Gastrulation is very important stage development. In all multicellular animals, the body consists of at least two layers of cells, and this fundamental feature of animals is acquired by the embryo during the gastrulation stage. The most primitive “real” (i.e., having a nervous system and muscles) animals are considered to be coelenterates. Their body, even in adulthood, is formed by two layers - ecto- and endoderm.

Thus, gastrulation is the process of gastrula formation, during which a hollow cellular ball is transformed into a multilayer, bilaterally symmetrical structure with an intestinal tube (primary intestine - archenteron) located in the center. This is the stage of embryogenesis, including difficult process reproduction, movement and differentiation of the cellular material of the embryo, leading to the formation of germ layers - ectoderm, endoderm and mesoderm

Gastrulation was studied in embryos sea ​​urchin. The starting material for gastrulation is the blastula, consisting of approximately 1000 cells, covered with a thin layer of extracellular matrix. Gastrulation begins with the separation of several dozen so-called primary mesenchyme cells from the blastoderm at the vegetative pole (Fig. 16-6). In all likelihood, these cells lose the ability to communicate with other cells and the extracellular matrix of the outer surface of the embryo and gain an affinity for the fibronectin-rich matrix that lines the blastocoel. These cells enter the cavity of the blastula and move along its wall, being pulled up by the long thin processes (filopodia) they produce with “sticky” ends (Fig. 16-7). When the tip of the filopodia comes into contact with a surface to which it can firmly attach, the filopodia contracts and pulls the cell with it. The formed filopodia are apparently retracted, and new ones appear in their place in other places, so that the cell can move in one direction or another. Ultimately, the cells occupy a clearly defined position, which is apparently due to their specific affinity for certain areas of the surface of the blastocoel. This was shown in experiments with monoclonal antibodies, which demonstrated highly specialized differences between cells in different parts of the embryo in terms of their surface chemistry. After the primary mesenchymal cells have taken their place, they begin to form the skeleton.

With the beginning of the migration of primary mesenchyme cells, the blastoderm begins to invaginate (invaginate) in the region of the vegetative pole, forming the primary gut (Fig. 16-6). In this case, the shape of the cells first changes: the inner end of the cell, facing the blastocoel, becomes wider than the outer one, and therefore the cell layer bends inside the blastocoel (Fig. 16-8). The next stage of invagination occurs due to the redistribution of cells; the invaginating cells are actively rearranged, but their shape does not change. As a result, the initially rather wide gastrula cavity turns into a long narrow intestinal tube. At the same time, certain cells at the top of this intestinal tube release long filopodia into the blastocoel, which come into contact with the walls of the cavity, adhere to them and contract, as if helping to direct the process of intussusception (Fig. 16-6). Later, at the point of contact of two contacting layers, the wall of the embryo breaks through, and a secondary mouth forms at the site of the break. Since the cells that directed the intussusception with their filopodia have completed their task, they separate from the blastoderm, move into the space between the intestinal tube and the body wall and turn into the so-called secondary mesenchyme, which will eventually give rise to the coelom wall and musculature.

As a result of gastrulation, the hollow spherical blastula turns into a three-layer structure: the inner layer, i.e. the wall of the primary intestine is called endoderm, the outer layer that remains outside is called ectoderm, and the intermediate loose layer of tissue, consisting of primary and secondary mesenchyme, is mesoderm. These are the three primary germ layers, characteristic of all higher animals. The organization of a three-layer embryo generally corresponds to the organization of an adult animal with a digestive tube inside, an epidermis outside and organs of connective tissue origin between them. (Fink R.D., McClay D.R., 1985, Gustafson T., Wolpert L., 1967, Hardin J.D., Cheng L.Y., 1986, McClay D.R., Wesel G.M., 1985, Wilt F.H., 1987, Ettensohn C.A., 1985, McClay D.R., Ettensohn C.A. , 1987, Odell G.M. et al., 1981.)

U different groups In coelenterates, gastrulation occurs in different ways; Using their example, we will analyze its methods. (The same methods of gastrulation are also found in other groups of animals). The blastula of many coelenterates has flagella and floats in the water column. One of the ways of gastrulation is that some blastula cells lose their flagella, acquire pseudopods and crawl into the internal cavity. The cells remaining on the surface change their shape (flatten) and close together, so that the wall of the blastula remains solid. The internal cells form loose tissue - parenchyma. This developmental stage is called parenchymula. Then the central cells partially die, and in their place a cavity is formed, and the remaining cells “line up”, forming the inner epithelial layer - endoderm (Fig. 148, A). This is how the larva characteristic of coelenterates is formed - planula. Soon it settles to the bottom, its front end turns into the sole of a polyp, and at the back a mouth forms and tentacles located around the mouth grow.

According to the theory developed by one of the largest embryologists of the 19th century. I.I. Mechnikov, approximately the same sequence of events was observed in the evolution of multicellular organisms. According to Mechnikov, the outer cells of the ancestral form of multicellular organisms were capable of phagocytosis and intracellular digestion. The “fed” cells crawled inside and digested the prey. Gradually, a division of functions arose between the cells of the outer and inner layers. The outer cells were responsible for movement and perception of signals, while the inner cells were responsible for capturing and digesting food. In another method, blastula cells simply divide so that the division planes are parallel to the surface of the ball (Fig. 148, B). In this case, immediately, bypassing the parenchyma, a planula is formed. In the third method, an invagination is formed at the posterior end of the blastula body, which turns into a deep canal. The hole formed at the site of invagination closes, and we get the same planula (Fig. 148, B). So in different ways development achieves the same result. This interesting feature embryonic development, observed in other processes, is called equifinality (from the Latin equi - equal and final - end). For example, everyone knows about the striking similarity of the embryos of different groups of vertebrates. But this similarity is achieved at a rather late stage of development with completely different initial stages (

Appendix 2. Embryological dictionary

Acrosome(from Greek. acros - extreme and soma - body; synonym - perforator) - a cytoplasmic sheath at the anterior end of the sperm head, developing as a result of a complex transformation of elements of the Golgi complex, as well as by condensation of granules of the acrosomal substance.

Allantois(from Greek alias - sausage and eideos - similar) - a temporary, provisional organ of the embryos of higher vertebrates. It is formed as a blind (sausage-like) outgrowth of the ventral wall of the terminal intestine, or from the wall of the yolk sac. The main role of A. is to serve as a container for isolating toxic products of nitrogen metabolism (urea and uric acid). In humans, A. is visible from the first days of the 3rd week of embryonic development as a finger-like outgrowth from the posterior wall of the yolk sac; in the 2nd month of embryogenesis it is reduced. At the same time, the role of A. is great in the formation of the extraembryonic bloodstream, since vessels grow along its body towards the chorion.

Amnion(from Greek amnion - a cup for the blood of an animal sacrificed by the ancient Greeks; here is the peri-embryonic vesicle. Synonyms - water shell, amniotic membrane ) - temporary, provisional organ of embryos of higher vertebrates (amniotes), creating aquatic environment certain chemical composition and pressure necessary for the development of the embryo. The high rate of exchange of amniotic fluid, which washes the embryo and then the fetus, provides it with a sterile aquatic environment, similar in composition to sea water.

Animal pole – part of the egg containing the largest amount of cytoplasm, where it is located cell nucleus before fertilization and where the reduction bodies are displaced.

Apoptosis(programmed cell death) - a method of cell death in embryonic or postnatal histogenesis as a result of activation internal program self-destruction.

Biogenetic law(from Greek bios - life and genesis - development, origin) - a law reflecting the relationship between the individual and historical development, which boil down to the fact that during individual development (ontogenesis) and especially in the embryonic period, living beings repeat main stages development of the entire series of ancestral forms.

Blastomeres(gr. blastos - sprout, germ + meros - part) - very large cells formed as a result of fragmentation of the zygote. The human preembryo consists of them during the period of fragmentation before the onset of implantation and until the blastocyst “sheds” the zona pellucida. Feature B. - lack of growth during the period between divisions, due to which the volume of cells with each successive division is halved (hence - fragmentation). There are large blastomeres called macromeres and small ones called micromeres. The main biological result of fragmentation is the somatization of the cells of the embryo, i.e., bringing the blastomeres to the “somatic standard,” which allows the nucleus to control the processes of synthesis and differentiation of body cells.

Blastopore:(gr. blastos - sprout + poros - opening) - an opening (primary mouth) leading into the closed cavity of the primary intestine (gastrocoel) in the embryo at the gastrulation stage.

Blastocyst(gr. blastos - sprout, rudiment + kystis - bubble; germinal vesicle) - an early stage of development of the human embryo and other mammals. The outer wall of the vesicle is formed by light blastomeres - trophectoderm. The latter take part in the education of the most important structural element chorion (placenta) - trophoblast. The cavity of the vesicle - the blastocoel - is filled with liquid. Adjacent to the inner side of the trophectoderm in the area of ​​the implantation pole is a cluster of dark blastomeres called the inner cell mass (embryoblast), from which the body of the embryo and some extraembryonic organs develop during mammalian embryogenesis: the amnion, yolk sac and allantois.

