formation), systemogenesis (formation of functional systems of the body).

Splitting up.

The lifespan of a new organism in the form of one cell (zygote) lasts in different animals from several minutes to several hours and even days, and then fragmentation begins.

Cleavage is the process of mitotic division of the zygote into daughter cells (blastomeres). Cleavage differs from ordinary mitotic division in the following ways:

blastomeres do not reach the original size of the zygote;

2) blastomeres do not diverge, although they are independent cells.

The following types of crushing are distinguished:

1) complete, incomplete;

2) uniform, uneven;

3) synchronous, asynchronous.

The eggs and the zygotes formed after their fertilization, containing a small amount of lecithin (oligolecithal), evenly distributed in the cytoplasm (isolecithal), are completely divided into two daughter cells (blastomeres) of equal size, which then simultaneously (synchronously) divide again into blastomeres.

This type of crushing is complete, uniform and synchronous. Eggs and zygotes containing a moderate amount of yolk are also crushed completely, but the resulting blastomeres have different sizes and are crushed non-simultaneously - the crushing is complete, uneven, asynchronous. As a result of fragmentation, an accumulation of blastomeres is first formed, and the embryo in this form is called a morula. Then fluid accumulates between the blastomeres, which pushes the blastomeres to the periphery, and a cavity filled with fluid is formed in the center.

At this stage of development, the embryo is called blastula.

The blastula consists of:

1) blastoderm - shells of blastomeres;

2) blastocoel - a cavity filled with liquid.

The human blastula is a blastocyst.

After the formation of the blastula, the second stage of embryogenesis begins - gastrulation.

Gastrulation- the process of formation of germ layers, formed through the reproduction and movement of cells. The process of gastrulation occurs differently in different animals.

The following methods of gastrulation are distinguished:

delamination (splitting of a cluster of blastomeres into plates);

2) immigration (movement of cells inside the developing embryo);

3) intussusception (invagination of a layer of cells into the embryo);

4) epiboly (overgrowth of slowly dividing blastomeres with rapidly dividing ones with the formation of an outer layer of cells).

As a result of gastrulation, three germ layers are formed in the embryo of any animal species:

1) ectoderm (outer germ layer);

2) endoderm (inner germ layer);

3) mesoderm (middle germ layer).

Each germ layer is a separate layer of cells.

Between the sheets there are initially slit-like spaces into which process cells soon migrate, collectively forming the germinal mesenchyme (some authors consider it as the fourth germ layer). Germinal mesenchyme is formed by the eviction of cells

from all three germ layers, mainly from the mesoderm.

The embryo, consisting of three germ layers and mesenchyme, is called gastrula.

The process of gastrulation in the embryos of different animals differs significantly both in methods and in time. The germ layers and mesenchyme formed after gastrulation contain presumptive tissue rudiments. After this, the third stage of embryogenesis begins - histo- and organogenesis.

Histo- and organogenesis(or differentiation of germ layers) is the process of transformation of tissue primordia into tissues and organs, and then the formation of functional

body systems.

The basis of histo- and organogenesis are the following processes: mitotic division (proliferation), induction, determination, growth, migration and differentiation of cells.

As a result of these processes, axial rudiments of organ complexes (notochord, neural tube, intestinal tube, mesodermal complexes) are first formed. At the same time, various tissues are gradually formed, and from the combination of tissues, anatomical organs are laid down and developed, combining into functional systems - digestive, respiratory, reproductive, etc. At the initial stage of histo- and organogenesis, the embryo is called an embryo, which later turns into a fetus.

At present, it has not been definitively established how cells completely different in morphology and function are formed from one cell (zygote), and subsequently from identical germ layers, and from them tissues are formed (from ectoderm

epithelial tissues, horny scales, nerve cells and glial cells).

Presumably, genetic mechanisms play a leading role in these transformations.

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Without having in your disposal of early embryos humans, showing some of the most important stages of the formation of germ layers, we tried to trace their formation in other mammals. The most noticeable feature of early development is the formation of many cells from a single fertilized egg through successive mitoses. Even more important is the fact that even during the early phases of rapid proliferation, the cells thus formed do not remain an unorganized mass.

