The formation of specialized local populations is most characteristic of organisms that remain attached during most of their life cycle. The reason is that mobile organisms largely control the conditions of their existence: they can avoid disastrous or unfavorable habitats or move away from them and begin to actively search for new ones; motionless organisms - higher plants, many seaweeds, corals - do not have such freedom; at the end of the dispersal stage of the life cycle, they either have to live in the conditions that exist where they settled, or die. As Bradshaw (1972) noted, "...the plant is unable to run to a new place or hide in a secluded corner." All that a higher plant can do is, growing and “growing” from place to place, to seek out resources or get out of an unfavorable area; uproot itself and transplant itself of its own choice to another place, it cannot. Its offspring (seeds, pollen or gametes) are subject to all the vicissitudes of passive propagation by wind, water or animals (on the surface or inside the body). For these reasons, populations of immobile organisms are subjected to a particularly strong influence of natural selection.[ ...]

The formation of spores from the sporogenous tissue (archesporia) of the sporangium is preceded by meiosis. In this case, as we already know, the number of chromosomes is halved, and the spore has a single set of chromosomes, it is haploid. Haploid are the protonema, gametophores, organs of sexual reproduction (archegonia and antheridia) and, of course, gametes. All these structures belong to the sexual generation - ha-metophase.[ ...]

The formation of zoospores and gametes in brown algae occurs in receptacles of two main types: single-nested (Fig. 121, 2) and multi-nested (Fig. 128, 1 a). Multi-cell receptacles can function as sporangia and as gametangia. Outwardly, they do not differ in the same way as zoospores and gametes. Single-celled receptacles are more often sporangia. Meiosis in brown algae occurs during the formation of spores in single-celled sporangia, only in cyclospores it occurs at the time of formation of gametes.[ ...]

With the formation of a spore, the haploid phase in the fern life cycle begins, which ends with the formation of gametes. Gametes are formed on the gametophyte (sexual generation, or outgrowth) of a fern that arises from a germinating spore. But out of the colossal number of spores (usually several tens of millions) produced each time by a sporophyte, only a relatively small part falls into sufficiently favorable conditions for germination, and of the germinated spores, far from all reach the stage of a mature gametophyte.[ ...]

Meiosis during the formation of pollen and the embryo sac in haploids proceeds with great disturbances, which contributes to the sterility of gametes. In such plants, the chromosomes in the metaphase of the first division of meiosis during microsporogenesis remain univalent, are arranged incorrectly and diverge unevenly in the anaphase of the first division, as a result of which nuclei with different numbers of chromosomes are formed. There have been cases of the appearance of restorative nuclei as a result of the loss of reduction division and the formation of dyads with a diploid number of chromosomes.[ ...]

After the fusion of gametes and the formation of a zygote, a long dormant period begins, which can last all winter or more. In the zygote, two out of four or one out of two chloroplasts are destroyed (depending on how many there are in the cell). The gamete nuclei lie side by side during the dormant period and then fuse just before the zygote germinates. The process of germination of the zygote was observed very rarely, and therefore it is still poorly understood.[ ...]

In the first minutes of zygote formation, numerous contracting vacuoles appear in it. Small vacuoles merge into larger ones. Each vacuole pulsates for 4-7 minutes, then contracts, and its contents come out. The action of pulsating vacuoles lasts from 4 to 32 minutes, until the transparent places of the zygote are no longer visible. During this period (from 1 to 3.5 hours), the zygote is significantly reduced. Inside it, chloroplasts and pyrenoids are located in the peripheral layer. Then the first signs of zygote ornamentation begin to appear - cone-shaped tubercles scattered over its surface. Gradually they stretch out and take the form of spikes. During the growth of spikes, which lasts 2-3 hours, the zygote again increases in size and becomes the same as during the fusion of gametes.[ ...]

As a result of the fusion of gametes, a spherical zygote is formed, while the flagella fall off and a shell appears. The zygotes of some algae retain flagella for some time, then a planozygote is obtained, which is able to swim from several days to three weeks. In the zygote, the fusion of two nuclei of gametes occurs, and it becomes diploid. In the future, the zygotes of different algae behave differently. Some zygotes develop a thick membrane (hyppozygotes) and fall into a dormant period lasting up to several months. Other zygotes germinate without a dormant period. In some cases, new thalli grow directly from the zygotes. In others, zygotes divide with meiosis and zoospore formation; such zygotes pre-grow, and 4-32 zoospores emerge from them, depending on the size.[ ...]

