Embryonic development is a complex and lengthy morphogenetic process, during which a new multicellular organism is formed from the paternal and maternal germ cells, capable of independent life in the environment. Underlies sexual reproduction and ensures the transmission of hereditary traits from parents to offspring.
Fertilization consists in the connection of the spermatozoon with the egg. The most important stages of the fertilization process are:
1) penetration of the joint venture into the egg;
2) activation of various synthetic processes in the egg;
3) the fusion of the nuclei of the egg and the SP with the restoration of the diploid set of chromosomes.
In order for fertilization to occur, the convergence of female and male germ cells is necessary. It is achieved through insemination.
The penetration of SP into the egg is facilitated by the enzyme hyaluronidiase and other biologically active substances (spermolysin), which increase the permeability of the main intercellular substance. Enzymes are secreted by the acrosome in the process acrosome reaction. Its essence is as follows, at the moment of contact with the egg at the top of the sperm head, the plasma membrane and the membrane adjacent to it dissolve, and the adjacent portion of the egg membrane dissolves. The acrosomal membrane protrudes outward and forms an outgrowth in the form of a hollow tube. or a tubercle of fertilization. After that, the plasma membranes of both gametes merge and the union of their contents begins. From this moment, the SP and I represent a single zygote cell.
Activation I or cortical response, which develops as a result of contact with SP, has morphological and biochemical manifestations. The manifestation of activation are changes in the superficial cortical layer of the ooplasm and the formation fertilization membranes. The fertilization membrane protects the yatsyo from the penetration of oversized spermatozoa.
Zygote- diploid (containing a complete double set of chromosomes) cell resulting from fertilization (the fusion of an egg and a sperm cell). The zygote is totipotent(that is, able to give rise to any other) cell.
In humans, the first mitotic division of the zygote occurs approximately 30 hours after fertilization, due to the complex processes of preparation for the first division. crushing.
Splitting up - this is a series of successive mitotic divisions of the zygote and ending with the formation of a multicellular embryo - blastula. The first cleavage division begins after the union of the hereditary material of the pronuclei and the formation of a common metaphase plate. Cells formed during cleavage are called blastomeres(from Greek. blaste- sprout, germ). A feature of mitotic cleavage divisions is that with each division, the cells become smaller and smaller until they reach the ratio of the volumes of the nucleus and cytoplasm that is usual for somatic cells. First, the blastomeres adjoin each other, forming a cluster of cells called morula . Then a cavity is formed between the cells - blastocoel , filled with liquid. Cells are pushed to the periphery, forming the wall of the blastula - blastoderm. The total size of the embryo by the end of cleavage at the blastula stage does not exceed the size of the zygote.
Progenesis - gametogenesis (spermato- and ovogenesis) and fertilization. Spermatogenesis is carried out in the convoluted tubules of the testes and is divided into four periods: 1) the reproduction period - I; 2) growth period - II; 3) ripening period - III; 4) period of formation - IV. Ovogenesis is carried out in the ovaries and is divided into three periods: 1) the reproduction period (in embryogenesis and during the 1st year post embryonic development); 2) a period of growth (small and large); 3) maturation period. The egg consists of a nucleus with a haploid set of chromosomes and a pronounced cytoplasm, which contains all organelles, with the exception of the cytocenter.
Splitting up. crushing characteristics. The main types of eggs according to the location of the yolk. The relationship of the structure of the egg with the type of crushing. Blastomeres and embryonic cells. Structure and types of blastula.
Splitting up - This process is based on mitotic cell division. However, the daughter cells formed as a result of division do not diverge, but remain closely adjacent to each other. In the process of crushing, daughter cells progressively decrease. Each animal is characterized by a certain type of crushing, due to the amount and nature of the distribution of the yolk in the egg. The yolk inhibits crushing, therefore, the part of the zygote overloaded with yolk splits more slowly or does not split at all.