Video: Embryogenesis: Development of the embryo

Almost immediately they are located in the form of a hollow formation called a blastoderm vesicle.

At one pole, a group of cells known as the inner cell mass gathers. As soon as it is formed, cells begin to emerge from it, lining a small internal cavity - the primary gut, or archenteron. From these cells the endoderm is formed.

Ta part of the original group The cells from which the integuments of the embryo and the outermost layer of its membranes are formed are called ectoderm.

Soon, between the first two germ layers, a third layer is formed, called, quite aptly, mesoderm.

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Germ layers are of interest to the embryologist from several points of view.

The simple structure of the embryo, when it first contains one, then two and finally three primary layers of cells, is a reflection of the phylogenetic changes that took place in lower animals - the ancestors of vertebrates. From the point of view of possible ontogenetic recapitulations, some facts fully allow this.

Nervous system of embryos vertebrates arise from the ectoderm - a layer of cells through which primitive organisms that do not yet have a nervous system are in contact with the external environment.

The lining of the vertebrate digestive tube is formed from the endoderm, a layer of cells that in very primitive forms lines their gastrocoel-like internal cavity.

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Skeletal, muscular and circulatory systems originate in vertebrates almost exclusively from the mesoderm - a layer that is relatively unnoticeable in small, low-organized creatures, but whose role increases as their size and complexity increase due to their increasing needs for support and circulatory systems.

Along with the possibility interpretation of germ layers from the point of view of their phylogenetic significance, it is also important for us to establish the role they play in individual development.

The germ layers are the first organized groups of cells in the embryo, which are clearly distinguished from each other by their features and relationships. The fact that these relationships are essentially the same in all vertebrate embryos strongly suggests a common origin and similar heredity in the various members of this huge group of animals.

One might think that in these germ layers For the first time, differences between different classes begin to be created over the general plan of body structure, characteristic of all vertebrates.

Formation of embryonic leaflets The period when the main process of development is only an increase in the number of cells ends, and the period of differentiation and specialization of cells begins.

Differentiation occurs in the germ layers before we can see its signs using any of our microscopic techniques. In the leaf, which has a completely homogeneous appearance, localized groups of cells with different potencies for further development constantly appear.

We have known about this for a long time, for we can see how from the germ layer various structures arise. At the same time, no visible changes are noticeable in the germ layer due to which they arise.

Recent experimental studies indicate how early this invisible differentiation precedes the visible morphological localization of cell groups that we easily recognize as the rudiment of the definitive organ.

So, for example, if you cut from any place of Hensen's node a narrow transverse strip of the ectoderm of a twelve-hour embryo and grown in tissue culture, then at a certain time specialized cellular elements of a type that is found only in the eye will be discovered, although the rudiment of the optic vesicle of a chick embryo does not appear before 30 hours of incubation.

A strip taken from another area, although it appears the same, when grown in culture does not form cells characteristic of the eye, but exhibits a different specialization.

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Experiments show how early in the germ layers groups of cells with different potencies for development are determined.

As development progresses, these cell groups become more and more prominent. In some cases, they are separated from the mother leaf by protrusion, in other cases - by migration of individual cells, which later accumulate somewhere in a new place.

From the primary groups of cells thus formed, gradually definitive organs are formed.

Therefore, the origin of various parts of the body in embryogenesis depends on the growth, division and differentiation of the germ layers. This diagram shows us the general path along which the early processes discussed above develop. If we follow the process of development further, we see that each normal division of an object is more or less clearly centered around a certain branch of this family tree of germ layers.

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Main article: Sexual reproduction

Fertilization

Human life begins from the moment of fusion in the mother’s body of two sex cells - an egg and a sperm, and one new cell is formed, that is, a new organism. Each of the female and male germ cells contains 23 pairs of chromosomes, 22 of which transmit the hereditary characteristics of the father and mother to the fetus.

In both of these germ cells there are about 100 thousand genes that determine the structural and functional characteristics of the newly formed organism.

