Zygote
A zygote (Greek zygote paired) is a diploid (containing a complete double set of chromosomes) cell formed as a result of fertilization (the fusion of an egg and a sperm). A zygote is a totipotent (that is, capable of giving birth to any other) cell. The term was introduced by the German botanist E. Strassburger.
In humans, the first mitotic division of the zygote occurs approximately 30 hours after fertilization, which is due to complex processes of preparation for the first act of cleavage. The cells formed as a result of fragmentation of the zygote are called blastomeres. The first divisions of the zygote are called “fragmentations” because the cell is fragmented: the daughter cells become smaller after each division, 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 covered with a dense shell and for some time turns into a resting spore (often called a zygospore) - characteristic of many fungi and algae.
Splitting up
The 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 of 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 fragmentation, 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 division during the development of a fertilized egg determined the appearance of the figurative term - fragmentation of the zygote.
In different animal species, eggs differ in the quantity and nature of 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 yolk in the cytoplasm, the entire mass of the zygote is divided with the formation of identical blastomeres - complete uniform fragmentation (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 the part free of yolk is crushed. Thus, in reptiles and birds, only the disc-shaped portion of the zygote at one of the poles, where the nucleus is located, is subject to fragmentation - incomplete, discoidal fragmentation. Finally, in insects, only the surface layer of the zygote cytoplasm is involved in the process of crushing - incomplete, superficial crushing.
As a result of fragmentation (when the number of dividing blastomeres reaches a significant number), a blastula is formed. In a typical case (for example, in a lancelet), the blastula is a hollow ball, the wall of which is formed by a single layer of cells (blastoderm). The cavity of the blastula - blastocoel, otherwise called the primary body cavity, is filled with fluid. In amphibians, the blastula has a very small cavity, and in some animals (for example, arthropods), the blastocoel may be completely absent.
Fertilization - This is the process of fusion of germ cells. The diploid cell formed as a result of fertilization - the zygote - represents the initial stage of the development of a new organism. The fertilization process consists of three successive phases:
a) bringing together gametes(gamones (hormones of gametes), on the one hand, activate the movement of sperm, and on the other, their gluing.) At the moment of contact of the sperm with the shell of the egg, an acrosome reaction occurs, during which, under the action of proteolytic enzymes of the acrosome, the egg shells dissolve. Next, the plasma membranes of the egg and sperm merge and, through the resulting cytoplasmic bridge, the cytoplasm of both gametes are combined. Then the nucleus and centriole of the sperm pass into the cytoplasm of the egg, and the sperm membrane is embedded in the membrane of the egg. The tail part of the sperm in most animals also enters the egg, but then separates and dissolves without playing any role in further development;
b) activation of the eggDue to the fact that a section of the sperm membrane is permeable to sodium ions, the latter begin to enter the egg, changing the membrane potential of the cell. Then, in the form of a wave propagating from the point of contact of the gametes, an increase in the content of calcium ions occurs, after which the cortical granules also dissolve in a wave. The specific enzymes released in this process promote the detachment of the vitelline membrane; it hardens, this is the fertilization membrane. All the described processes represent the so-called cortical reaction.;
c) fusion of gametes, or syngamyWhen the egg meets the sperm, it is usually at one of the stages of meiosis, blocked by a specific factor. In most vertebrates, this block occurs at the metaphase II stage; in many invertebrates, as well as in three species of mammals (horses, dogs and foxes), the block occurs at the stage of diakinesis. In most cases, the meiosis block is removed after the activation of the egg due to fertilization. While meiosis completes in the egg, the nucleus of the sperm that penetrates it is modified. It takes the form of an interphase and then a prophase nucleus. During this time, the DNA doubles and the male pronucleus receives an amount of hereditary material corresponding to p2c, i.e. contains a haploid set of reduplicated chromosomes. The nucleus of the egg, having completed meiosis, turns into the female pronucleus, also acquiring p2c. Both pronuclei undergo complex movements, then come closer and merge (synkaryon), forming a common metaphase plate. This, in fact, is the moment of the final fusion of gametes - syngamy. The first mitotic division of the zygote leads to the formation of two embryonic cells (blastomeres) with a set of chromosomes 2n2c in each.
Zygote - a diploid (containing a complete double set of chromosomes) cell formed as a result of fertilization (the fusion of an egg and a sperm). A zygote is a totipotent (that is, capable of giving birth to any other) cell.
In humans, the first mitotic division of the zygote occurs approximately 30 hours after fertilization, which is due to complex processes of preparation for the first act of cleavage. The cells formed as a result of fragmentation of the zygote are called
blastomeres. The first divisions of the zygote are called “fragmentations” because the cell is fragmented: the daughter cells become smaller after each division, and there is no stage of cell growth between divisions.
Splitting up - this is a series of successive mitotic divisions of the zygote and then blastomeres, ending with the formation of a multicellular embryo - blastulas. Between successive divisions, cell growth does not occur, but DNA is necessarily synthesized. All DNA precursors and necessary enzymes are accumulated during oogenesis. First, the blastomeres are adjacent to each other, forming a cluster of cells called Morula. Then a cavity forms 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 at the end of cleavage at the blastula stage does not exceed the size of the zygote. The main result of the cleavage period is the transformation of the zygote into multicellular single-layer embryo.