In isolecithal, a poor yolk fertilized lancelet egg, the first cleavage furrow in the form of a gap begins at the animal pole and gradually spreads in the longitudinal meridional direction towards the vegetative one, dividing the egg into 2 cells - 2 blastomeres. The second furrow runs perpendicular to the first - 4 blastomeres are formed. As a result of a series of successive divisions, groups of cells are formed that are closely adjacent to each other. In some animals, such an embryo resembles a mulberry or raspberry. He got the name morula(lat. morum - mulberry) - a multicellular ball without a cavity inside.
IN telolecithal eggs , overloaded with yolk - crushing can be complete uniform or uneven and incomplete. Blastomeres of the vegetative pole, due to the abundance of inert yolk, always lag behind the blastomeres of the animal pole in the rate of cleavage. Complete but uneven cleavage is characteristic of amphibian eggs.. In fish, birds, and some other animals, only the part of the egg located at the animal pole is crushed; incomplete discoidal cleavage occurs. In the process of crushing, the number of blastomeres increases, but the blastomeres do not grow to the size of the original cell, but become smaller with each crushing. This is explained by the fact that the mitotic cycles of the crushing zygote do not have a typical interphase; the presynthetic period (G1) is absent, and the synthetic period (S) begins as early as the telophase of preceding mitosis.
Cleavage of the egg ends with the formation blastula.
In polylecithal oocytes bony fish, reptiles, birds, as well as monotreme mammals crushing partial, or meroblastic, those. covers only the cytoplasm free from yolk. It is located in the form of a thin disk on the animal pole, therefore this type crushing called discoidal . When characterizing the type of crushing, the relative position and rate of division of blastomeres are also taken into account. If the blastomeres are arranged in rows one above the other along the radii, crushing called radial.
Fragmentation can be: deterministic and regulatory; complete (holoblastic) or incomplete (meroblastic); uniform (blastomeres are more or less the same in size) and uneven (blastomeres are not the same in size, two to three size groups are distinguished, usually called macro- and micromeres)
Types of eggs:
Amount of yolk - Oligolecetal (lancelet) Mesolacetal (amphibians) Polylecetal (fish, birds)
Location- Isolatecetal(located diffusely, evenly). They contain a little yolk, evenly distributed throughout the cell. Characteristic of echinoderms, lower chordates and mammals. In mammals, these are alleciatal eggs (there is practically no yolk)
Telolecetal(with moderate amounts of yolk at the lower vegetative pole)
Sharply telolecital (with a large amount of yolk, occupies the entire egg, except for the upper pole. There is a lot of yolk, concentrated on the vegetative pole. There are 2 groups: moderately telolecital (molluscs, amphibians) and sharply lecithal (reptiles and birds). Cytoplasm and nucleus are concentrated on the anomalous pole .
Centrolecetal(the yolk is a little, but dense in the center). Little yolk, located in the center. characteristic of arthropods
Blastomeres- cells formed as a result of divisions of crushing eggs in multicellular animals. A characteristic feature of B. is the absence of growth in the period between divisions, as a result of which, during the next division, the volume of each B. is halved. With holoblastic crushing in telolecithal eggs B. differ in size: large B. - macromeres, medium - mesomers, small - micromeres. During synchronous cleavage divisions B., as a rule, are homogeneous in form, the structure of their cytoplasm is very simple. Then the superficial B. flatten, and the egg proceeds to conclude, the crushing phase - blastulation.
The structure of the blastula. If a solid ball is formed without a cavity inside, then such a nucleus is called morula. The formation of a blastula or morula depends on the properties of the cytoplasm. Blastula is formed at sufficient viscosity of the cytoplasm, morula - at low viscosity. With sufficient viscosity of the cytoplasm, the blastomeres retain a rounded shape and only slightly flatten at the points of contact. As a result, a gap appears between them, which, as crushing increases, is filled with liquid and turns into a blastocoel. With a low viscosity of the cytoplasm, the blastomeres are not rounded and are located closely next to each other, there is no gap and no cavity is formed. Blastulas are different in structure and depend on the type of crushing.