Blastula(Gr. blastos - sprout, rudiment) - an embryo at the end of the crushing period. In a typical case, it has the shape of a vesicle, the walls of which consist of blastomere cells, surrounding a cavity (blastocoel) - the primary body cavity.

Vegetative pole – part of the telolecithal egg, opposite the place of separation of the reduction bodies, rich in yolk, from which the organs of “plant” life of the embryo are formed, in particular the digestive ones.

Visceral layer of splanchnotome – a layer of unsegmented mesoderm adjacent to the endoderm.

Extraembryonic ectoderm - continuation of the ectoderm as part of some extra-embryonic organs, mainly in the embryonic membranes.

Gamons(from Greek gamos - marriage) - substances produced by the egg and sperm and interacting during fertilization according to the “antigen-antibody” type. There are gynogamon - fertilizin and androgamon - antifertilizin. The term “gamones,” by analogy with “hormones,” was proposed by M. Hartmann (1940).

Ganglion plate- an unpaired plate of neural material formed during neurulation when the neural tube closes at the site of fusion of the neural folds. The ganglion plate is located between the cutaneous ectoderm and the neural tube. The material of the ganglion plate is the source of the development of nerve ganglia.

Gastrocel - the cavity of the primary intestine, which arises in the embryo of many animals at the gastrula stage, filled with fluid and communicating with the external environment through an opening - the blastopore. Later it becomes the intestinal cavity.

Gastrula – a three-layer embryonic form formed as a result of gastrulation. Inside the gastrula there is a cavity - the gastrocoel, which communicates with the environment through the blastopore, or primary mouth.

Gastrulation(from Latin gaster - stomach) - a complex process of chemical and morphogenetic changes, accompanied by reproduction, growth, directed movement and differentiation of cells, resulting in the formation of germ layers: outer (ectoderm), middle (mesoderm) and internal (endoderm) - sources rudiments of tissues and organs.

Hyperplasia(from Greek hyper - over and plasis - formation) - an excessive increase in the number of cells as a result of their intensive reproduction.

Hypertrophy(from Greek hyper - over and trophe - nutrition) - an increase in the volume of cells and tissues due to an increase in cell mass, and not due to their reproduction (see hyperplasia).

Hypoblast(from Greek hypo - below, under and blastos - rudiment, embryo) - bottom layer germinal shield. In fish and birds, it grows in the form of a tongue between the outer layer of the blastodisc and the periblast at the beginning of gastrulation. The hypoblast material subsequently gives rise to endoderm.

Hypogenesis(from Greek hypo - below, under and genesis - development) - underdevelopment of organs or their parts.

Hypoplasia(from Greek hypo - below, under and plasis - formation) - underdevelopment of cells, their complexes and tissues due to disturbances in the normal course of histogenesis.

Histoblastic potencies(from Greek histos - tissue and blastos - rudiment, embryo) - the ability of the material of embryonic rudiments to give tissue as a result of subsequent development.

Histogenesis(from Greek histos - tissue and genesis - development) - the development of tissue from the cellular material of embryonic rudiments. By natural processes histogenesis are: cell reproduction, cell growth, cell movements (migration), differentiation of cells and their non-cellular derivatives, determination, intercellular and intertissue interactions, programmed cell death, etc. During embryonic histogenesis, various tissues are formed.

Holoblastic (complete) crushing – a process characteristic of isolecithal eggs in which cleavage planes divide the egg completely into equal parts, as in the starfish or sea urchin, or into unequal parts, as in the gastropod Crepidula. The fragmentation of the moderately telolecithal egg of the lancelet occurs according to the holoblastic type, however, the unevenness of division appears only after the stage of four blastomeres.

Cell degeneration(from lat. degenerare - degeneration) - degeneration of cells as a consequence of their differentiation (physiological degeneration) or the action of damaging factors (pathological degeneration).

Delamination – one of the methods of gastrulation, when the blastoderm splits into two layers of cells - outer and inner, which correspond to the ectoderm and endoderm.

Dermatome –(gr. derma - skin + tome - section) - the dorsolateral part of the somite, which is the rudiment of the connective tissue part of the skin (the skin itself).

Determination(from lat. determinare - determine) - the process of determining (fixing) the development path of the material of embryonic rudiments in the direction of the formation of specific tissue structures.

Discoblastula(gr.diskos - flat circle + blastos - sprout) - a blastula formed from a multiyolk egg as a result of incomplete crushing and discoidal crushing of blastomeres. It has the shape of a disk spread on the yolk. Discoblastula is characteristic of fish, birds and reptiles, the fragmentation of which is incomplete, partial, discoidal, meroblastic. The blastocoel in the discoblastula looks like a narrow gap between the outer layer of cells and the yolk.

Splitting up- the early stage of embryo development, following fertilization and the formation of a zygote, which is a series of successive mitotic divisions of a fertilized egg, not accompanied by the growth of resulting cells (blastomeres). In humans, the embryo at this stage is often called not an embryo, but a preembryo or conceptus. D. ends with the formation of a blastula, so the cells of the cleaving embryo are blastomeres. Biological results of D.: 1) acquisition of multicellularity; 2) the optimal ratio of nucleus and cytoplasm creates the prerequisites for normal cell differentiation; 3) acquisition of the primary internal environment. The course of D. is clearly determined and species-specific. It is determined by the amount of yolk in the egg and the characteristics of its distribution.

Vitelline endoderm – part of the endoderm lining the cavity of the yolk sac (absent in the human embryo).

Yolk sac- extra-embryonic (provisional) organ that performs a trophic function. It is formed in the embryos of fish, reptiles, birds and mammals. The source of development of these tissues is the extraembryonic endoderm and mesoderm. Inside the yolk sac there is a yolk, which serves as trophic material, due to which the embryo is nourished in the first stages of its development. The yolk sac is preserved even in placental mammals, probably due to some of its important properties: 1) the endoderm of the mammary sac serves as the site of localization of primary germ cells; 2) the mesoderm adjacent to the breast is the source of the formation of blood cells (both red and white), as well as the primary circulatory network, which serves to transport nutrients extracted from the yolk; 3) the vitelline endoderm takes part in a number of major inductive events that lead to the formation of individual important structures.

Germ layers- layers of the body of animal embryos that appear during gastrulation. There are three germ layers: the outer - ectoderm, the middle - mesoderm and the inner - endoderm. The germ layers consist of cells that differ in size, shape, relative position and other morphophysiological characteristics. The material of each germ layer is gradually determined and differentiated, differing specific features of its development.

Germinal shield- the thickened central section of the blastodisc of birds, from the material of which the body of the embryo is formed. The extraembryonic material of the blastodisc is located along the periphery of the scutellum.

Presumptive rudiments(from lat. presumptio - assumption, outline) - supposed, future rudiments of organs, isolated in the blastula on the basis of experimental data. From the presumptive primordia during the gastrulation period, actual embryonic primordia are formed.

Zygote(gr. zygotos - yoked, double-harnessed) - a fertilized egg with a diploid set of chromosomes. Essentially, it is a new single-celled organism with genetic information, inherited both paternally and maternally, which, as a result of fertilization, is genetically and ootypically determined to develop into a specific multicellular form.

Immigration(from Latin immigro - I move in) - one of the methods of gastrulation, in which individual cells immigrate inside the embryo and are located under its surface layer.

Intussusception- a method of gastrulation characteristic of isolecithal eggs, for which holoblastic cleavage is typical - invagination of cells of the vegetative pole, which leads to the formation of a two-layer embryo shaped like a cup.

Induction(from lat. inductio - guidance, motivation) - the mutual influence of some cells (or rudiments) on other cells (or rudiments), as a result of which the direction of their development is determined.

Mesenchyme(from sin, mesos - middle and enchyma - poured in) - embryonic tissue (according to other authors, embryonic rudiment) of the internal environment, which is a loose accumulation of process cells. Mesenchyme is formed mainly from mesoderm material (sclerotome, dermatome, visceral and parietal layers of splanchnotome). Part of the mesenchyme arises from the ectoderm. Connective tissues, cartilage, bones, and some muscles develop from mesenchyme. At the early stages of development, the mesenchyme is labilely determined, and the direction of its differentiation can be changed under the influence of various factors.

Mesoderm(gr. mesos - middle, intermediate + derma - skin) - the middle germ layer in multicellular animals (except sponges and coelenterates) and humans. Differentiated into segmented (somites) and non-segmented ( splanchnotome).The somites are distinguished: 1) outer segment - dermatome, 2) middle - myotome and 3) inner - sclerotome. The somites are connected to the splanchnotome by a segmental leg - nephrotome. The splanchnotome contains a secondary body cavity - the coelom. Derivatives of the mesoderm are tissues of the internal environment, the circulatory system, muscle tissue, epithelium of the renal and coelomic types.

Meroblastic (incomplete, partial) fragmentation – a series of successive mitotic divisions typical of telolecithal eggs rich in yolk; it is limited to a relatively small area at the animal pole. The cleavage planes do not extend through the entire egg and do not include the yolk, so that as a result of division, a small disc of cells (blastodisc) is formed at the animal pole. This crushing, also called discoidal, characteristic of reptiles and birds.