Morphology of crushing.As a rule, blastomeres are located in strict order relative to each other and the polar axis of the egg. The order, or method, of crushing depends on the quantity, density and nature of the distribution of the yolk in the egg. According to the Sachs-Hertwig rules, the cell nucleus tends to be located in the center of the yolk-free cytoplasm, and the cell division spindle tends to be located in the direction of the greatest extent of this zone.
In oligo- and mesolecithal In eggs, crushing is complete, or holoblastic. This type of cleavage occurs in lampreys, some fish, all amphibians, as well as in marsupials and placental mammals. With complete crushing, the plane of the first division corresponds to the plane of bilateral symmetry. The plane of the second division runs perpendicular to the plane of the first. Both grooves of the first two divisions are meridian, i.e. begin at the animal pole and spread to the vegetative pole. The egg cell turns out to be divided into four more or less equal in size blastomeres. The plane of the third division runs perpendicular to the first two in the latitudinal direction. After this, uneven cleavage appears in mesolecithal eggs at the stage of eight blastomeres. At the animal pole there are four smaller blastomeres - micromeres, at the vegetative pole there are four larger ones - macromeres. Then the division again occurs in the meridian planes, and then again in the latitude planes.
In polylecithals In the eggs of bony fish, reptiles, birds, as well as monotreme mammals, fragmentation is partial, or meroblastic, i.e. covers only yolk-free cytoplasm. It is located in the form of a thin disk at the animal pole, therefore this type of crushing is called discoidal. When characterizing the type of fragmentation, the relative position and rate of division of the blastomeres are also taken into account. If blastomeres are arranged in rows above each other along radii, cleavage is called radial. It is typical of chordates and echinoderms. In nature, there are other variants of the spatial arrangement of blastomeres during crushing, which determines such types as spiral in mollusks, bilateral in roundworms, anarchic in jellyfish.
A relationship was observed between the distribution of yolk and the degree of synchrony in the division of animal and vegetative blastomeres. In oligolecithal eggs of echinoderms, cleavage is almost synchronous; in mesolecithal egg cells, synchrony is disrupted after the third division, since vegetative blastomeres divide more slowly due to the large amount of yolk. In forms with partial cleavage, divisions are asynchronous from the very beginning and blastomeres, occupying a central position, divide faster.
By the end of crushing, a blastula is formed. The type of blastula depends on the type of cleavage, and therefore on the type of egg.
Features of molecular genetic and biochemical processes during crushing.As noted above, mitotic cycles during the cleavage period are greatly shortened, especially at the very beginning.
For example, the entire division cycle in sea urchin eggs lasts 30-40 minutes, with the S-phase lasting only 15 minutes. The GI and G2 periods are practically absent, since the necessary reserve of all substances has been created in the cytoplasm of the egg cell, and the larger the cell, the greater the supply. Before each division, DNA and histones are synthesized.
The rate at which the replication fork moves along DNA during cleavage is normal. At the same time, more initiation points are observed in the DNA of blastomeres than in somatic cells. DNA synthesis occurs in all replicons simultaneously, synchronously. Therefore, the time of DNA replication in the nucleus coincides with the doubling time of one, and shortened, replicon. It has been shown that when the nucleus is removed from the zygote, fragmentation occurs and the embryo reaches in its development almost to the blastula stage. Further development stops.
At the beginning of cleavage, other types of nuclear activity, such as transcription, are practically absent. In different types of eggs, gene transcription and RNA synthesis begin at different stages. In cases where there are many different substances in the cytoplasm, as, for example, in amphibians, transcription is not immediately activated. Their RNA synthesis begins at the early blastula stage. On the contrary, in mammals, RNA synthesis already begins at the stage of two blastomeres.
During the fragmentation period, RNA and proteins are formed, similar to those synthesized during oogenesis. These are mainly histones, cell membrane proteins and enzymes necessary for cell division. The named proteins are used immediately along with proteins previously stored in the cytoplasm of the eggs. Along with this, during the period of fragmentation, the synthesis of proteins that were not there before is possible. This is supported by data on the presence of regional differences in the synthesis of RNA and proteins between blastomeres. Sometimes these RNAs and proteins begin to act at later stages.
An important role in fragmentation is played by the division of the cytoplasm - cytotomy. It has a special morphogenetic significance, as it determines the type of fragmentation. During cytotomy, a constriction is first formed using a contractile ring of microfilaments. The assembly of this ring occurs under the direct influence of the poles of the mitotic spindle. After cytotomy, the blastomeres of the oligolecithal eggs remain connected to each other only by thin bridges. It is at this time that they are easiest to separate. This occurs because cytotomy leads to a decrease in the contact zone between cells due to the limited surface area of the membranes. Immediately after cytotomy, the synthesis of new areas of the cell surface begins, the contact zone increases and the blastomeres begin to come into close contact. Cleavage furrows run along the boundaries between individual sections of ovoplasm, reflecting the phenomenon of ovoplasmic segregation. Therefore, the cytoplasm of different blastomeres differs in chemical composition.