Sexual reproduction is the formation of a new organism with the participation of two parent individuals. During sexual reproduction, the fusion of germ cells - the gametes of the male and female body. The new organism carries hereditary information from both parents. Sex cells are formed as a result of a special type of division. In this case, unlike the cells of an adult organism, which carry a diploid (double) set of chromosomes, the resulting gametes have a haploid (single) set. As a result of fertilization, the paired, diploid set of chromosomes is restored. One chromosome from a pair is paternal, and the other is maternal. Gametes are formed in the gonads or in specialized cells during meiosis.[ ...]

Fertilization begins with the formation and growth of the pollen tube, which actively makes its way through the tissues of the megasporapgium (nucellus) towards the archegonium. Approximately a week before the act of fertilization, the nucleus of the spermatogenic cell divides, forming two identical or unequal in size (in araucaria and pine) male gametes. The tip of the pollen tube makes its way between the cervical cells of the archegonium and reaches the egg. Here it ruptures, releasing male gametes into the cytoplasm of the egg. Following this, one of the two male gametes enters the egg. The fusion of the two nuclei occurs very slowly, but in the end they unite, forming the first diploid nucleus of the sporophyte.[ ...]

Fertilization is the union of gametes, accompanied by the formation of a zygote.[ ...]

Gametophyte - the stage associated with the formation of gametes in the life cycle of plants.[ ...]

In the animal kingdom, meiosis leads to the formation of germ cells - gametes, and usually only these cells contain a hayloid set of chromosomes. In plants, meiosis can occur at different stages of the life cycle, and as haploid products, they form both germ cells - gametes, and asexual spores. The lifespan of gametes is limited and ranges from several minutes to several days, after which unfertilized gametes disappear.[ ...]

In desmids, the formation of double zygotes can also occur. They develop during the copulation of four cells that have arisen through the successive division of one individual (Fig. 256, 7). In closterium species, the method of formation of double zygotes is different: in each cell, two special gametes first develop, which, merging, give double zygotes. Very rarely, three or four individuals take part in the formation of desmidian zygotes.[ ...]

Female gametes are large, immobile; male - smaller, biflagellated. During the transition to sexual reproduction, part of the cells in the upper half of the thallus undergoes sequential division and gives rise to a multilayered sexual tissue. Within this tissue, dark and light areas are well distinguished, arranged in pairs and corresponding to clusters of female and male cells. Each cell eventually produces one gamete. The zygote, like aplanospores, germinates into a lamellar thallus. The gametophyte, up to the formation of germ cells, is morphologically similar to the sporophyte, so the development cycle of prazioly can be considered isomorphic. Its originality lies in the unusual structure of the genital tissue.[ ...]

Since during the sexual process, as a result of the fusion of gametes and their nuclei, the set of chromosomes in the nucleus is doubled, then at some point in the development cycle, a reduction nuclear division (meiosis) occurs, as a result of which the daughter nuclei receive a single set of chromosomes. The sporophytes of many algae are diploid, and meiosis in the cycle of their development coincides with the moment of formation of spores, from which haploid gametosporophytes or gametophytes develop. Such meiosis is called sporadic reduction (Fig. 2b, 1).[ ...]

As a rule, mutations in the number of chromosomes occur in the gametes of one of the parents. Therefore, all cells of the organism, in the conception of which one of the mutant gametes took part, will contain an abnormal chromosome set. However, sometimes quantitative chromosomal mutations can occur during the first divisions of a zygote formed by normal gametes. An organism will develop from such a zygote, some of the cells of which will have a normal diploid set, while the other part will have an abnormal one. This phenomenon is called chromosomal mosaicism, and individuals with mosaicism are called chromosomal mosaics. Mosaicism is more frequent along the sex chromosomes. Such mosaics have the genotype X/XX, X/XY, XX/XY, XXY/XX.[ ...]

In higher animals, in the case of males, meiosis is accompanied by the formation of four functionally active gametes (Fig. 80). In contrast, in females, each ovocyte of the second order produces only one egg. Other nuclear products of female meiosis are three reduction bodies that do not participate in reproduction and degenerate.[ ...]

Isogametes develop in special sessile "sporangia". These are spherical formations occupying different positions on the threads: they can be apical, lateral, intercalary. They develop mainly on creeping threads. These sessile "sporangia" form as a result of cell enlargement and, unlike stalked sporangia, do not separate from the filament when the gametes mature. The biflagellated reproductive cells that do not always develop in them behave like gametes, they often germinate without fusion or give aplanospores.[ ...]