Myotome(from Greek mys, myos - muscle and tome - part) - a layer of cells between the dermatome and sclerotome in the middle part of the dorsal segment of the mesoderm, from which somatic muscles develop.

Myoepicardial plate(from Greek mys, myos - muscle, epi - above and kardia - heart) - areas of the visceral leaves of the splanchnotomes, from which cardiac muscle tissue and epicardium develop (the serous membrane covering the outside of the myocardium).

Morula– (gr. moron – mulberry) – stage of crushing, when the embryo is a compact bunch of blastomeres that does not contain a cavity inside.

Neurulation(from Greek neuron - nerve) is the process of separation of neural plate material from the skin ectoderm and the formation of the neural tube during the development of embryos of chordate-type animals.

Neural plate - thickening of the neural ectoderm on the dorsal side of the embryo as an early rudiment of the central nervous system in chordates and humans.

Neural tube- the rudiment of the nervous system. It is formed from the neural plate, which bends along the midline, turning into a groove. The latter turns into a tube by fusion of the right and left edges of the groove. In this case, the neural tube plunges under the skin ectoderm and is detached from it.

Nervous, or medullary, ridges- paired comb-like thickened folds of ectoderm located on the border between the neural plate (neuroectoderm) and cutaneous ectoderm. Nerve ganglia develop from the material of the neural folds.

Nephrotomes(from Greek nephros - kidney and tomos - segment) - paired segmented sections of mesoderm connecting the dorsal segments with the unsegmented splanchnotome. Nephrotomes are often called segmental legs. From the cephalic nephrotomes, the pronephros develops, while the trunk nephrotomes form the primary kidney (mesonephros). The non-segmented caudal metanephrogenic bud gives rise to a secondary bud.

Legs of somites - a small section of mesoderm connecting the somites with the splanchnotome, divided into segments - segmental legs (nephrogonotome), giving rise to the epithelium of the kidneys, gonads and vas deferens.

Fertilization(syn. - fertilization) - the most important biological phenomenon, consisting of a series of events of interaction and fusion of haploid male and female germ cells. O. can be external, occurring outside the body of the organism (most aquatic organisms) and internal, occurring in special organs of the body (in land animals and amphibians). As a result of fertilization, the diploid set of chromosomes characteristic of a given animal species is restored, and a qualitatively new cell appears—a fertilized one, which is a single-celled organism called a zygote. There are three stages of O.: 1) distant interaction, the essential aspects of which are positive taxis and capacitation of the sperm, as well as the production of peptide factors of distant interaction - gamones - by approaching sex cells (gametes); 2) contact interaction, during which, due to the movements of the sperm tails, the rotation of the oocyte begins and its “exposure” - denudation, shedding of follicular cells. An acrosomal reaction begins in sperm - acrosome enzymes destroy the zona pellucida of the oocyte, which facilitates the penetration of sperm into the periviteline space. The cortical reaction of the oocyte occurs. The zona pellucida becomes compacted and a fertilization membrane is formed, which ensures the blockade of polyspermy; 3) the last stage of O. - the synkaryon stage - the entry of the sperm (or its nucleus) into the body of the egg, leads to the subsequent fusion of the (haploid) nuclei of two parent cells - syngamy. The resulting zygote now has a double set of chromosomes and contains the genetic characteristics of the paternal and maternal organisms.

Organogenesis – the process of formation and development of organs during individual life - ontogenesis.

Axial complex of organ primordia- a set of rudiments of the nervous system, axial skeleton and somatic muscles in the embryogenesis of chordates. These rudiments are located symmetrically to the axis passing through the notochord or spine; the neural plate is located above the notochord, the intestinal endoderm is below, and the mesoderm is located on the sides.

Differentiation(lat. differentiatia - difference) - a process during which those parts of the genetically programmed properties of a maturing cell that are already determined by the vector (direction) of its determination are manifested and realized in time and space and characterized by the fact that initially identical young (undifferentiated) cells undergo gradual changes and acquire persistent internal differences from each other, become diverse - differentiated, and become capable of performing special functions.

Parietal layer of splanchnotome – a layer of unsegmented mesoderm adjacent to the ectoderm.

Primary streak- a section of the germinal disc of birds or the germinal shield of mammals from which the mesoderm is formed. During the process of gastrulation, the material of the primitive streak, folding over the edges of the primary groove, is immersed between the outer and inner layers of the embryo, forming the middle, mesodermal layer.

Primary Hensen's node - a thickening at the cephalic end of the primitive streak at the second stage of gastrulation, from which the prechordal plate and notochord develop.

Placenta hemochorial(from Greek haima - blood) - the placenta, in which the chorion destroys the epithelium, connective tissue and vascular wall of the uterine mucosa. The chorionic villi find themselves immersed in the lacunae through which maternal blood circulates.

Placenta desmochorionic(from Greek desmos - connection, connection; here - connective tissue) - the placenta, in which chorionic villi penetrate the connective tissue of the uterine mucosa, destroying its epithelium. A desmochorionic placenta is characteristic of ruminants.

Placenta discoidal- a placenta in which the villous portion of the chorion has the shape of a disk. The rest of the chorion is smooth. Such a placenta is found in baboons, great apes and humans. In marmosets and macaques, a bidiscoid placenta is formed, containing two disc-shaped sections of the villous chorion.

Placenta endotheliochorionic- placenta, as part of the cortex, the chorionic villi are adjacent to the endothelium of the uterine vessels, destroying the epithelium and connective tissue of the uterine mucosa, as well as part of the vascular wall along the way of their growth.

Placenta epitheliochorionic- semi-placenta, in which the chorionic epithelium is only in contact with the uterine epithelium. The chorionic villi enter the dilated openings of the uterine glands and are freely pulled out of them during childbirth (like fingers from a glove). Epitheliochorial placenta is found in some ungulates (eg, pig, camel, hippopotamus).

Postnatal period(gr. post - after + lat. natus - birth) – a developmental process that arises or occurs immediately after birth.

Prechordal plate(lat. pre - in front + gr. chorde - string) - a section of ectoderm immersed inside the embryo and located anterior to the head end of the chord. The PP material is used to build the external muscles of the eyeball and serves as a rudiment for the epithelium of the foregut (esophagus) and respiratory tract.

Provisional authorities(from lat. provisorich - preliminary, temporary) - temporary, non-definitive organs of the embryo that have adaptive significance for its development. These include: the yolk sac, amnion, allantois and serous membrane - in birds and reptiles; yolk sac, amnion, allantois and chorion - in mammals and humans.

Progenesis(gr. pro – forward + genesis – origin, emergence) is a term that summarizes the set of processes that take place during the emergence and differentiation of gametes up to the moment of their fusion during fertilization.

Rheotaxis – expedient movement of cells towards the flow of pipe fluid or positioning of the body parallel to it.

Serosa, or serous membrane- is one of the provisional (temporary, extra-embryonic) organs in birds and reptiles. It develops from the outer layer of the amniotic fold, consisting of extraembryonic ectoderm and parietal mesoderm. The material of these rudiments turns into serous epithelium and mesenchyme with blood vessels. The serous membrane performs respiratory and trophic functions.

Sincarion(sincaryon; syn - + Greek karyon - core; synonym: amphinucleus, amphikarion) – the nucleus of a zygote, formed by the fusion of the nuclei of male and female germ cells during the process of fertilization and containing a diploid set of chromosomes.

Trunk fold- a ring-shaped interception that separates the body of the developing embryo from the yolk sac.

Sclerotome –(Gr. scleros - hard + tome - cutting, dissection) - a segmented rudiment of the axial skeleton in the embryos of chordates and humans. Develops from the inner part of the somites. During embryonic histogenesis, it gives rise to skeletal-type tissues.

Somites(gr. soma - body; syn. - dorsal segment) - symmetrical paired metameric sections of the dorsal mesoderm. Each pair of S. is located on the sides of the neural groove (neural tube) and notochord. During subsequent differentiation, each of them is the basis for development: a) external - dermatome, b) middle - myotome and c) internal - sclerotome. Each new couple somites are formed during embryogenesis behind an already formed pair at certain intervals (on average 6.5 hours).

Sperm(gr. sperma - seed + zoon - animal; syn. - sperm, lively) - mature male sex cell, formed as a result of spermatogenesis, capable of fertilizing an egg. It is haploid, but can, unlike the female reproductive cell, contain either an X or a Y sex chromosome. Cells in higher animals and humans have a highly elongated shape, and the size (volume) of these cells in most male animals is significantly less than any somatic cell: its length = 50-60 µm, and maximum diameter = 6-7 µm; its volume = 4.7 µm 3 (volume of the human egg = 1.4 * 10 6 µm).

Splanchnotome(from Greek splanchnon - insides and tomos - segment) - the unsegmented ventral part of the mesoderm in vertebrate embryos. It consists of two layers: the outer - parietal and the inner - visceral. Between the leaves there is a slit-like space - the secondary body cavity (coelom). From the leaves of the SP arise the mesothelium, myocardium and epicardium, the adrenal cortex, the gonadal epithelium, and mesenchymal cells in the embryonic period of development.