Characteristics and significance of the main stages of embryonic development: gastrulation, histo- and organogenesis. Formation of 2 and 3 layer embryos. Methods of formation of mesoderm. Derivatives of germ layers. Regulatory mechanisms of these processes at the gene and cellular levels.
Histogenesis- (from the Greek histos - tissue it ... genesis), a set of processes that has developed in phylogenesis, ensuring in the ontogenesis of multicellular organisms the formation, existence and restoration of tissues with their inherent organ-specificity. features. In the body, tissues develop from certain embryonic primordia (derivatives of germ layers), formed as a result of proliferation, movement (morphogenetic movements) and adhesion of embryonic cells in the early stages of its development in the process of organogenesis. Beings, factor G. is the differentiation of determined cells, leading to the appearance of various morphol. and physiol. types of cells that are regularly distributed in the body. Sometimes G. is accompanied by the formation of intercellular substance. An important role in determining the direction of G. belongs to intercellular contact interactions and hormonal influences. A set of cells that perform certain functions. G., is divided into a number of groups: ancestral (stem) cells, capable of differentiation and replenishing the loss of their own kind by division; progenitor cells (so-called semi-stem) - differentiate, but retain the ability to divide; mature differentiated cells. Reparative hygiene in the postnatal period underlies the restoration of damaged or partially lost tissue. G.'s qualities and changes can lead to the appearance and growth of a tumor.
Organogenesis(from the Greek organon - organ, genesis - development, education) - the process of development, or formation, of organs in the embryo of humans and animals. Organogenesis follows earlier periods of embryonic development (see Embryo) - egg fragmentation, gastrulation and occurs after the main rudiments (anlage) of organs and tissues have separated. Organogenesis proceeds in parallel with histogenesis (see), or tissue development. Unlike tissues, each of which has its source in one of the embryonic rudiments, organs, as a rule, arise with the participation of several (from two to four) different rudiments (see Germ layers), giving rise to different tissue components of the organ. For example, as part of the intestinal wall, the epithelium lining the organ cavity and glands develop from the internal germ layer - endoderm (see), connective tissue with blood vessels and smooth muscle tissue - from mesenchyme (see), mesothelium covering the serous membrane of the intestine - from the visceral layer of the splanchnotome, i.e., the middle germ layer - mesoderm, and the nerves and ganglia of the organ - from the neural rudiment. The skin is formed with the participation of the outer germ layer - ectoderm (see), from which the epidermis and its derivatives (hair, sebaceous and sweat glands, nails, etc.) develop, and dermatomes, from which mesenchyme arises, differentiating into the connective tissue basis of the skin (dermis ). Nerves and nerve endings in the skin, as elsewhere, are derivatives of the neural rudiment. Some organs are formed from one primordium, for example, bone, blood vessels, lymph nodes - from mesenchyme; however, here too, derivatives of the rudiment of the nervous system—nerve fibers—grow into the anlage, and nerve endings are formed.
If histogenesis consists mainly in the reproduction and specialization of cells, as well as in the formation of intercellular substances and other non-cellular structures by them, then the main processes underlying organogenesis are the formation of folds, invaginations, protrusions, thickenings, uneven growth, fusion or division by the germ layers (separation), as well as mutual germination of various bookmarks. In humans, organogenesis begins at the end of the 3rd week and is generally completed by the 4th month of intrauterine development. However, the development of a number of provisional (temporary) organs of the embryo - chorion, amnion, yolk sac - begins already from the end of the 1st week, and some definitive (final) organs form later than others (for example, lymph nodes - from the last months of intrauterine development to onset of puberty).
Gastrulation –a single-layer embryo - blastula - turns into a multi-layer - two- or three-layer, called gastrula (from the Greek gaster - stomach in the diminutive sense).
In primitive chordates, for example, the lancelet, a homogeneous single-layer blastoderm during gastrulation is transformed into an outer germ layer - ectoderm - and an inner germ layer - endoderm. The endoderm forms the primary gut with a cavity inside the gastrocoel. The opening leading to the gastrocoel is called the blastopore or primary mouth. Two germ layers are the defining morphological features of gastrulation. Their existence at a certain stage of development in all multicellular animals, from coelenterates to higher vertebrates, allows us to think about the homology of the germ layers and the unity of origin of all these animals. In vertebrates, in addition to the two mentioned during gastrulation, a third germ layer is formed - the mesoderm, which occupies a place between the ecto- and endoderm. The development of the middle germ layer, which is chordomesoderm, is an evolutionary complication of the gastrulation phase in vertebrates and is associated with the acceleration of their development in the early stages of embryogenesis. In more primitive chordates, such as the lancelet, chordomesoderm is usually formed at the beginning of the next phase after gastrulation - organogenesis. A shift in the time of development of some organs relative to others in descendants compared to ancestral groups is a manifestation of heterochrony. Changes in the time of formation of the most important organs in the process of evolution are not uncommon.
The process of gastrulation is characterized by important cellular transformations, such as directed movements of groups and individual cells, selective proliferation and cell sorting, the beginning of cytodifferentiation and inductive interactions.
Methods of gastrulationare different. There are four types of spatially directed cell movements that lead to the transformation of the embryo from a single-layer to a multi-layer.