Let's consider this using an example with genomic formulas. PrCBO plants produce gametes with the AVBEB2 genomic structure (from 0 to 7). When fertilized with a common wheat gamete with ABB genomes, P3(B2) plants arise, which have the following genomic structure: AA + BB + BB + E (from 0 to 7) and B2 (from 0 to 7). The chromosomes of the E and B2 genomes, as having no partners, will be eliminated already during the formation of P3 gametes. And in the next, fourth generation, many plants will have the genome structure AA + BB + BB, i.e., these will be plants of the common wheat type, which still contain single chromosomes of the B2 and E genomes and therefore some features of wheatgrass will be preserved. But in the future, these couch grass chromosomes will be eliminated. Wheatgrass hereditary! material in wheat-couch hybrids of the common wheat type may be preserved in the form of separate couch grass segments in wheat chromosomes, or, as some researchers now note, individual couch grass chromosomes can be in wheat-couch hybrids as additional or replacing wheat chromosomes. But in all these cases, the plants belong to the type of annual soft wheats, and their couch grass features are manifested in a weak form.[ ...]

The peculiarity of conjugation in mucosity lies in the fact that even before the formation of the membrane, the eigota is separated from the cells that formed it by two, three or four partitions.[ ...]

For the exit of gametes, there is a hole with uneven edges in the outer wall of the cell (Fig. 219.7). The zygote develops into a single-celled sporophyte. At the same time, it greatly increases in size and is covered with a thick shell (Fig. 219, 5). After a more or less long time, the contents of this unicellular plant divides with the formation of zoospores. Thus, here the change of forms of development is heteromorphic.[ ...]

In multicellular organisms (plants and animals), sexual reproduction is associated with the formation of germ or germ cells (gametes), fertilization and the formation of zygotes.[ ...]

In the ephedra megasporangium, a massive female gametophyte develops from the megaspore as a result of free nuclear fission and the subsequent formation of cell walls. Each of them usually contains two archegonia. Archegonium has a long neck, consisting of 32 or more cells. When the nucleus of the central cell of the archegonium divides, no cell septa is formed between the abdominal tubular nucleus and the nucleus of the egg.[ ...]

Two parental individuals take part in sexual reproduction, each of which participates in the formation of a new organism, introducing only one sex cell - a gamete (egg or sperm). Each gamete carries half a set of chromosomes. As a result of the fusion of two gametes, a zygote is formed, from which a new organism develops. The zygote as a result receives the hereditary characteristics of both parents.[ ...]

In modern representatives of the order, only sexual reproduction is known. The sexual process is isogamy. Gametes are formed in special gametangia. In most Dasycladic gametangia, they appear as spherical outgrowths on the top or side of the branches of the first order and correspond to modified branches of the second order (Fig. 232, 3, 4). In acetabularia, gametangia are formed on short segments of the first order as special outgrowths (Fig. 232, 9). Thick-walled multinuclear cysts with a special lid are formed inside the gametangium (Fig. 232, 10). Mature cysts contain many gametes. When the walls of the gametangium are destroyed, the cysts enter the external environment and already here gametes are released from them. Cysts rather than zygotes may serve as resting stages. There is an opinion that the formation of cysts is associated with calcification of the thallus. Some decalcified genera do not have them and gametes are formed directly in gametangia.[ ...]

Subsequently, anisogamy developed (from the Greek anisos - unequal, games - marriage), characterized by the differentiation of gametes that differ only in size. An example of anisogamy is the formation of gametes also in a number of species of protozoa.[ ...]

As a result of meiosis, microspore tetrads are formed from the mother cells of microspores, which subsequently develop into male gametes. There are three types of microspore tetrad formation (Fig. 94): successive (successive), intermediate and simultaneous (simultaneous).[ ...]

Meiosis is essentially two divisions in which cells divide twice and chromosomes divide only once. This leads to the formation of four cells, each of which has a haploid number of chromosomes, that is, half the set of chromosomes of somatic cells. Each of these four cells is potentially a gamete. Fertilization (fusion of two gametes) restores the diploid number of chromosomes.[ ...]

The genus is characterized by an isomorphic change in the forms of development and it is impossible to distinguish sexual plants from asexual plants in appearance. Mature gametes and zoospores enter the environment through one rounded opening - a pore formed at the top of the papillary bend in the outer wall of the cell (Fig. 218, 2). During the germination of zoospores and zygotes, the enteromorph, like all algae of the Ulva family, passes through the stage of a single-row thread (Fig. 218, 3-7). The cell from which development begins is divided into two - apical and basal. As a result of transverse divisions, the first one forms a vertical thread, the second turns into a primary rhizoid. Later, the thread is transformed into a tubular thallus. When environmental conditions change, the first divisions can lead to the formation of creeping filaments arranged in the form of a disk, from the cells of which one or more vertical single-row filaments grow later.[ ...]