The beginning of a new organism is given by a fertilized egg (with the exception of cases of parthenogenesis and vegetative reproduction). Fertilization is the process of fusion of two germ cells (gametes) with each other, during which two different functions are carried out: sexual (combining the genes of two parental individuals) and reproductive (the emergence of a new organism). The first of these functions includes the transfer of genes from parents to offspring, the second - the initiation in the cytoplasm of the egg of those reactions and movements that allow further development. As a result of fertilization, a double (2p) set of chromosomes is restored in the egg. The centrosome, introduced by the sperm, after doubling forms a fission spindle, and the zygote enters the 1st stage of embryogenesis - the stage of crushing. As a result of mitosis, 2 daughter cells - blastomeres - are formed from the zygote.
Prezygotic period
The prezygotic period of development is associated with the formation of gametes (gametogenesis). The formation of oocytes begins in women even before they are born and is completed for each given oocyte only after its fertilization. By the time of birth, the female fetus in the ovaries contains about two million first-order oocytes (these are still diploid cells), and only 350 - 450 of them will reach the stage of second-order oocytes (haploid cells), turning into eggs (one at a time during one menstrual cycle). Unlike women, germ cells in the testes (testicles) in men begin to form only with the onset of puberty. The duration of the period of sperm formation is approximately 70 days; for one gram of testicle weight, the number of spermatozoa is about 100 million per day.
Fertilization
Fertilization - the fusion of a male reproductive cell (sperm) with a female (egg, ovum), leading to the formation of a zygote - a new unicellular organism. The biological meaning of fertilization is the unification of the nuclear material of male and female gametes, which leads to the unification of paternal and maternal genes, the restoration of the diploid set of chromosomes, as well as the activation of the egg, that is, its stimulation for embryonic development. The connection of the egg with the sperm usually occurs in the funnel-shaped part of the fallopian tube during the first 12 hours after ovulation.
The seminal fluid, entering the woman's vagina during sexual intercourse, usually contains from 60 to 150 million spermatozoa, which, thanks to movements at a speed of 2-3 mm per minute, constant undulating contractions of the uterus and tubes and an alkaline environment, already after 1-2 minutes after intercourse, they reach the uterus, and after 2-3 hours - the end sections of the fallopian tubes, where they usually merge with the egg. There are monospermic (one sperm enters the egg) and polysperm (two or more sperm enter the egg, but only one sperm nucleus fuses with the egg nucleus) fertilization. Preservation of sperm activity during their passage in the genital tract of a woman is facilitated by the slightly alkaline environment of the cervical canal of the uterus, filled with a mucous plug. During orgasm during sexual intercourse, the mucous plug from the cervical canal is partially pushed out, and then retracted into it and thereby contributes to a faster entry of spermatozoa from the vagina (where it is normal for healthy woman slightly acidic environment) to a more favorable environment of the cervix and uterine cavity. The passage of spermatozoa through the mucous plug of the cervical canal is also facilitated by the sharp increase in mucus permeability on the days of ovulation. On the remaining days of the menstrual cycle, the mucous plug has a significantly lower permeability for spermatozoa.
Many spermatozoa located in the genital tract of a woman can retain the ability to fertilize for 48-72 hours (sometimes even up to 4-5 days). An ovulated egg remains viable for approximately 24 hours. Given this, the most favorable time for fertilization is the period of rupture of a mature follicle, followed by the birth of an egg, as well as the 2-3rd day after ovulation. Women using a physiological method of contraception should be aware that the timing of ovulation may fluctuate, and the viability of the egg and sperm can be significantly longer. Shortly after fertilization, zygote cleavage and embryo formation begin.
ZygoteZygote (Greek zygote paired) is a diploid (containing a complete double set of chromosomes) cell resulting from fertilization (the fusion of an egg and a sperm cell). The zygote is a totipotent (that is, capable of producing any other) cell. The term was introduced by the German botanist E. Strasburger.
In humans, the first mitotic division of the zygote occurs approximately 30 hours after fertilization, which is due to the complex processes of preparation for the first act of crushing. Cells formed as a result of crushing the zygote are called blastomeres. The first divisions of the zygote are called "crushing" because the cell is crushed: after each division, the daughter cells become smaller and smaller, and there is no stage of cell growth between divisions.