Intussusception- invagination of one of the sections of the blastoderm inward as a whole layer. In the lancelet, the cells of the vegetative pole invaginate; in amphibians, invagination occurs at the border between the animal and vegetative poles in the region of the gray falx. The process of invagination is only possible in eggs with a small or medium amount of yolk.
Epiboly- overgrowth of small cells of the animal pole with larger cells of the vegetative pole that lag behind in the rate of division and are less mobile. This process is clearly expressed in amphibians.
Denomination-stratification of blastoderm cells into two layers lying on top of each other. Delamination can be observed in the discoblastula of embryos with a partial type of cleavage, such as reptiles, birds, and oviparous mammals. Delamination occurs in the embryoblast of placental mammals, leading to the formation of the hypoblast and epiblast.
Immigration- movement of groups or individual cells that are not united into a single layer. Immigration occurs in all embryos, but is most characteristic of the second phase of gastrulation in higher vertebrates. In each specific case of embryogenesis, as a rule, several methods of gastrulation are combined.
Morphology of gastrulation.In the region of the blastula, from the cellular material of which, during gastrulation and early organogenesis (neurulation), completely defined germ layers and organs are usually formed. Intussusception begins at the vegetative pole. Due to faster division, the cells of the animal pole grow and push the cells of the vegetative pole into the blastula. This is facilitated by a change in the state of the cytoplasm in the cells forming the lips of the blastopore and adjacent to them. Due to invagination, the blastocoel decreases and the gastrocoel increases. Simultaneously with the disappearance of the blastocoel, the ectoderm and endoderm come into close contact. In the lancelet, as in all deuterostomes (these include the echinoderm type, the chordate type and some other small types of animals), the blastopore region turns into the tail part of the body, in contrast to protostomes, in which the blastopore corresponds to the head part. The oral opening in deuterostomes is formed at the end of the embryo opposite the blastopore. Gastrulation in amphibians has much in common with gastrulation of the lancelet, but since their eggs have much more yolk and it is located mainly at the vegetative pole, large amphiblastula blastomeres are not able to invaginate inward.Intussusception goes a little differently. At the border between the animal and vegetative poles in the gray falx region, the cells first strongly extend inward, taking on the appearance of “flask-shaped”, and then pull the cells of the superficial layer of the blastula along with them. A crescentic groove and a dorsal lip of the blastopore appear. At the same time, smaller cells of the animal pole, dividing faster, begin to move towards the vegetative pole. In the area of the dorsal lip they turn over and invaginate, and larger cells grow on the sides and on the side opposite the falciform groove. Then the processepiboly leads to the formation of the lateral and ventral lips of the blastopore. The blastopore closes into a ring, inside which large light cells of the vegetative pole are visible for some time in the form of the so-called yolk plug. Later they are completely immersed inside, and the blastopore narrows. Using the method of marking with intravital (vital) dyes in amphibians, the movements of blastula cells during gastrulation were studied in detail. It was established that specific areas of the blastoderm, called presumptive (from the Latin praesumptio - assumption), during normal development, first appear as part of certain organ primordia, and then within the organs themselves. It is known that in tailless amphibians, the material of the presumptive notochord and mesoderm at the blastula stage lies not on its surface, but in the inner layers of the amphiblastula wall, however, approximately at the same levels as shown in the figure. An analysis of the early stages of amphibian development allows us to conclude that ovoplasmic segregation, which is clearly manifested in the egg and zygote, is of great importance in determining the fate of cells that inherited one or another section of the cytoplasm. Gastrulation in embryos with a mepoblastic type of cleavage and development has its own characteristics. In birds, it begins after cleavage and the formation of the blastula during the passage of the embryo through the oviduct. By the time the egg is laid, the embryo already consists of several layers: the upper layer is called the epiblast, the lower layer is called the primary hypoblast. Between them there is a narrow gap - the blastocoel. Then a secondary hypoblast is formed, the method of formation of which is not entirely clear. There is evidence that primary germ cells originate in the primary hypoblast of birds, and the secondary one forms the extraembryonic endoderm. The formation of primary and secondary hypoblast is considered as a phenomenon preceding gastrulation. The main events of gastrulation and the final formation of the three germ layers begin after oviposition with the onset of incubation. An accumulation of cells occurs in the posterior part of the epiblast as a result of uneven cell division in speed and their movement from the lateral sections of the epiblast to the center, towards each other. A so-called primary stripe is formed, which extends towards the head end. A primary groove is formed in the center of the primary stripe, and primary ridges are formed along the edges. At the head end of the primary streak, a thickening appears - Hensen's node, and in it - the primary fossa. When epiblast cells enter the primary groove, their shape changes. They resemble in shape the “flask-shaped” gastrula cells of amphibians. These cells then become stellate in shape and submerge beneath the epiblast to form mesoderm. The endoderm is formed on the basis of the primary and secondary hypoblast with the addition of a new generation of endoderm cells migrating from the upper layers of the blastoderm. The presence of several generations of endodermal cells indicates that the gastrulation period is extended over time. Some of the cells migrating from the epiblast through Hensen's node form the future notochord. Simultaneously with the initiation and elongation of the notochord, Hensen's node and the primitive streak gradually disappear in the direction from the head to the caudal end. This corresponds to the narrowing and closure of the blastopore. As the primitive streak contracts, it leaves behind formed areas of the axial organs of the embryo in the direction from the head to the tail sections. It seems reasonable to consider the movements of cells in the chick embryo as homologous epiboly, and the primitive streak and Hensen's node as homologous to the blastopore in the dorsal lip of the gastrula of amphibians. It is interesting to note that the cells of mammalian embryos, despite the fact that in these animals the eggs have a small amount of yolk and complete fragmentation, during the gastrulation phase they retain the movements characteristic of the embryos of reptiles and birds. This supports the idea that mammals descended from an ancestral group in which eggs were rich in yolk.