Trophoblast(gr. trophe - nutrition + blastos - rudiment) - the rudiment of extra-embryonic contact-barrier structures in placental mammals - chorion and placenta; first forms a single-layer wall of the embryonic vesicle, a kind of mammalian blastula - blastocyst). On early stages T. ensures contact of the embryo with the maternal body, participates in its implantation into the wall of the uterus and then in the formation of the chorion and placenta. In humans and primates, this is a complex multilayered epithelial-like structure, which is considered as a collective concept and includes:

1) cytotrophoblast of chorionic villi and 2) syncytiotrophoblast.

Chemotaxis(gr. chemeia - from the word “chemical” + taxys - location) - the expedient movement of cells along the concentration gradient of chemical stimuli (chemoattractants).

Chord(Gr. chorde - gut, string; syn. dorsal string, notochord) - a cellular cord located along the axis of the embryo, around which the spine is formed. During ontogenesis, the notochord is reduced, remaining in the adult only in the form of the nucleus pulposus of the intervertebral discs.

Chorion(Gr. chorion - skin) is an extra-embryonic organ of placental mammals, clearly differentiated, starting from the period of gastrulation. The type of its differentiation depends on what method of placentation will be formed - with or without implantation into the mucosa. In any case, the trophoblast and a derivative of the mesoderm - differentiating mesenchyme with blood vessels - take part in the formation of X. From the moment of placentation, X. acquires the features of its structural and functional organization inherent in one or another form of the placenta. In humans and primates there are: a) villous and b) smooth X., which closely interact with the main and capsular membranes of the uterine mucosa. Subsequently, the villous X. is transformed into the placenta. Smooth X. forms a plate that from the inside, through the amniochorial space, contacts the amniotic membrane, and, on the other hand, forms a zone of long-term contact of genetically heterogeneous tissues (peripheral cytotrophoblast and decidual cells), is a continuation of the main plate outside the marginal sinus of the discoidal placenta .

Coeloblastula – a single-layer thin-walled vesicle with a large cavity (blastocoel), which is formed by low-yolk eggs.

Overall(gr. zoiloma - depression, cavity; synonym - secondary cavity) - a cavity inside the embryo that arises by moving apart the cells of the dense mesodermal rudiment. In mammals and humans, the pericardial, pleural, and peritoneal cavities lined with mesothelium (intraembryonic cavities) are formed from c. In addition to intraembryonic, in embryology there is an extraembryonic c. (syn. - exocoelom, exocoelomic cavity), which arises in the early stages of development of vertebrate mammals and humans due to the filling of the blastocyst cavity with elements of the extraembryonic mesoderm moving out from the embryonic disc.

Ectoderm(rp. ectos - outside + derma - skin) - the outer germ layer, differentiated at the stage of gastrulation (three-layer embryo). There are germinal and extraembryonic E. Germinal E. is divided into: 1) neuroectoderm, represented, in turn, by: a) the neural tube and b) neural crest, and: 2) integumentary E., consisting of: a) cutaneous E., covering the embryo from the outside and forming the epithelium of the skin and its derivatives; b) E. oral bay, invaginating under the outer cover (taking part in the formation of: the oral bay itself, the epithelium of the airways and the respiratory part of the lungs); c) on the sides of the head, two paired thickenings of E. are formed - placodes, which then sink into the underlying tissue and lose connection with the outer cover. The otic vesicle and the lens of the eye are formed from the placodes. Extraembryonic E. forms the epithelium of the amnion and umbilical cord.

Embryoblast(gr. embryon - embryo + blastos - rudiment) - the old name for the inner cell mass - an accumulation of blastomeres - blastocytes protruding into its cavity. In fact, these are just blastomeres of the inner cell mass, used to build the body of the embryo.

Embryogenesis– the early period of individual development of the organism, lasting from the moment of fertilization to birth (in viviparous organisms) or before hatching from the egg (in oviparous organisms). Embryogenesis is divided into the following main periods: fertilization, fragmentation, gastrulation, formation of axial organ primordia, histo- and organogenesis.

Embryology- (gr. em bryo - in shells) - the science of the development of animals from the moment of fertilization to birth (for viviparous animals), hatching from an egg (for oviparous animals), the end of the period of metamorphosis (for animals with the larval stage of development) or cessation of nutrition for counting the reserves of nutritional material in the egg. In other words, embryology is usually considered as the science of embryo development.

Endoderm(gr. entos - inside, + dermis - skin) - the inner layer of the embryo of multicellular animals and humans in the early stages of development - the inner germ layer. There are: a) germinal E., which forms the epithelium of the gastrointestinal tract and its glands; b) extraembryonic E., forming the epithelium of the yolk sac and, possibly, in some types of allantois. The proximal part of the latter goes to build the epithelium of the bladder.

Epiblast(gr. epi - on, above, over + blastos - sprout, embryo) - arises as a result of the transformation of blastomeres of the inner cell mass in the second week of development in humans and primates. As a result of the splitting of the internal cell mass in the blastocyst and its differentiation, the following are formed: 1) the outer layer - E., adjacent to the trophectoderm, and 2) the inner layer - the hypoblast, facing the cavity of the blastocyst. E. is represented by orderly arranged cells resembling multirow prismatic epithelium. Small cavities appear between E. cells, which, merging, form a common cavity - the future amniotic cavity (cavitation amnion). Due to the divergence of the roof cells of the cavity to the sides, the trophoblast section turns out to be temporarily its roof (Rauberian trophoblast). Then the lateral sections of the growing ectoderm form upward-directed folds (amniotic extraembryonic ectoderm), which, merging, form the roof of the amniotic cavity and, in general, the amniotic vesicle. The rest of the E., which makes up the bottom of the formed amniotic sac, is the material of the developing germinal disc.

Epiboly(from Greek epibolé - throwing, laying, layer) – one of the methods of gastrulation, in which the cells of the animal region of the embryo grow onto its vegetative region - the future endoderm.

Egg(Latin ovum - egg) - a mature female reproductive cell formed in the ovary as a result of oogenesis. The most significant properties of the chromosome include: 1) a haploid set of chromosomes (with an X sex chromosome); 2) very large size, due to an unusually extensive cytoplasm with a relatively small nucleus; 3) the presence of yolk in the cytoplasm - a complex of protein-lipid granules intended to nourish the embryo. In humans, the egg has a diameter of about 130 microns. In addition to the plasma membrane, the egg also has secondary membranes: a) a shiny or transparent zone (zona pellucida); and outside b) follicular - a layer of follicular cells.

BLASTOPORE BLASTOPORE

(from blasto... and Greek poros - passage, opening), primary mouth, gastropore (obsolete), opening, through which in embryos plural. In multicellular animals, the gastrocoel communicates with the environment. In most animals, B. is laid at the vegetative pole or at a certain distance from it (varying depending on the amount of yolk in the egg); in hydroids and ctenophores - on the animal. In animals, the gastrula is formed by invagination, the edges of the gastrula are usually called. lips; There are dorsal (dorsal), lateral (lateral) and ventral (ventral) lips. In lower vertebrates, chordomesoderm material is rolled into the embryo through the dorsal and lateral lips; As the endoderm or yolk becomes overgrown with ectoderm, the lips of the B. close (see Fig.). The yolk-rich blastomeres remaining on the outside form a yolk plug that closes the yolk plug. In birds and mammals, the homologue of the amphibian B. is the primitive streak. In protostomes, one part of the B. becomes the definitive mouth, and the other - the anus; in deuterostomes, the definitive mouth arises independently of the B. in another place, and the B. turns into an anus or into a provisional organ - neuroenteric. channel.

.(Source: “Biological Encyclopedic Dictionary.” Editor-in-chief M. S. Gilyarov; Editorial Board: A. A. Babaev, G. G. Vinberg, G. A. Zavarzin and others - 2nd ed., corrected - M.: Sov. Encyclopedia, 1986.)

Synonyms:

See what "BLASTOPORE" is in other dictionaries:

Blastopore... Spelling dictionary-reference book

- (from other Greek. βλαστός sprout + other Greek. πόρος passage, opening; Primary mouth) an opening through which the cavity of the primary intestine of an animal embryo at the gastrula stage communicates with the environment. Most... ... Wikipedia

BLASTOPORE- (from the Greek blastos rudiment and poros opening), primary mouth (German Urmund), an embryological term proposed by Haeckel to designate the opening connecting the cavity of the gastrorule (primary gut) with the external environment. Blastopore is clear... ... Great Medical Encyclopedia

Hole, gastropore, hole Dictionary of Russian synonyms. blastopore noun, number of synonyms: 2 gastropores (1) ... Synonym dictionary

- (from the Greek blastos embryo and poros opening) (primary mouth), the opening through which the embryonic gut of the gastrula (see GASTRULA) communicates with the external environment ... encyclopedic Dictionary

Blastopore- * blastapore * blastopore is an embryonic structure that forms the endoderm during gastrulation during invagination... Genetics. encyclopedic Dictionary

- (gr. blastos sprout + poros passage, hole) biol. otherwise, gastropore is the opening in the animal embryo at the gastrula stage, through which its cavity communicates with the environment. New dictionary foreign words. by EdwART, 2009. blastopore... ... Dictionary of foreign words of the Russian language

BLASTOPORE- (blastopore) a hole resulting from invagination of the surface layer of the embryo (gastrula) at an early stage of its development. In humans, the blastopore is very small in size; subsequently, the primary gut is formed from it (see Archenteron) ... Explanatory dictionary of medicine

blastopore- ANIMAL EMBRYOLOGY BLASTOPORE, PRIMARY MOUTH - the opening through which the embryonic gut of the gastrula of multicellular animals communicates with the environment ... General embryology: Terminological dictionary

- (blastoporus, LNE; blasto + Greek poros opening; syn. primary mouth) opening through which the gastrula cavity communicates with the environment; in higher vertebrates it looks like a long narrow depression (primary groove) ... Large medical dictionary

The embryonic period has next steps: fragmentation, gastrulation, histo- and organogenesis.