Development of the zygote The zygote either begins to develop immediately after fertilization, or is dressed in a dense shell and for some time turns into a resting spore (often called a zygospore) - typical of many fungi and algae.
Splitting upThe period of embryonic development of a multicellular animal begins with the fragmentation of the zygote and ends with the birth of a new individual. The cleavage process consists in a series of successive mitotic divisions of the zygote. The two cells formed as a result of a new division of the zygote and all subsequent generations of cells at this stage are called blastomeres. During crushing, one division follows another, and the resulting blastomeres do not grow, as a result of which each new generation of blastomeres is represented by smaller cells. This feature of cell divisions during the development of a fertilized egg determined the appearance of a figurative term - crushing of the zygote.
At different types Animal eggs differ in the quantity and nature of the distribution of reserve nutrients (yolk) in the cytoplasm. This largely determines the nature of the subsequent fragmentation of the zygote. With a small amount and uniform distribution of the yolk in the cytoplasm, the entire mass of the zygote divides with the formation of identical blastomeres - complete uniform crushing (for example, in mammals). When the yolk accumulates predominantly at one of the poles of the zygote, uneven fragmentation occurs - blastomeres are formed that differ in size: larger macromeres and micromeres (for example, in amphibians). If the egg is very rich in yolk, then its part, free of yolk, is crushed. So, in reptiles, birds, only the disc-shaped section of the zygote at one of the poles, where the nucleus is located, undergoes crushing - incomplete, discoidal crushing. Finally, in insects, only the surface layer of the cytoplasm of the zygote is involved in the crushing process - incomplete, superficial crushing.
As a result of crushing (when the number of dividing blastomeres reaches a significant number), a blastula is formed. In a typical case (for example, in the lancelet), the blastula is a hollow ball, the wall of which is formed by a single layer of cells (blastoderm). The blastula cavity - blastocoel, otherwise called the primary body cavity, is filled with fluid. In amphibians, the blastula has a very small cavity, and in some animals (such as arthropods), the blastocoel may be completely absent.
gastrulationAt the next stage of the embryonic period, the process of gastrula formation takes place - gastrulation. In many animals, the formation of the gastrula occurs by invagination, i.e. protrusions of the blastoderm at one of the blastula poles (with intensive reproduction of cells in this zone). As a result, a two-layer, bowl-shaped embryo is formed. The outer layer of cells is the ectoderm, and the inner layer is the endoderm. The internal cavity that occurs when the wall of the blastula protrudes, the primary intestine, communicates with the external environment through an opening - the primary mouth (blastopore). There are other types of gastrulation. For example, in some coelenterates, the endoderm of the gastrula is formed by immigration, i.e. "eviction" of a part of blastoderm cells into the cavity of the blastula and their subsequent reproduction. The primary mouth is formed by rupture of the wall of the gastrula. With uneven crushing (in some worms, mollusks), the gastrula is formed as a result of fouling of macromeres with micromeres and the formation of endoderm due to the first. Often different methods of gastrulation are combined.
In all animals (except for sponges and coelenterates - bilayer animals), the gastrulation stage ends with the formation of another layer of cells - the mesoderm. This "cell layer is formed between the ento- and ectoderm. There are two known ways of laying the mesoderm. In annelids, for example, two large cells (teloblasts) are isolated in the blastopore region of the gastrula. Reproducing, they give rise to two mesodermal stripes, of which (partly due to divergence of cells, partly as a result of the destruction of part of the cells inside the mesodermal strips), coelomic sacs are formed - the teloblastic method of laying the mesoderm.In the enterocelous method (echinoderms, lancelet, vertebrates), as a result of protrusion of the wall of the primary intestine, lateral pockets are formed, which then separate and become coelomic In both cases of mesoderm formation, coelomic sacs grow and fill the primary body cavity. The mesodermal layer of cells lining the inside of the body cavity forms the peritoneal epithelium. The cavity that thus replaced the primary one is called the secondary body cavity, or coelom. In the case of the teloblastic method of closing The blastopore mesoderm develops into the oral opening of an adult animal. Such organisms are called protostomes. In deuterostomes (with the enterocoelous method of laying the mesoderm), the blastopore overgrows or turns into an anus, and the mouth of an adult occurs a second time, by protrusion of the ectoderm.