Features of the gastrulation stage.Gastrulation is characterized by a variety of cellular processes. Mitotic cell reproduction continues, and it has different intensity in different parts of the embryo. However, the most characteristic feature of gastrulation is the movement of cell masses. This leads to a change in the structure of the embryo and its transformation from blastula to gastrula. Cells are sorted according to their belonging to different germ layers, within which they “recognize” each other. The gastrulation phase marks the beginning of cytodifferentiation, which means the transition to the active use of biological information from one’s own genome. One of the regulators of genetic activity is the different chemical composition of the cytoplasm of embryonic cells, established as a result of ovoplasmic segregation. Thus, the ectodermal cells of amphibians are dark in color due to the pigment that entered them from the animal pole of the egg, and the endoderm cells are light, since they originate from the vegetative pole of the egg. During gastrulation, the role of embryonic induction is very important. It has been shown that the appearance of the primitive streak in birds is the result of an inductive interaction between the hypoblast and the epiblast. The hypoblast is characterized by polarity. A change in the position of the hypoblast relative to the epiblast causes a change in the orientation of the primitive streak. All of these processes are described in detail in the chapter. It should be noted that such manifestations of the integrity of the embryo as determination, embryonic regulation and integration are inherent in it during gastrulation to the same extent as during cleavage.
Formation of mesoderm - In all animals, with the exception of coelenterates, in connection with gastrulation (in parallel with it or at the next stage caused by gastrulation), a third germ layer - mesoderm - appears. This is a set of cellular elements lying between the ectoderm and endoderm, i.e. in the blastocoele. Like this. Thus, the embryo becomes not two-layered, but three-layered. In higher vertebrates, a three-layered structure of embryos appears already during the process of gastrulation, while in lower chordates and all other types, as a result of gastrulation proper, a two-layer embryo is formed.
It is possible to establish two fundamentally different pathways for the emergence of mesoderm:teloblastic, characteristic of Protostomia, and enterocoelous, characteristic of Deute-rosiomia. in protostomes, during gastrulation, at the border between the ectoderm and endoderm, on the sides of the blastopore, there are already two large cells that separate small cells from themselves (due to divisions). Thus, the middle layer is formed - the mesoderm. Teloblasts, giving rise to new generations of mesoderm cells, remain at the posterior end of the embryo. For this reason, this method of mesoderm formation is called teloblastic (from the Greek telos - end).
With the enterocoel method, a set of cells of the developing mesoderm appears in the form of pocket-like protrusions of the primary intestine (protrusion of its walls into the blastocoel). These protrusions, into which parts of the primary intestinal cavity enter, are separated from the intestine and separated from it in the form of pouches. The cavity of the sacs turns into a whole, i.e., into a secondary body cavity; the coelomic sacs can be divided into segments of the middle germ layer does not reflect the whole variety of variations and deviations that are strictly natural for individual groups of animals. Similar to the teloblastic method, but only externally, is the method of formation of mesoderm not by dividing teloblasts, but by the appearance at the edges of the blastopore of an unpaired dense primordium (group of cells), subsequently dividing into two symmetrical stripes of cells. With the enterocoel method, the mesoderm rudiment can be paired or unpaired; in some cases, two symmetrical coelomic sacs are formed, and in others, one common coelomic sac is first formed, which is subsequently divided into two symmetrical halves.
Derivatives of germ layers.The further fate of the three germ layers is different.
From the ectoderm develop: all nervous tissue; the outer layers of the skin and its derivatives (hair, nails, tooth enamel) and partially the mucous membrane of the oral cavity, nasal cavity and anus.
The endoderm gives rise to the lining of the entire digestive tract - from the oral cavity to the anus - and all its derivatives, i.e. thymus, thyroid gland, parathyroid glands, trachea, lungs, liver and pancreas.
From the mesoderm are formed: all types of connective tissue, bone and cartilage tissue, blood and the vascular system; all types of muscle tissue; excretory and reproductive systems, dermal layer of skin.
In an adult animal there are very few organs of endodermal origin that do not contain nerve cells originating from the ectoderm. Each important organ also contains derivatives of the mesoderm - blood vessels, blood, and often muscles, so that the structural isolation of the germ layers is preserved only at the stage of their formation. Already at the very beginning of their development, all organs acquire a complex structure, and they include derivatives of all germ layers
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 sex cells (gametes) with each other, during which two different functions are carried out: sexual (combining the genes of two parents) and reproductive (the emergence of a new organism). The first of these functions includes the transfer of genes from parents to offspring, the second is the initiation in the cytoplasm of the egg of those reactions and movements that allow development to continue. As a result of fertilization, a double (2n) set of chromosomes is restored in the egg. The centrosome introduced by the sperm, after duplication, forms a fission spindle, and the zygote enters the 1st stage of embryogenesis - the cleavage stage. As a result of mitosis, 2 daughter cells are formed from the zygote - blastomeres.