. Splitting up. As a result of crushing, a blastula is formed;

. Gastrulation. As a result of gastrulation, germ layers are formed;

. Histo- and organogenesis- formation of tissues and axial organs of the embryo.

In the larval form of ontogenesis, the embryonic period begins with the formation of the zygote and ends with the emergence from the egg.

cervical shells. In the non-larval form of ontogenesis, this period begins with the formation of the zygote and ends with the emergence from the embryonic membranes. In the intrauterine form of ontogenesis - from the formation of the zygote to the birth of the individual.

Splitting up

As a result of fertilization, a zygote is formed, which begins to fragment. Cleavage is accompanied by mitotic divisions. There is no post-mitotic period between divisions in a short interphase, and DNA synthesis begins in the telophase of the previous mitotic division. The embryo does not grow. The total volume of the embryo does not change. The cells formed during the cleavage process are called blastomeres, and the embryo - blastula. The type of crushing depends on the quantity and distribution of yolk in the egg (Fig. 67).

Rice. 67.Types of eggs and methods of their crushing (eggs and embryos are depicted with the animal pole facing up).

I- isolecithal egg (a) and its uniform crushing (b); II And III- telolecithal eggs (c, e), and their crushing - discoidal (d) and uneven (f); IV- centrolecithal egg (g) and its superficial crushing (h).

Types of crushing

Splitting upcan be complete uniform, complete uneven, incomplete discoidal, superficial (Fig. 68).

Full uniform crushing is characteristic of isolecithal eggs - for example, lancelet. The zygote nucleus divides by mitosis into

Rice. 68.Types of eggs and their corresponding types of crushing.

two, then the cytoplasm divides. The cleavage furrow runs along the meridian, and two blastomeres are formed. Then the nucleus divides again and a second cleavage furrow appears on the surface of the embryo, running along the meridian, perpendicular to the first. 4 blastomeres are formed, the 3rd groove runs along the equator and divides it into 8 parts. Then there is an alternation of meridional and equatorial fragmentation. The number of blastomeres increases. The embryo at the stage of 32 blastomeres is called Morula. Crushing continues until the formation of an embryo, similar to a vesicle, the walls of which are formed by a single layer of cells called blastoderm. The blastomeres diverge from the center of the embryo, forming a cavity called primary, or blastocoel. The blastomeres are the same size. As a result of such fragmentation, coeloblastula.

Complete uneven crushing is characteristic of telolecithal eggs with a moderate yolk content, for example in frogs. The 1st and 2nd cleavage furrows run along the meridians and completely divide the egg into 4 parts, the 3rd furrow is shifted towards the animal pole, where there is no yolk. Blastomeres are of unequal size: at the animal pole they are smaller (micromeres), on vegetative more (macromeres). The yolk makes crushing difficult, and therefore the crushing of macromeres is slower than micromeres. The wall of the blastula consists of several rows of cells. The primary cavity is small and shifted towards the animal pole. Formed amphiblastula.

Incomplete discoid Telolecithal eggs with a high yolk content, for example in reptiles and birds, undergo crushing. Cleavage occurs only at the animal pole. The 1st and 2nd cleavage furrows run along the meridian perpendicular to each other, the 3rd furrow is shifted towards the animal pole. As a result of this, it is formed germinal disc. The blastocoel is located under the blastoderm layer in the form of a slit. Blastula is called discoblastula.

Incomplete surface crushing characteristic of centrolecithal eggs, for example in arthropods. Centrolecithal nuclei

New eggs divide repeatedly and move to the periphery, where there is no yolk in the cytoplasm. The cytoplasm forms blastomeres. The blastula has one layer of blastomeres. The blastocoel is filled with yolk. This blastula is called periblastula.

Gastrulation

At the end of the period of fragmentation in multicellular animals, the period of formation of germ layers begins - gastrulation. Gastrulation is associated with the movement of embryonic material. First it is formed early gastrula, having two germ layers (ectoderm and endoderm), then late gastrula, when the 3rd germ layer is formed - mesoderm. The embryo formed as a result of gastrulation is called gastrula.

The formation of early gastrula occurs in the following ways:

. immigration(cell eviction), in coelenterates;

. intussusception(invagination), at the lancelet;

. epiboly(fouling), in a frog;

. delamination(cleavage), in coelenterates.

At immigration Some blastoderm cells from the surface of the embryo go into the blastocoel. An outer layer is formed - ectoderm and an inner layer - endoderm. The blastocoel is filled with cells.

At intussusception a certain portion of the blastoderm (vegetative pole) bends inward and reaches the animal pole. A two-layer gastrula embryo is formed. The outer layer of cells is ectoderm, the inner layer is endoderm, lining the cavity of the primary intestine. (gastrocoel). The opening through which the cavity communicates with the external environment is called the primary mouth - blastopore. U protostomes(worms, mollusks, arthropods) it turns into mouth opening, at deuterostomes- V anal hole, and the mouth is formed at the opposite end (chordates).

Epibolycharacteristic of animals that develop from telolecithal eggs. The formation of gastrula occurs due to the rapid

rapid reproduction of micromeres that overgrow the vegetative pole. Macromeres end up inside the embryo. Blastopore formation does not occur and there is no gastrocoel.

Delaminationfound in coelenterates, whose blastula is similar to the morula. Blastodermal cells are divided into outer and inner layers. The outer layer forms the ectoderm, the inner layer forms the endoderm (Fig. 69).

In all multicellular organisms, except sponges and coelenterates, a 3rd germ layer appears - mesoderm. Mesoderm formation occurs in two ways: teloblastic or enterocoelous.

Teloblastic the method is characteristic of protostomes. At the border between the ectoderm and endoderm on each side of the blastopore

Rice. 69. Types of gastrulae.

a - intussusception gastrula; b, c - two stages of development of the immigration gastrula; d, e - two stages of development of the delamination gastrula; e, and- two stages of development of the epibolic gastrula; 1 - ectoderm; 2 - endoderm; 3 - blastocoel.

cells - teloblasts- begin to divide and give rise to mesoderm.

Enterocelic method characteristic of deuterostomes. The cells that form the mesoderm are isolated in the form of pockets of the primary gut. Pocket cavities are transformed into a whole. The mesoderm is divided into separate sections - somites, from which certain tissues and organs are formed.

Histo- and organogenesis

After the formation of mesoderm, the process of histo- and organogenesis begins. First, the axial organs are formed - the neural tube, notochord, then all other organs (Fig. 70).

In the lancelet, from the ectoderm on the dorsal side of the embryo, neural tube. The rest of the ectoderm forms the skin epithelium and its derivatives. From ento- and mesoderm under the neural tube, a notochord is formed. Below the chord is intestinal tube, on the sides of the chord - somite mesoderm. The outer part of the somite, adjacent to the ectoderm, is called the dermotome. It forms the connective tissue of the skin. Interior - sclerotome- gives rise to the skeleton. Between the dermotome and sclerotome is myotome, giving rise to striated muscles. Under the somites are the legs (nephrogonotome), from which the genitourinary system is formed.

Coelomic bags are formed symmetrically on the sides. The walls of the coelomic sacs facing the intestine are

Rice. 70.Formation of mesoderm (according to Shimkevich, 1925, modified).

a - in protostomes; b- in deuterostomes;

1 - ectoderm; 2 - mesenchyme; 3 - endoderm; 4 - teloblast (a) and coelomic mesoderm ( b).

they call splanchnopleura, towards the ectoderm - somatopleura. These leaves are involved in the formation of the cardiovascular system, pleura, peritoneum, and pericardium (Fig. 71).

Thus, from ectoderm skin epithelium, skin glands, tooth enamel, hair, nails, and claws are formed.

From endoderm the epithelium of the midgut, liver, pancreas, thyroid, thymus, epithelium of the respiratory system.

Mesodermparticipates in the formation of muscles, connective tissue, bone tissue, channels of the excretory system, circulatory system, and part of the tissue of the gonads.

Topic 3

Gametogenesis. Embryogenesis

1. Crushing. Types of crushing.

2. Blastula. Types of blastulas. Influence of factors external environment on crushing processes.

3. Gastrulation.

4. Histogenesis and organogenesis.

5. Theory of germ layers.

6. Derivatives of germ layers.

Antipchuk, Yu.P. Histology with the basics of embryology / Yu.P. Antipchuk. – M.: Education, 1983. – 240 p.