The formation of three germ layers (ecto-, ento- and mesoderm) completes the stage of gastrulation, and from this moment the processes of histo- and organogenesis begin. As a result of cell differentiation of the three germ layers, various tissues and organs of the developing organism are formed. As early as the end of the last century (largely thanks to the studies of I. I. Mechnikov and A. O. Kovalevsky) it was established that in different animal species the same germ layers give rise to the same organs and tissues. From the ectoderm, the epidermis with all derivative structures and the nervous system are formed. Due to the endoderm, the digestive tract and associated organs (liver, pancreas, lungs, etc.) are formed. The mesoderm forms the skeleton, vascular system, excretory apparatus, gonads. Although today the germ layers are not considered strictly specialized, nevertheless, their homology in the vast majority of animal species is obvious, which indicates the unity of the origin of the animal kingdom.
During the embryonic period, there is an increase in the rate of growth and differentiation in developing organisms. If growth does not occur during cleavage and the blastula (in terms of mass) can be significantly inferior to the zygote, then, starting from the process of gastrulation, the mass of the embryo rapidly increases (due to intensive cell reproduction). The processes of cell differentiation begin at the earliest stage of embryogenesis - crushing and underlie primary tissue differentiation - the emergence of three germ layers (embryonic tissues). Further development of the embryo is accompanied by an ever-increasing process of differentiation of tissues and organs. As a result of the embryonic period of development, an organism is formed that is capable of independent (more or less) existence in the external environment. A new individual is born either as a result of hatching from an egg (in oviparous animals) or exit from the mother's body (in viviparous).
Histo - and organogenesisHisto - and organogenesis of the embryo are carried out as a result of reproduction, migration, differentiation of cells, its components, the establishment of intercellular contacts and the death of some cells. The 317th to the 20th day continues the presomitic period from the 20th day the somite period of development begins. On the 20th day of embryogenesis, through the formation of trunk folds (cephalocaudal and lateral), the embryo itself is separated from extraembryonic organs, as well as its flat shape is changed to a cylindrical one. At the same time, the dorsal parts of the mesoderm of the embryo are divided into separate segments located on both sides of the chord - somites. On the 21st day, there are 2-3 pairs of somites in the body of the embryo. Somites begin to form from the III pair, I and II pairs appear somewhat later. The number of somites gradually increases: on the 23rd day of development there are 10 pairs of somites, on the 25th - 14 pairs, on the 27th - 25 pairs, at the end of the fifth week the number of somites in the embryo reaches 43-44 pairs. Based on the calculation of the number of somites, it is possible to approximately determine the timing of development (somitic age) of the embryo.
From the outer part of each somite, a dermatome arises, from the inner - a sclerot, from the middle - a myot. The dermatome becomes the source of the skin dermis, the sclerote becomes the source of cartilage and bone tissue, and the myotome becomes the source of the skeletal muscles of the dorsal part of the embryo. The ventral sections of the mesoderm - splanchnotome - are not segmented, but are divided into visceral and parietal sheets, from which serous membranes of internal organs develop, muscle heart and adrenal cortex. Blood vessels, blood cells, connective and smooth muscle tissue of the embryo are formed from the mesenchyme of the splanchnotome. The section of the mesoderm that connects the somites with the splanchnotome is divided into segmented legs - the nephrogonoth, which serve as a source for the development of the kidneys and gonads, as well as the paramesonephric ducts. Of the latter, the epithelium of the uterus and oviduct is formed.