The gender of the unborn child depends on the 23rd pair of chromosomes of the female and male germ cells. The 23rd pair of chromosomes of the female germ cell is designated as X-X (XX), and the 23rd pair of chromosomes of the male germ cell is designated X-Y (XY).

If the X chromosome of a male cell merges with a female cell, a girl is born, and when the Y chromosome of a male cell merges with a female cell, a boy is born.

Thus, the sex of the unborn child depends on the father’s reproductive cell, but not on his will or desire.

The female and male reproductive cells, merging in the fallopian tube, form one cell, that is, a new organism that has 46 pairs of chromosomes. As soon as such a cell is formed, it begins to multiply by division within one week, while gradually moving towards the uterus. Once in the uterine cavity, it attaches to its wall and continues its development in the form of an embryo, or fetus.

Fetal development

A new organism that arises in the womb develops in the oviduct in the first week of its life and, starting from the second week, its development proceeds in the uterine cavity and continues for 9 months.

And all this time the fetus is nourished by the blood of the mother’s body. From the 23rd day of development of the embryo, its heart and systemic circulation begin to function. But his lungs and pulmonary circulation do not work during the period of embryonic development, and the fetus is provided with oxygen through the umbilical vessels at the expense of the maternal body.

As soon as the baby is born, the umbilical cord is cut and separated from the mother's body. From this moment, his lungs and pulmonary circulation begin to function.

Afterbirth

From the outer part of the embryo in the uterine cavity, a special tissue is formed, rich in blood vessels and consisting of special cells - the so-called afterbirth, with the help of which the embryo is attached to the wall of the uterus (Fig.

82). The umbilical cord is formed from its vessels, through the arteries and veins of which the fetus is connected to the vessels of the mother’s body. The afterbirth provides nutrition to the fetus and, in addition, protects it from the effects of harmful chemicals and microbes that have entered the mother’s body.

Damage to the placenta and its detachment from the uterine wall poses a danger to the fetus. Material from the site http://wiki-med.com

Amnion

The fetus is surrounded by a thin membrane (amnion), the internal cavity of which is filled with amniotic fluid.

This fluid plays an important role in metabolic processes in the fetus’s body, in protecting it from adverse external influences and facilitating its free movement (Fig. 83).

Layers of the embryo

In the third week of intrauterine life, the cells of the embryo form three layers. The outer one is called ectoderm, the middle one is called mesoderm and the inner one is called endoderm.

Each of them gives rise to different tissues and organs of the embryo.

On this page there is material on the following topics:

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Questions for this article:

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The process of human embryonic development has 4 stages and lasts 8 weeks. It begins from the moment the male and female reproductive cells meet, their fusion and the formation of a zygote, and ends with the formation of an embryo.

What stages does embryogenesis consist of?

After the sperm merges with the egg, education It is she who moves through the fallopian tubes over the course of 3-4 days and reaches the uterine cavity. In this case, a period is observed. It is characterized by strong intensive cell division. At the end of this stage of embryo development blastula is formed- a cluster of individual blastomeres, in the form of a ball.

The third period, gastrulation, involves the formation of the second germ layer, resulting in gastrula is formed. After this, the third germ layer, the mesoderm, appears. Unlike vertebrates, embryogenesis in humans is complicated by the development of the axial complex of organs - the formation of the rudiments of the nervous system, as well as the axial skeleton and with it the muscles, occurs.

At the fourth stage of human embryonic development, isolation of the rudiments of future organs and systems formed at this moment. Thus, from the first germ layer the above-mentioned nervous system and partly the sense organs are formed. From the second endoderm, the epithelial tissue lining the digestive canal and the glands located in it. Connective, cartilage, bone tissue, as well as the vascular system are formed from mesenchyme.

What could cause the sequence of these stages to be disrupted?

The stages of human embryonic development presented in the table below do not always occur in the order in which it is necessary. Thus, under the influence of certain types of factors, mainly exogenous, the course of development of individual organs and systems may be disrupted. Among these reasons are:

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