Almazov, I.V., Sutulov L.S. Atlas of histology and embryology / I.V. Almazov, L.S. Sutulov. – M.: Medicine, 1978. – 148 p.

Histology / ed. Yu.I. Afanasyeva. – M: Medicine, 1989. – 361 p.

Ryabov, K.P. Histology with the basics of embryology / K.P. Ryabov. – Mn.: Higher. school, 1991. – 289 p.

Biological encyclopedic dictionary / ed. M.S. Gilyarov. – M.: Sov. Encycl., 1989. – 864 p.

Workshop on histology, cytology and embryology / ed. ON THE. Yurina, A.I. Radostina. – M.: Higher. school, 1989. – 154 p.

Ham A., Cormick D. Histology / A. Ham, D. Cormick. – M.: Mir, 1983. – 192

1. CRUSHING

After fertilization, the egg begins to divide. Aboutthe process of egg division, leading to an increase in the number of cells without their growth, is called crushing. The nature of crushing is determined by the amount of yolk in the egg and its distribution. Depending on this, crushing can be complete or partial. Crushing, in which eggcompletely divided, called holoblastic , but not completely, partially– meroblastic.

Complete crushing eggs may be uniform Anduneven.

Complete uniform crushing characteristic eggs of the lancelet, whose eggs have little yolk.With this type of fragmentation, the resulting cells, or blasto measures,have approximately the same dimensions.

Totally unequaldimensional crushing characteristic of eggscartilaginous fish, amphibians, whose eggs have painabove the yolk. With this type of cleavage, blastomeres of unequal size are formed, with the first two cleavagesgive equal blastomeres, and then the blastomeres located at the animal pole divide faster, become smaller,than in the vegetative one.

Incomplete crushing eggs may be superficial And discoidal.

At incomplete surface crushing, towhich is characteristic of many arthropods, is crushed only on topnal part of the egg, and its central part, richyolk, does not divide.

At incomplete discoidal crush research institute, which is typical, for example, of presma for penitent birds, only a small area is crushed on topity of the egg, where there is little yolk, and the rest of it, morecoated with yolk, does not divide. Due to the fact that with this typecrushing, the surface area of ​​the crushing egg hasthe shape of a disk, then crushing received the name discoi distant (Fig. 1).

Rice. 1. Types of egg crushing (original):

1 – full uniform; 2 – completely uneven; 3 – incomplete superficial; 4 – incomplete discondal.

The nature of crushing is determined not only by the quantityyolk and its distribution in the egg, but also the relative arrangement of cells that are formed as a result of crushing.

According to crushing direction radial, spiral, bisymmetrical, or bilateral crushing is distinguished.

At radial crushing, which is characteristic of coelenteratesnym, echinoderms and many lower chordates, top row blaThe stomer is located exactly above the lower one.

At spiral crushing,which is observed in most worms and piersLyuskov, the upper row of blastomeres is located between the blastomeres of the lower row.

At bilateral crushing, which is typical roundworms, ascidians, blastomeres locatedare located symmetrically on the sides of the original blastomere. Followsnote that in the process of egg fragmentation there is oftenplace combination various types crushing.

2. BLASTULA, OR SINGLE-LAYER EMBRYOM

During the crushing process, a multicellular single layer appearsembryo, which looks like a ball, in many casesteas with a cavity in the middle. Such an embryo is called blast loy . The layer of cells that forms the wall of the blastula is called blah stoderma (derma – skin), and the cavity of the blastula – blastocoele ( koilon cavity) or primary body cavity. Whenwhen crushed, a spherical embryo without a cavity appearsinside, similar to a mulberry, it is called pestilence loy (morula mulberry). This blastula is characteristicon, for example, placental mammals.

Depending on the type of egg fragmentation distinguish besides morula, there are 5 more types of blastula: coeloblastula, amphiblastula, steroblastula, periblastula and discoblastula(Fig. 2).

Blastula, the blastoderm of which is formed byit is a layer of cells, and the blastocoel occupies the central positiontion is called a typical blastula or coeloblastula . Such a blastula is characteristic, for example, of the lancelet.

Amphiblastula differs from a typical blastula in that its blastocoel ispuppy to the animal pole - the zone of small blastomeres, and bla The stoderma is represented by several rows of cells. Suchblastula is characteristic of amphibians, from which it received its Name.

Steroblastula (sterros – dense) consists of large blastoderm cells, which are so deep behindgo into the cavity of the blastocoel, so that nothing remains of the latterYes. Some arthropods have such a blastula.

IN periblastula , arising from superficial crushing of the egg, the blastoderm cells are locatedalong the periphery of the uncrushed yolk and blastocoelNo. This type of blastula is characteristic of some insects.

Disco ideal crushing leads to the formation discoblastula , the blastocoel of which looks like a narrow slit located undercells of the blastoderm, called in this case embryoyour disk. This type of blastula is observed in bony fishes,reptiles, birds.

Rice. 2. Types of blastulas (original):

/ – morula; 2 – csloblastula; 3 – amphnblastula; 4 – discoblastula; 5– sterrobla of the chair; 6 – periblastula; 7 – crying.

The considered diversity of blastulae shows that ondifferentiation takes place at this stage of development of organismscellular material of the embryo, associated with the amount and location of the yolk, with the content and synthesis of DNA, with the timedivision of substances in the cytoplasm of cells.

For crushing processes and early stages development of organizationsmov influence environmental conditions , because thethe development of the organism occurs in the environment in which it took placeits formation in historical development. Insignificant significant deviations of environmental conditions from typical ones affect thenew only at the rate of crushing.

To environmental factors , which affect crushingeggs and early stages of organism development, follows inprimarily include the presence of moisture, chemical compositionand the reaction of the environment (pH), luminous flux, temperature, presencenutrients, oxygen, etc.

3. GASTRULATION

Gastrulation – this is a process of educationformation of a two-layer embryo.

In a broader view, astrulation – complex process of chemical and morphological changes, accompanied by reproduction, growth, directed movement and differentiation of cells, resulting in the formation of germ layers, sources of tissue and organ primordia, complexes of axial organs.

Gastrulation in most animals occurs in two stages. At the first stage, 2 germ layers are formed ( ectoderm and endoderm), and on the second the third 3 germ layer is formed ( mesoderm) and further embryo formation occurs.

There are four types, or methods, of the formation of a two-layer embryo: possession, or immigration; invagination, or intussusception; fouling, or epiboly; separation or delamination .

The initial one in the process of phylogenetic development and the mostThe most primitive form of gastrulation is move-in , orimmigration . This way of educationbilayer embryo is that individual cellsor groups of cells actively move from the blastoderm toblastocoel. These cells, resulting from cell divisionblastoderm current, settle on its inner surface, formchewing the inner layer of cells, or endoderm, and blasto cellsThe dermis located on the outside are already called ectoderm.If the invasion of cells into the blastocoel takes place only with a hundredrons of one vegetative pole of the blastula, then such immigrace is called unipolar or single-pole, and whenyes from various parts of the blastoderm, – multipolar or multi-pole. Unipolar immigration is not typical which hydroid polyps, jellyfish and almost all hydromasDuzam. Multipolar immigration is a rare phenomenon and is observed in some jellyfish. Upon immigration gastral nay cavity,or gastrocel (cavity of the primary intestine), obrais formed either simultaneously with the appearance of a two-layerbirth, as is the case, for example, with hydromedusas, ormuch later, as in many species of coelenterates.In the latter, such a massive invasion of cells is observed that the blastocoel is completely filled with them, the cavity disappears, and only subsequently, with the separation of the endoderm, thethe gastrocele disappears, naturally, having no connection with the externalenvironment. This method of gastrulation is especially common amongdi bilayer animals– coelenterates. All other vocational trainingGastrulation patterns are derivatives of immigration.

If gastrulation is carried out by invaginations, orintussusception (in - penetration, vagina - shell, moisture lishche), then the single-layer wall of the blastula– blastoderm – pro bends inside the blastocoel and reaches the opposite wallki. Invagination begins from the vegetative pole, whichrecognized by larger cells, and goes in the directionanimal pole. At the end of intussusception, the embryonew bilayer, since the blastoderm forms two layerscells: outer - ectoderm and inner - endoderm.After invagination, a cavity is formed, which is surrounded by endoderm cells. This cavity is called the gastric cavityloss. The gastrocoel communicates with the external environment through an openingwhich is called primary mouth or blastopore). The edges of this hole are called lips.

In progress evolutionary development representatives of the bellyof the kingdom, the formation of a blastopore during gastrulation byserved as the basis for dividing animals into two largegroups based on characteristics such as formation of the primary mouth. In worms, mollusks, arthropods, the primary mouth developschanging and differentiating, it turns into a constant, or definitive, mouth of an adult. Such animals are called lane second-mouthed. In echinoderms, ki cervically breathing, chordate blastopores transform into posteriorpassage, or anal, opening, or into the neurointestinalcanal, which is located at the posterior end of the embryo, and the mouththe opening appears anew at the anterior end of the abdominal cavitysurface of the embryo. Such animals are called deuterostomes mi . Gastrulation by invagination is very common inlower deuterostomes and chordates.