In the process of differentiation of the germinal ectoderm, the neural tube, neural crests, placodes, skin ectoderm and prechordal plate are formed. The process of neural tube formation is called neurulation. It consists in the formation of a slit-like depression on the surface of the ectoderm; the thickened edges of this depression (neural folds) fuse to form the neural tube. Brain vesicles form from the cranial part of the neural tube, which is the rudiment of the brain. On both sides of the neural tube (between the latter and the skin ectoderm), groups of cells are separated, from which neural crests are formed. Neural crest cells are capable of migrating. Cells migrating in the direction of the dermatome give rise to pigment cells - melanocytes; neural crest cells that migrate towards the abdominal cavity give rise to the sympathetic and parasympathetic ganglia, the adrenal medulla. From the cells of the neural crests, which did not migrate, ganglionic plates are formed, from which the spinal and peripheral autonomic nerve ganglia develop. The ganglia of the head and the nerve cells of the organ of hearing and balance are formed from the placodes.
Splitting up(segmentation) in individual representatives of the category of vertebrates generally has the same course; however, as already mentioned above, it was influenced by factors that during phylogenesis influenced development in the form of consequences of the influence of the internal and external environment in which organisms lived during their ancestral development (cenogenetic factors).
When observing changes, occurring in eggs according to the phylogenetic development of eggs of individual representatives of the vertebrate category, it can be seen that the egg cells differ significantly from each other in the content of the nutrient and building substance - the yolk. The egg cells of the lancelet (Amfioxus), an organism that is phylogenetically considered the lowest organized creature, but which already has a strong dorsal region, are among the oligolecital.
However, in accordance with phylogenetic development, the amount of yolk in the eggs of vertebrates, which are phylogenetically the most highly organized organisms, increases more and more, reaching a maximum amount in avian eggs, which are relatively very large and polylecital. Under the influence of coenogenetic factors (factors influencing from the external environment and determined by changes in lifestyle and, consequently, development), the amount of yolk in the process of phylogenetic development towards humans decreases more and more, due to which the eggs of humans and higher mammals become again (secondarily) oligolecithal .
Having a variable amount yolk has, as mentioned above, a significant impact on the process of crushing the egg. Egg cells with a low yolk content (oligolecital) are completely crushed, that is, the entire substance of a fertilized egg is divided into new cells, blastomeres (holoblastic type eggs) during crushing. On the contrary, in eggs containing more yolk, or even a large amount of yolk (polylecital), cleavage furrows continuously crush only a smaller part of the ooplasm, located at the so-called animal pole, where there are fewer yolk granules (meroblastic type eggs).
In accordance with this, individual representatives of the category vertebrates, the following types of crushing are distinguished.
1. Complete crushing. Complete, total crushing includes those cases when, in the process of crushing division, the entire fertilized egg cell is divided and crushing furrows spread over its entire surface. According to this type, egg cells of the holoblastic species are crushed. Depending on the content of a greater or lesser amount of yolk in the ooplasm, as well as depending on its distribution in the ooplasm, during crushing, blastomeres of either relatively the same size (complete uniform, equal, or adequal cleavage) or blastomeres of various sizes, namely larger in the area with a high content of yolk and smaller in the place where the yolk is less (complete uneven, inequal crushing). Larger blastomeres are called macromeres, smaller ones are called micromeres.
Full equal, or adequal, crushing is characteristic of oligolecithal, isolecithal eggs (lancelet, higher mammals and humans); according to the complete inequal type, mesolecithal egg cells of an anisolecital and moderately telolecithal species (some lower fish and amphibians) are crushed.
2. Partial, partial, crushing. By partial type, egg cells are crushed, containing a significant amount of yolk (polylecithal eggs), which, due to their large sizes cleavage furrows during cell division penetrate only into the region of the animal pole, where the cell nucleus is located and where the ooplasm layer contains fewer yolk granules (higher fish, reptiles, birds and some lower mammals, oviparous).
With such crushing on the animal pole of a relatively large egg, only a round field (disk) is crushed, while the remainder of the egg cell (yolk ball) remains unshattered (partial disc-shaped crushing). In insects, their polylecital centrolecithal oocytes, although they are crushed over the entire surface, but the center of the cell, containing a large amount of yolk, remains not crushed (partial surface crushing).
In the above figure individual types of egg cells are shown depending on the content and distribution of the yolk in the ooplasm, as well as depending on the corresponding type of crushing.
The process when a zygote is transformed into a multicellular organism is called crushing. This period is the next after fertilization and includes a number of numerous successive divisions.