Prezygotic period
The prezygotic period of development is associated with the formation of gametes (gametogenesis). The formation of eggs begins in women even before their birth and is completed for each given egg only after its fertilization. By the time of birth, a 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, sex cells in the testes (testes) in men begin to form only with the onset of puberty. The duration of sperm formation is approximately 70 days; per gram of testicle weight, the number of sperm is about 100 million per day.
Fertilization
Fertilization - the fusion of a male reproductive cell (sperm) with a female (egg, egg), leading to the formation of a zygote - a new single-celled 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, restoration of the diploid set of chromosomes, as well as activation of the egg, that is, stimulation of its embryonic development. The union of the egg with the sperm usually occurs in the funnel-shaped dilated part of the fallopian tube during the first 12 hours after ovulation.
Seminal fluid, entering a woman’s vagina during sexual intercourse, usually contains from 60 to 150 million sperm, which, thanks to movements at a speed of 2-3 mm per minute, constant wave-like contractions of the uterus and tubes and an alkaline environment, already after 1-2 minutes after sexual intercourse they reach the uterus, and after 2-3 hours - the end sections of the fallopian tubes, where fusion with the egg usually occurs. There are monospermic (one sperm penetrates the egg) and polyspermic (two or more sperm penetrate the egg, but only one sperm nucleus fuses with the egg nucleus). The preservation of sperm activity while passing through the woman’s genital tract is facilitated by the slightly alkaline environment of the cervical canal of the uterus, filled with a mucus plug. During orgasm during sexual intercourse, the mucous plug from the cervical canal is partially pushed out and then retracted into it again, thereby facilitating the faster entry of sperm from the vagina (where normally in a healthy woman the environment is slightly acidic) into the more favorable environment of the cervix and uterine cavity. The passage of sperm through the mucous plug of the cervical canal is also facilitated by the sharply increasing mucus permeability on the days of ovulation. On the remaining days of the menstrual cycle, the mucus plug has significantly less permeability to sperm.
Many sperm found in a woman’s genital tract 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. Taking this into account, the most favorable time for fertilization is considered to be 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 remember that the timing of ovulation may fluctuate, and the viability of the egg and sperm may be significantly longer. Soon after fertilization, the zygote begins to fragment and form an embryo.
ZygoteA zygote (Greek zygote paired) is a diploid (containing a complete double set of chromosomes) cell formed as a result of fertilization (the fusion of an egg and a sperm). A zygote is a totipotent (that is, capable of giving birth to any other) cell. The term was introduced by the German botanist E. Strassburger.
In humans, the first mitotic division of the zygote occurs approximately 30 hours after fertilization, which is due to complex processes of preparation for the first act of cleavage. The cells formed as a result of fragmentation of the zygote are called blastomeres. The first divisions of the zygote are called “fragmentations” because the cell is fragmented: the daughter cells become smaller after each division, 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 covered with a dense shell and for some time turns into a resting spore (often called a zygospore) - characteristic 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 of 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 fragmentation, 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 division during the development of a fertilized egg determined the appearance of the figurative term - fragmentation of the zygote.
In different animal species, eggs differ in the quantity and nature of 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 yolk in the cytoplasm, the entire mass of the zygote is divided with the formation of identical blastomeres - complete uniform fragmentation (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 the part free of yolk is crushed. Thus, in reptiles and birds, only the disc-shaped portion of the zygote at one of the poles, where the nucleus is located, is subject to fragmentation - incomplete, discoidal fragmentation. Finally, in insects, only the surface layer of the zygote cytoplasm is involved in the process of crushing - incomplete, superficial crushing.
As a result of fragmentation (when the number of dividing blastomeres reaches a significant number), a blastula is formed. In a typical case (for example, in a lancelet), the blastula is a hollow ball, the wall of which is formed by a single layer of cells (blastoderm). The cavity of the blastula - blastocoel, otherwise called the primary body cavity, is filled with fluid. In amphibians, the blastula has a very small cavity, and in some animals (for example, arthropods), the blastocoel may be completely absent.
GastrulationAt the next stage of the embryonic period, the process of gastrula formation occurs - gastrulation. In many animals, gastrula formation occurs through intussusception, i.e. protrusion of the blastoderm at one of the poles of the blastula (with intensive proliferation of cells in this zone). As a result, a two-layer, cup-shaped embryo is formed. The outer layer of cells is ectoderm, and the inner layer is endoderm. The internal cavity that appears 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 gastrula endoderm is formed by immigration, i.e. “eviction” of some blastoderm cells into the cavity of the blastula and their subsequent reproduction. The primary mouth is formed by rupture of the gastrula wall. With uneven fragmentation (in some worms and mollusks), the gastrula is formed as a result of the overgrowth of macromeres with micromeres and the formation of endoderm at the expense of the former. Often different methods of gastrulation are combined.