The fate of the blastocoel during invagination can be twofold:it can be preserved if the cell layers of the ectoderm and endoderm do not touch each other and thentwo cavities are observed simultaneously in the embryo– blasto goal and gastro goal. If the cell layers of ectoderm and entoWhen invaginated, the dermis touches along its entire length, thenthe blastocoel disappears and the embryo has only one cavity – the cavity of the primary intestine, or gastrocoel.

In some vertebrates, due to an increase in amount of yolk in the egg and insignificant sizemi blastocoel, pushed towards the animal pole, gastrulation only by the type of intussusception becomes impossible. Tog yes, in some cyclostomes and amphibians to invagination when connects fouling, or epiboly . Gastrulation by epiboly consists in the fact that cellsblastoderms of the animal pole of the blastula divide significantlybut faster than the cells of the vegetative pole. Due to thissmall cells of the animal pole gradually from outside tocrawl onto large cells of the vegetative pole, overgrowing themand forming the ectoderm, and the cells of the vegetative pole formingthey contain endoderm. During gastrulation by epiboly in its pure form,as is the case in some invertebrates, at first there is noneither blastopore nor gastrocoel, which arise significantlyLater. In those cases when epiboly accompanies invagi nations, as is observed in some bony fishes, terrestrial aquatic, then a blastopore appears, as well as the cavity of the primary intestine, which exist along with the blastocoel.

In bony fish, reptiles, birds and oviparous fish mammals whose eggs have a lot of yellowHowever, in the first phase of gastrulation, a two-layer embryo is formedby delaminations, or delaminations, blastoderm cell layer. This type of formation of a two-layer embryo is observedfound in coelenterates, as well as in higher placentalsmammals whose eggs have lost large reservessy yolk. Gastrulation by delamination consists of divisionblastoderm cells, and the resulting daughter cellscells are pushed into the blastocoel and form the inner layercells, or endoderm, and the outer layer of cells, or the former blastoderm, is transformed into ectoderm. When daughter cells are pushed into the blastocoel, we are dealing withcalled primary delamination, and with secondary dividingnations, the cells of the future ectoderm, on the contrary, are pushed asideshoot. With this method of gastrulation, a blastopore is not formedand the gastrocoel, naturally, does not communicate with the external environment.

It should be noted that, despite the originality of the departmentdifferent methods of gastrulation, in the process of formation of a multilayerof a new embryo, there is often a combination of differenttypes of gastrulation– two or even three types.

Formation of the third layer of cells, or third embryo th leaf– mesoderm, during embryonic developmentanimals is carried out in four ways: teloblastic ski, enterocelous, ectodermal and mixed, or transitional .

In protostomes, the formation of mesoderm is carried outis formed due to several large cells locatedduring gastrulation at the border between the ectoderm and endoderm on the sides of the blastopore. The origin of these cells is not clear, nor is their relationship to any of the germsneck leaves, since they are separated already at the crushing stage. The daughter cells of these cells are locatedbetween the ectoderm and endoderm and form the third layer cells – mesoderm. Since the original cells formingmesoderm, throughout the entire time located on the backend of the embryo, then according to their localization they arebeamed the name teloblasts , and the method of meso formation itself dermis – teloblastic . Secondary body cavity of the embryosha, or in general, formed by mesoderm cells due to their bundles.

In deuterostomes, including lower chordates, mesodermformed from endoderm cells. With this methodformation of the mesoderm of the cell wall of the primary intestine, or endoderWe, intensively multiplying, form on the sides of the primaryintestines are saccular protrusions into the blastocoel. These bulgesvania, growing into the blastocoel, are located between the ectoderesmine and endoderm. Subsequently, the endodermal protrusionsnia are detached from the endoderm, and the integrity of the wall of the featherThe intestine and endodermal outgrowths are restored by cell proliferation. After separation from the primaryIntestines, cellular material of endodermal origin is called the third germ layer or mesoderm. Oncemelting, the mesoderm fills the entire cavity of the blastocoel, andthe loss of bag-like outgrowths, being in origin by The cavity of the gastrocoel is called the secondary body cavity.This method of mesoderm formation is calledenterocoelous , since mesodermoriginates from the endoderm with the simultaneous formation of ce scrap

In reptiles, birds, mammals, etc.human, the third germ layer is formed from the ectodermduring the second phase of gastrulation.During the second phase of gastrulation, invagination of ectocells is observeddermis into the blastocoel with simultaneous immigration of ec cellstoderm into the space between the ectoderm and endoderm. Cells that move out of the ectoderm and are located betweenthe last and layer of endoderm cells are meso cellsdermis. This method of mesoderm formation is called ecto dermal.

In cartilaginous fish and amphibians it is observed mixed ny,or transition, method of formation of mesoderm, whichis a reflection of the evolutionary transformations of the embryonal development from lower chordates to higher ones. These womenVotive mesoderm is formed during gastrulation onetemporarily with ectoderm and endoderm.

4. HISTOGENESIS AND ORGANOGENESIS

The gastrulation process is completed the formation of 3 germ layers: outer – ectoderm; inner – endoderm and middle – mesoderm. Subsequently, the embryo develops a neural tube, lays down axial organs (notochord and somites), and separates other embryonic rudiments.

Embryonic primordia – sources of development of tissues and organs in ontogenesis, represented by groups of more or less numerous poorly differentiated (unspecialized) cells; rudiments do not have intercellular substance.

The embryonic rudiments include:

1) Dorsal string, or chord ( chorda – chord);

2) Segmented mesoderm, or somites;

3) Segmented legs, or nephrotomy ( nephros - kidney);

4) Unsegmented mesoderm (lateral plates) or splanchnotome.

Let us consider how the formation of the neural tube, the laying of axial organs and the separation of embryonic rudiments occur using the example of birds (Fig. 3).

Rice. 3. Formation of axial organs and embryonic rudiments in birds(orig.):

1 - ectoderm- 2 - neural tube; 3 – somite; 4 – nephrotomy; 5 – parietal splanchnotome drain; 6– in general; 7 – visceral layer of splanchnotome; 8– blood island; 9 – endoderm; 10 – chord.

Neural tubeformed from ectoderm cells. Process of formation of the neural tubein the embryos of chordates and humans century called neurulation. After the formation of a cluster of mesoderm cells, correspondingcorresponding to the future notochord, cellular material of the ectoderm,located above the chord, forms medullary( medulla - brain), or nervous, plate. This vast layer of cellsthe ectoderm covering the notochord and somites bends slightly,and at its edges two are formed nervous, or medullary, va Lika,representing the edges neural plate. By bending, the neural plate forms nervous jelly side,and the neural folds close with their ridges,forming neural tubesku. In skullless animals there is only one neural tube along its entire length. kova, it turns into the spinal cord. In vertebratesIn animals and humans, the anterior part of the neural tube is formedlater due to the closure of higher neural folds,as a result of which the head section of the neural tube appearsexpanded and from it the brain subsequently developsbubbles and then the brain,

Chordlocated between the endodermine and the neural tube and has the appearance of an undivided continuouscell cord. In most chordates and humans but it exists only in embryos. Throughoutthe chord of life is preserved in the lancelet, and some fish - wholeheads, sturgeons and lungfishes.

A cluster of mesoderm cells located symmetricallybut on the sides of the chord, is segmentedmesoderm, or somites. This embryonic rudiment has its ownnatural metamerism, i.e., dismemberment of the dorsal part of the mesoderminto identical parts, or segments, which are locatedone after another along the main axis of the body. This structure ismented mesoderm is a characteristic feature,common to all chordates.

Unsegmented mesoderm, or splanchnotome, cleftfalls into two layers of cells, or two leaves, – outer, or parietal, or parietal, whichadjacent to the ectoderm, and interior, or visceral , which is adjacent to the endoderm. Simple a hole formed between the outer and inner sheetkami splanchnotoma, is a secondary cavitybodies. For some time the somites are associated with the splanchnotome segmental legs, or nephrotomes, but soonconnection between segmented and non-segmented mesodere mine is interrupted.

The formation of tissue primordia is based on the processes of determination and commitment.

Determination- a genetically programmed path of development of cells and tissues. It is based on persistent changes in repression (blocking) and derepression (unblocking) of genes that determine the specificity of the synthesis of mRNA and proteins.

Committing– limitation possible ways cell development.

At the same time, differentiation processes continue in the primary primordia of embryonic and extraembryonic organs.

Differentiation – these are changes in the structure of cells associated with their functional specialization, caused by the activity of certain genes.

During embryonic development, the following are distinguished: stages of differentiation:

I . Ootypic differentiation – the material of future primordia is represented by separate sections of the cytoplasm of the egg or zygote.

II . Blastomeric differentiation– differences in cellular material are established in blastomeres.

III . Rudimentary differentiation– stage of early gastrulation.

IV . Histogenic differentiation rudiments of tissues, when the rudiments of different tissues appear within one germ layer.

Embryonic histogenesis - the process of emergence of specialized tissues from poorly differentiated cellular material of embryonic rudiments, occurring during the embryonic development of organs.