The process of crushing the zygote takes about six days. All the cells that make up the embryo are called blastomeres in medicine. Cleavage of the zygote is characterized by its individual characteristics.
Interphase, as a period, is minimal in its duration. This is followed by two full-fledged mitoses, which explains the progressive decrease in the zygote. By the end of the sixth day, after fertilization, the formed multicellular organism does not exceed the size of the zygote. But the process of crushing ends at the moment when the cells of the embryo become similar to the somatic cells of the human body.
Final and First stage cleavage zygotes are unique in their structure. In the process of colossal changes, a full-fledged asynchronous and subequal division takes place. Such data indicate the fact that cleavage affects all parts of the zygote, and blastomeres appear of the same size. If the cells are different in volume, then during the crushing of the zygote, non-simultaneous mitotic division occurs.
A not too dense conglomerate is created by blastomeres at about the eight-cell stage of zygote development. Although on the sixth day after fertilization, approximately after the third stage of division, the cells create a dense structure inside the embryo. This work is called compaction and provokes detachment of the internal blastomeres from the external ones. The types of cleavage of the zygote differ in their period, and the aforementioned stage is the morula. Such central formations create the main cell mass. And cells connected by tight contacts serve as a kind of barrier that is designed to protect the internal structure of the morula. That is, peripheral cells create a trophoblast - a cell mass of an external type.
The final processes of crushing the zygote
As a result of crushing the zygote, the cellular organism turns into an embryo. Only on the fourth day after fertilization does the zygote enter the uterine cavity. In the embryo, a peculiar liquid cavity is formed - the blastocoel. Now the embryo is a bubble and bears the name - blastocyst. Inside the body there is a cellular embryoblast - this is an internal mass. It is from this “matter” that the embryo itself and some of its external organs are formed, which are visualized outside the embryo. If the inner cell mass begins to divide, then this fact will lead to the formation of twins.
The embryonic part of the placenta is formed on the basis of the trophoblast, it is he who creates the chorion. Around the fourth day after fertilization, the cells destroy the membrane, that is, they change the transparent part of the embryo. This is how the zygote prepares for the next stage of its transformation.
A zygote is formed, capable of further development. The division of a zygote is called cleavage. Splitting up- This is the repeated division of the zygote after fertilization, as a result of which a multicellular embryo is formed.
The zygote divides very quickly, the cells decrease in size and do not have time to grow. Therefore, the embryo does not increase in volume. The resulting cells are called blastomeres, and the constrictions separating them from each other are called cleavage furrows.
The following crushing furrows are distinguished in direction: meridional - these are furrows that divide the zygote from the animal to the vegetative pole; the equatorial furrow divides the zygote along the equator; latitudinal furrows run parallel to the equatorial furrow; tangential grooves run parallel to the surface of the zygote.
The equatorial furrow is always one, but there can be many meridional, latitudinal and tangential furrows. The direction of the crushing furrows is always determined by the position of the division spindle.
Crushing always takes place according to certain rules:
The first rule reflects the location of the cleavage spindle in the blastomere, namely:
- the cleavage spindle is located in the direction of the greatest extent of the cytoplasm, free from inclusions.
The second rule reflects the direction of the crushing furrows:
- crushing furrows always run perpendicular to the fission spindle.
The third rule reflects the speed of crushing furrows:
- the speed of passage of the cleavage furrows is inversely proportional to the amount of yolk in the egg, i.e. in that part of the cell where there is little yolk, the furrows will pass at a higher speed, and in the part where there is more yolk, the speed of passage of the cleavage furrows slows down.
Cleavage depends on the amount and location of the yolk in the egg. With a small amount of yolk, the entire zygote is crushed, with a significant amount, only a part of the zygote free of yolk is crushed. In this regard, the eggs are divided into holoblastic (completely crushed) and meroblastic (with partial crushing). Consequently, crushing depends on the amount of yolk and, taking into account a number of features, is subdivided: according to the completeness of the coverage of the zygote material by the process, into complete and incomplete; according to the ratio of the sizes of the formed blastomeres to uniform and uneven, and according to the consistency of blastomere divisions - synchronous and asynchronous.