In all animals (except for sponges and coelenterates - two-layered animals), the gastrulation stage ends with the formation of another layer of cells - mesoderm. This “cellular layer is formed between the ento- and ectoderm. There are two known ways of laying down the mesoderm. In annelids, for example, in the region of the gastrula blastopore, two large cells (teloblasts) are separated. When multiplying, 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 stripes) coelomic pouches are formed - a teloblastic method of laying down the mesoderm. In the enterocoelous method (echinoderms, lancelets, vertebrates), as a result of protrusion of the wall of the primary intestine, lateral pockets are formed, which then separate and become coelomic. bags. In both cases of mesoderm formation, coelomic bags grow and fill the primary body cavity. The mesodermal layer of cells lining the body cavity forms the peritoneal epithelium, which thus replaces the primary one, is called the secondary body cavity, or coelom. the blastopore turns into the mouth opening of an adult animal. Such organisms are called protostomes. In deuterostome animals (with the enterocoelous method of laying down the mesoderm), the blastopore overgrows or turns into the anus, and the mouth of an adult individual appears a second time, by protrusion of the ectoderm.
The formation of three germ layers (ecto-, ento- and mesoderm) completes the gastrulation stage, and from this moment the processes of histo- and organogenesis begin. As a result of differentiation of the cells of the three germ layers, various tissues and organs of the developing organism are formed. Even at the end of the last century (largely thanks to the research 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 its derivative structures and the nervous system are formed. The endoderm forms the digestive tract and associated organs (liver, pancreas, lungs, etc.). The mesoderm forms the skeleton, vascular system, excretory apparatus, and gonads. Although today the germ layers are not considered strictly specialized, their homology in the vast majority of animal species is obvious, which indicates the unity of origin of the animal kingdom.
During the embryonic period, the rate of growth and differentiation in developing organisms increases. If during the process of fragmentation growth does not occur and the blastula (in its mass) can be significantly inferior to the zygote, then, starting with 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 - fragmentation and underlie primary tissue differentiation - the emergence of three germ layers (embryonic tissues). The 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. The birth of a new individual occurs either as a result of hatching from an egg (in oviparous animals) or leaving the mother’s body (in viviparous animals).
Histo- and organogenesisThe histo- and organogenesis of the embryo is carried out as a result of reproduction, migration, differentiation of cells and its components, the establishment of intercellular contacts and the death of some cells. The presomitny period continues from the 317th to the 20th day; the somitny period of development begins on the 20th day. On the 20th day of embryogenesis, through the formation of trunk folds (cephalocaudal and lateral), the embryo itself is separated from the extraembryonic organs, as well as a change in its flat shape to a cylindrical one. At the same time, the dorsal portions of the mesoderm of the embryo are divided into separate segments located on both sides of the notochord - somites. On the 21st day, the embryo has 2-3 pairs of somites. Somites begin to form from the third pair; the first and second 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 counting the number of somites, it is possible to approximately determine the development time (somitic age) of the embryo.
A dermatome arises from the outer part of each somite, a sclerotome from the inner part, and a myotome from the middle part. The dermatome becomes the source of the dermis of the skin, the sclerotome - cartilage and bone tissue, the myotome - 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 layers, from which the serous membranes of the internal organs, the muscular tissue of the heart and the adrenal cortex develop. From the mesenchyme of the splanchnotome, blood vessels, blood cells, connective and smooth muscle tissue of the embryo are formed. The area of mesoderm connecting the somites with the splanchnotome is divided into segmental legs - nephrogonotome, which serve as a source of development of the kidneys and gonads, as well as paramesonephric ducts. The latter forms the epithelium of the uterus and oviduct.
During the process of differentiation of the embryonic ectoderm, the neural tube, neural crests, placodes, cutaneous 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) grow together to form the neural tube. From the cranial part of the neural tube, the brain vesicles are formed - the rudiment of the brain. On both sides of the neural tube (between the latter and the cutaneous ectoderm), groups of cells are separated from which the neural crests are formed. Neural crest cells are capable of migration. Cells migrating towards 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 neural crest cells that do not migrate, ganglion plates are formed, from which spinal and peripheral autonomic nerve ganglia develop. The placodes form the head ganglia and nerve cells of the organ of hearing and balance.
The period of embryonic development is the most complex in higher animals and consists of several stages.
The period begins with the stage zygote fragmentation(Fig. 1), i.e., a series of successive mitotic divisions of a fertilized egg. The two cells formed as a result of division (and all subsequent generations) at this stage are called blastomeres. One division follows another, and the resulting blastomeres do not grow and with each division the cells become smaller and smaller. This feature of cell division determined the appearance of the figurative term “fragmentation of the zygote.”
Rice. 1.Cleavage and gastrulation of lancelet egg (side view)
The figure shows: A- mature egg with a polar body; b- 2-cell stage; V- 4-cell stage; G- 8-cell stage; d- 16-cell stage; e- 32-cell stage (sectioned to show blastocoel); g - blastula; h - blastula section; and - early gastrula (at the vegetative pole - arrow - intussusception begins); j - late gastrula (intussusception has ended and a blastopore has formed; 1 - polar body; 2 - blastocoel; 3 - ectoderm; 4 - endoderm; 5 - cavity of the primary intestine; 6 - blastopore).