Histogenesis is accompanied by the reproduction and growth of cells, their movement - migration, differentiation of cells and their derivatives, intercellular and intertissue interactions - correlations, cell death.

In the process of histogenetic differentiation, specialization of tissue primordia and the formation of various types of tissues occur. When cells differentiate from the original stem cell, differons are formed - successive rows of cells. The number of differons in each type of tissue is different.

The result of histogenetic processes is the formation of the main groups of tissues. Their formation begins in the embryonic period and ends after birth.

In the process, the formation of a multilayer embryo is followed byThree cavities are formed: blastocoel, gastrocoel and in general. After the formation of the gastrocoel and coelom in the processgastrulation and neurulation of the blastocoel or fusion with the gastrocoele, as, for example, in amphibians, or so small seems to be taking on the appearance narrow cracks, which will transform are found in the cavity of the circulatory system. Gastrocel in progress development turns into the cavity of the midgut of the body, and as a whole forms a secondary body cavity.

5. THEORY OF GERM LAYERS

The first who paid attention to the emergence of organsfrom the germ layers, or layers, was K. F. Wolf (1759).Studying the development of the chick, he showed that from "disorganizednew, structureless" mass of the egg, germinal leaves appearki, which then give rise to individual organs. K. F. Wolfdistinguished between the nervous and intestinal layers from which they developrelevant authorities. Subsequently X . Pander (1817), after investigator K.F. Wolf also described the presence of germ layers in the chicken embryo. K. M. Baer (1828) discoveredthe presence of germ layers in other animals, due towith which he extended the concept of germ layers to all vertebrates. Thus, K. M. Baer distinguished between the primary germ layers, calling them animal and vegetative, from which subsequently, in the process of embryonic development, secondary germ layers arise, giving rise to certain organs.

The description of the germ layers greatly facilitated the study of the characteristics of the embryonic development of organisms and gavethe ability to establish phylogenetic relationships between livingvotive, seemed very distant in systematic terms. This was brilliantly demonstrated by A. O. Kovalevsky (1865, 1871), who is rightfully considered the foundermodern theory of germ layers. A. O. Kovalevskybased on broad comparative embryological comparisonsstudies showed that the two-layer stage of development goes throughalmost all multicellular organisms. He proved the similaritygerm layers in various animals, not only by origin, but also by derivatives of the germ layers. Underinfluence of the theory of germ layers by E. Haeckel (1866,1871) created the theory of gastrea, according to which everything is multicleexact animals descend from one common ancestor, the hypothetical organism gastrea - a two-layer animal, like removing the embryos of some modern animals on gastrula stage or modern coelenterates. So Thus, the germ layers were considered as the production nye of the original two-layer organization of the ancestors of modern multicellular animals. This is the essence of the theory behindbirth leaves.

However, in the theory of germ layers there are a number of exceptionsNI. According to this theory, the notochord develops from the endoderm,the nervous system is from the ectoderm, and the muscle tissue is from the meso dermis. However, in reptiles, birds, and mammals, the notochorddevelops from mesoderm arising from ectoderm. Ascidiy certain groups blastomeres are given simultaneously andchord, and the nervous system, i.e. organs originating from the theoryria of germ layers from various germ layerscov. Smooth muscle tissue of the iris, muscleshair follicles of mammalian skin develops not from mesoderm, as required by the theory of germ layers, but fromectoderm. Similar examples we could continue.

Exceptions to the germ layer theory can be explained byfrom the point of view of the theory of phylembryogenesis by A. N. Severtsov (1939).They should be considered as a result of changes in earlystages of development, in which not only the germ layers,but ontogenesis itself can change beyond recognition induring evolution under the influence of environmental influences.

Thus, the germ layer theory is athe greatest morphological generalization in the entire history of the embryo logy. Thanks to her, a new direction in embryology arosegies, namely evolutionary embryology, which showedthat the germ layers present in the vast majority animals, are one of the evidences of the commonality of origin and unity of the entire animal world.

6. DERIVATIVES OF GERM LAYERS

From the moment the germ layers appear, their cellsny material specializes in the field of educationcertain embryonic rudiments, as well as a wide circlehectares of tissues and organs. Already at the stage of embryo formationleaflets there are differences in their cellular composition. Thus, ectoderm cells are always smaller in size, more correctlynew shape and divide faster than endoderm cells. WHO dying in the process of embryonic development in the primaryhomogeneous material, as well as between the cells of the germ layers, differences are called differentiation . This is the final stage of embryogenesis.

Outer germ layer or ectoderm , in progress development gives such embryonic rudiments as neural tubesku, ganglion plate, placodes (local thickenings of thetoderm), skin ectoderm and extraembryonic ectoderm. Fromfrom these embryonic primordia the following tissues arise andorgans. The neural tube gives rise to neurons and macroglia of the brainand spinal cord, tail muscles of amphibian embryos,as well as the retina of the eye. Arose from the ganglion platethere are neurons and macroglia of the somatic and autonomic ganglianervous system, macroglia of nerves and nerve endings, chromatophores of lower vertebrates, birds and mammals, chromaffin cells, medulla adrenal glands, skeletonnye anlage of the jaw, hyoid, gill arches, cartilageslarynx, as well as ectomesenchyme. They develop from placodesrons and macroglia of some ganglia, or nerve ganglia,fishing, as well as the organs of balance, hearing and the lens of the eye.Cutaneous ectoderm gives rise to the epidermis of the skin and its derivatives– glands of the skin, hairline, nails, etc., epithelium of the mucous membrane of the vestibule of the oral cavity, vla galina, rectum and their glands, as well as tooth enamel. In addition, muscle tissues develop from the cutaneous ectoderm. hair fibers of the skin and the iris of the eye. The amnion epithelium arises from the extraembryonic ectoderm. chorion and umbilical cord, and in reptile embryos xia and birds - Epithelium of the serous membrane.

Inner germ layer or endoderm , V development forms such embryonic rudiments as kicervical and vitelline endoderm. From these embryonicrudiments the following tissues and organs develop. Intestinal entoderma is the starting point for the formation of the epithelium of the jellyintestinal tract and glands– glandular part of the liver,pancreas, salivary glands, as well as epithelium of breath organs and their glands. Vitelline endoderm differentiationpenetrates into the epithelium of the yolk sac. The extraembryonic endoderm develops into the corresponding vitelline membrane bag.

Middle germ layer or mesoderm , in progress development gives such embryonic rudiments as chordal rudiment, somites and their derivatives in the form of dermatome, myotome and sclerotoma ( scleros solid), as well as embryonicconnective tissue, or mesenchyme. In addition, mesoderm forms nephrotom, mesonephric, or Wolffian canals; Müllerian, or paramesonephric, canals; splanchnotoma;mesenchyme moving out of the splanchnotome; extraembryonicmesoderm. From the notochordal primordium at the appendiculars,turnips, cyclostomes, wholeheads, sturgeons and lungfishin the living groups, a notochord develops, which in the listed groupsin animals it remains for life, but in vertebrates it is replacedskeletogenic tissues. The dermatome provides connective tissuebasis of the skin, myotome– striated muscle tissueskeletal type, and the sclerotome forms skeletal tissues- cart cheek and bone. Nephrotomes give rise to the epithelium of the kidney, urinary tract, and Wolffian canals– sperm epitheliumcarrying paths. Müllerian channels form the epithelium of the eggwater, uterus and primary epithelial lining of the vagina.The coelomic epithelium, or me, develops from the splanchnotome.sotelium, adrenal cortex, cardiac muscle tissue ca and follicular epithelium of the gonads. Mesenchem, co. which is evicted from the splanchnotome and differentiates into cellsblood, connective tissue, blood vessels, smooth muscle tissuehollow internal organs and vessels. Extraembryonic mesothe dermis gives rise to the connective tissue base of the chorion, amnion, yolk sac, as well as exocoelomic epi telia.

The formation of some embryonic primordia occursdits before the formation of germ layers. These include trophoblast and gonoblast are formed. Trophoblast in placentalsmammals, animals and humans are formed even whenlenition. Sexual primordium, or gonoblast, also occurs beforeformation of three germ layers and therefore cannot be a derivative of any of them.

Control questions.

1. Name and characterize the main stages of embryonic development inherent in all animal species.

2. What is called crushing, its types. What determines the nature of crushing?

3. What is blastula called? What parts are distinguished in the blastula of the embryo? Name and describe the types of blastulas.

4. What environmental factors influence crushing processes?

5.What is gastrulation and what are the features of this stage of embryonic development in animals and humans?

6. What stages are distinguished during gastrulation, what is formed at each stage? What is formed during gastrulation?

7. List the main types of formation of a two-layer and three-layer embryo.

8. What caused the division of animals into protostomes and deuterostomes?

9. What is called neurulation?

10. Name the axial organs formed during the process of neurulation.

11. List the embryonic rudiments formed during embryogenesis.

12. Name the main features common to all chordates.

13. What germ layers are formed during embryogenesis?

15. What is called determination, commitment and differentiation, characterize them.

16. List the main stages of differentiation that are distinguished during embryonic development.