Complete crushing can be uniform and uneven. Complete uniform is characteristic of eggs with a small amount of yolk and its more or less uniform arrangement in. This type of crushed egg. In this case, the first furrow runs from the animal to the vegetative pole, two blastomeres are formed; the second furrow is also meridional, but runs perpendicular to the first, four blastomeres are formed. The third is equatorial, eight blastomeres are formed. After this, there is an alternation of meridional and latitudinal crushing furrows. The number of blastomeres after each division increases by a factor of two (2; 4; 16; 32, etc.). As a result of such crushing, a spherical embryo is formed, which is called blastula. The cells that form the wall of the blastula are called the blastoderm, and the cavity inside is called the blastocoel. The animal part of the blastula is called the roof, and the vegetative part is called the bottom of the blastula.
Complete uneven crushing is typical for eggs with an average content of yolk located in the vegetative part. Such eggs are characteristic of cyclostomes and. Wherein type of crushing blastomeres of unequal sizes are formed. In the animal pole, small blastomeres are formed, which are called micromeres, and in the vegetative pole - large ones - macromeres. The first two furrows, like those of the lancelet, run meridionally; the third furrow corresponds to the equatorial furrow, but is shifted from the equator to the animal pole. Since the cytoplasm free from yolk is located in the animal pole, fragmentation occurs faster here and small blastomeres are formed. The vegetative pole contains the bulk of the yolk, so the cleavage furrows pass more slowly and large blastomeres are formed.
Incomplete cleavage is characteristic of telolecithal and centrolecithal oocytes. Only the part of the egg that is free of yolk takes part in crushing. Incomplete crushing is divided into discoidal (bony, reptiles, birds) and superficial (arthropods).
Telolecithal oocytes are divided by incomplete discoidal cleavage, in which a large amount of yolk is concentrated in the vegetative part. In these eggs, the yolk-free part of the cytoplasm in the form of a germinal disc is spread out on the yolk at the animal pole. Cleavage occurs only in the region of the germinal disc. The vegetative part of the egg, filled with yolk, does not take part in crushing. The thickness of the germinal disc is negligible, so the cleavage spindles in the first four divisions are horizontal, and the cleavage furrows run vertically. One row of cells is formed. After several divisions, the cells become high and the cleavage spindles are located in them in a vertical direction, and the cleavage furrows run parallel to the surface of the egg. As a result, the germinal disc turns into a plate consisting of several rows of cells. Between the germinal disc and the yolk, a small cavity appears in the form of a gap, which is similar to the blastocoel.
Incomplete superficial crushing is observed in centrolecithal eggs with a large amount of yolk in its middle. The cytoplasm in such eggs is located along the periphery and a small part of it is in the center near the nucleus. The rest of the cell is filled with yolk. Thin cytoplasmic strands pass through the mass of the yolk, connecting the peripheral cytoplasm with the perinuclear one. Fragmentation begins with the fission of nuclei, as a result, the number of nuclei increases. They are surrounded by a thin rim of the cytoplasm, move to the periphery and are located in the yolk-free cytoplasm. As soon as the nuclei enter the surface layer, it divides into blastomeres according to their number. As a result of such crushing, the entire central part of the cytoplasm moves to the surface and merges with the peripheral one. Outside, a continuous blastoderm is formed, from which the embryo develops, and inside is the yolk. Superficial crushing is characteristic of arthropod eggs.
The nature of crushing is also influenced by the properties of the cytoplasm, which determine the relative position of the blastomeres. On this basis, radial, spiral and bilateral crushing are distinguished. With radial crushing, each upper blastomere is located exactly under the lower one (coelenterates, echinoderms, lancelet, etc.). During spiral crushing, each upper blastomere is displaced relative to the lower one by half, i.e. each upper blastomere is located between the two lower ones. In this case, the blastomeres are arranged as if in a spiral (worms, mollusks). With bilateral crushing, only one plane can be drawn through the zygote, on both sides of which identical blastomeres (roundworms, ascidians) will be observed.