As a result of fragmentation (when the number of blastomeres reaches a significant number), a blastula is formed (see Fig. 1, g, h). Often it is a hollow ball (for example, in a lancelet), the wall of which is formed by one layer of cells - the blastoderm. The cavity of the blastula, the blastocoel, or primary cavity, is filled with fluid.
At the next stage, the process of gastrulation occurs - the formation of the gastrula. In many animals, it is formed by invagination of the blastoderm inward at one of the poles of the blastula during intensive proliferation of cells in this zone. As a result, a gastrula appears (see Fig. 1, i, j).
The outer layer of cells is called ectoderm, and the inner layer is called endoderm. An internal cavity bounded by endoderm, the cavity of the primary gut communicates with the external environment by the primary mouth, or blastopore. There are other types of gastrulation, but in all animals (except sponges and coelenterates), this process ends with the formation of another cellular layer - mesoderm. It is located between the ento- and ectoderm.
At the end of the gastrulation stage, three cell layers (ecto-, endo- and mesoderm), or three germ layers, appear.
Next, the processes of histogenesis (tissue formation) and organogenesis (organ formation) begin in the embryo (embryo). As a result of differentiation of germ layer cells, various tissues and organs of the developing organism are formed. The integument and nervous system are formed from the ectoderm. Due to the endoderm, the intestinal tube, liver, pancreas, and lungs are formed. The mesoderm produces all other systems: musculoskeletal, circulatory, excretory, reproductive. The discovery of homology (similarity) of the three germ layers in almost all animals served as an important argument in favor of the point of view about the unity of their origin. The patterns outlined above were established at the end of the 19th century. I. I. Mechnikov and A. O. Kovalevsky and formed the basis of the “doctrine of germ layers” formulated by them.
During the embryonic period, there is an acceleration in the rate of growth and differentiation in the developing embryo. It is only during the process of fragmentation of the zygote that growth does not occur and the blastula (in its mass) may even be significantly inferior to the zygote, but starting from the process of gastrulation, the mass of the embryo rapidly increases.
The formation of different types of cells begins at the stage of fragmentation and underlies primary tissue differentiation - the emergence of three germ layers. Further development of the embryo is accompanied by an increasingly intensifying process of differentiation and morphogenesis. By the end of the embryonic period, the embryo already has all the main organs and systems that ensure viability in the external environment.
The embryonic period ends with the birth of a new individual capable of independent existence.
The concept of “the birth of a new life”, as a rule, is limited exclusively to associations about the conception of a child as the result of a passionate meeting of an egg and a sperm. Next, according to the majority, pregnancy occurs, the fetus develops and the expectant mother grows a big belly. What is there to be clever about, everything is banally simple... In fact, prenatal human development is a very important and subtle process that requires in-depth study. Let's try to understand the intricacies of one of its stages - crushing the zygote.
A zygote is an egg fertilized by a sperm. It is with fertilization, which can occur within 3 days after sexual intercourse, that the intrauterine development of a person begins. As a result of the penetration of the sperm into the egg, their nuclei merge with chromosome sets of 23 paternal and 23 maternal chromosomes and a nucleus is formed with a complete set of 46 chromosomes inherent in all cells of the body, with the exception of sex cells. After this, the zygote is fragmented.
Fragmentation of a human zygote is a 3-4-day process of dividing an embryo into small parts-cells by reproducing their structure similar to the structure of the mother cell (mitosis or cloning division) while maintaining its overall size (about 130 microns). Blastomers - cells formed during the fragmentation of the zygote, also divide, and at different rates, in other words, their division is not synchronous.
As a result of the first division of the zygote, two differentiated blastomeres emerge. One, larger, “dark” one, is the basis for the development of tissues and organs of the embryo. The totality of large blastomeres obtained during subsequent divisions is called an embryoblast. The second, small and “light” type of blastomere, the division of which occurs more quickly, forms a collection of its own kind - the trophoblast. With its help, finger-shaped villi arise, which are necessary for the subsequent attachment of the zygote to the uterine cavity. The blastomeres, without interacting with each other, are held in place only by the ovum pellucida. Its rupture can lead to the development of genetically identical embryos, such as identical twins.
Emergence of a multicellular embryoAs a result of fragmentation of the zygote, a multicellular embryo is formed, consisting of cell layers of the embryoblast (inside) and trophoblast (on the periphery). This is the morula stage - a period of embryonic development, during which the embryo contains up to hundreds of cells, fragmentation and 
the formation of which occurs as the embryo moves through the oviduct into the uterine cavity. Due to the lack of independent mobility, the movement of the crushed egg occurs under the influence of the hormones progesterone and estrogen due to the peristalsis of the muscles of the oviduct, the movement of the cilia of its epithelium, as well as the movement of the secretion of the glands in the fallopian tube. Somewhere on the 6th day after fertilization, the entry of the morula into the uterus leads to the beginning of the process of blastulation - the formation of a blastocyst, which is a hollow vesicle filled with liquid from well-developed layers of trophoblast and embryoblast.
Around day 9-10, the embryo grows (implanted) into the wall of the uterus, which is already completely surrounded by its cells. From this moment on, the woman’s menstrual cycle stops, and pregnancy can be determined.