The duration of haplophase and diplophase in different systematic groups of plants is different, while the processes directly related to the formation of gametes (meiotic division) are extremely similar to each other.[ ...]

seed plants. Today, gymnosperms are represented by approximately 700 species of shrubs and trees. These plants have seeds and the gametophyte is reduced. The formation of germ cells, fertilization and maturation of seeds occur on an adult plant - a sporophyte. The presence of seeds dramatically enhances the ability of plants to develop new spaces. Strictly speaking, the presence of seeds to some extent replaces the impossibility of plants to move, as if compensating for their immobility relative to animals. The seed also contributes to greater resistance of plants to the effects of adverse environmental factors. Gymnosperms are subdivided into conifers - about 560 modern species; cycads - known from the Carboniferous period, and ginkgo - also relict. The last two classes have a very limited distribution.[ ...]

Meiosis is a logically necessary part of the sexually reproducing life cycle. Meiosis provides for the splitting of genes - individual sections of DNA into individual gametes, resulting in a diverse combination of genes in gametes. With regard to maintaining the constancy of chromosomes in a cell, fertilization is the antithesis (opposite) of meiosis; in the process of fertilization, the haploid nuclei of two gametes of different sexes merge to form one cell - a zygote with a diploid nucleus.[ ...]

Dichotomous dictyota is the most widespread. Its thalli form large tufts up to 20 cm high, the width of the branches reaches 4-8 mm. Dichotomous dictyota is interesting for its distinct periods in the formation and maturation of gametangia. Off the coast of England, gametangia begin to develop during quadrature tides, and the final maturation and release of gametes occurs during several tides following the highest spring tide. The gametes mature every two weeks. Such a periodicity in reproduction and its connection with the lunar rhythm was preserved when the thalli were kept in the laboratory for several months. Off the Atlantic coast of North America, gamete maturation occurs at intervals of one month. Gamete development begins the day before full moon spring tide or the next day, gametes mature 6 or 8 days later. In the Gulf of Peter the Great (Sea of ​​Japan), plants with tetrasporangia are usually more common, which sometimes all turn into seedlings on the mother plants, and the branches of the latter become shaggy, since the seedlings, before separating, grow several millimeters high. Dichotomous dictyota grows in the upper sublittoral in places with water movement.[ ...]

The mechanisms of polyploidy are that they are the result of perversions of one or more mitotic cell divisions of the embryo or the result of non-disjunction during meiosis of the entire set of chromosomes, leading to the formation of diploid gametes. Nondisjunction of chromosomes in women occurs in 80% of cases, and in men it is observed in 20% of cases, and it is noted both in the first and second meiotic divisions.[ ...]

Unlike other multicellular algae, brown algae, along with the usual single-celled sporangia (Fig. 121, 2), have multi-celled sporangia and gametangia, which are incorrectly called multicellular (Fig. 128, 1 a). Before the formation of zoospores or gametes, the contents of multi-cell receptacles are divided by thin partitions into chambers, in which they are enclosed in one nucleus with a portion of the cytoplasm. Each chamber develops one, rarely two zoospores or gametes. On the surface of the thallus of many brown algae, special multicellular hairs develop, which look like a thread from one row of cells with a growth zone at the base; cells of the growth zone divide more often than others and therefore have small sizes (Fig. 121, 1 b).[ ...]

Sterols with 28 and 29 carbon atoms can be biogenetic precursors of various phytoecdysones (see above). The algae Achlya bisexual is and A. ambisexual is found to have a C29 steroid with hormonal activity - anteridiol.[ ...]

Pollen formation. In the anther, the pollen mother cells undergo meiosis and form microspores, haploid male spores, which are known as pollen at the end of development. A pollen grain can be considered as a separate plant, a male gametophyte (Fig. 117). Such a haploid "plant" producing male gametes is a survival of the gametophyte generation, which can be well developed in more primitive plants, such as ferns and mosses. In seed plants, this stage is greatly reduced. The haploid nucleus of the microspore divides mitotically, forming a generative nucleus and a nucleus in the pollen tube. Often the generative nucleus is associated with the cytoplasm, which looks like a cell within a cell. Generative nucleus 1 for the formation of two nuclei (male gametes) divides mitotically either in the pollen grain or in the pollen tube.[ ...]

Most algae have only one nucleus in the cell, but there are cases when there are two or three or more. Cells with several tens, like a cladophora, or hundreds, like a water mesh (hydrodiction), nuclei are called coenocytic. It is noteworthy that these algae return to a single-nuclear state when specialized cells of asexual (aplanospores, zoospores) and sexual (gametes) reproduction are formed. [...]

Asexual reproduction is absent in caulerpidae; apparently, it was lost in modern forms in the course of evolution. Sexual reproduction - anisogamy - is characterized by a number of features that distinguish caulerp from other families of the order. Firstly, the caulerpa does not have special reproductive organs - gametangia. Gametes are formed directly in assimilation threads in any part of them. In some areas, the cytoplasm thickens, acquires a dark green color, then a mesh structure, and finally divides with the formation of single-nuclear gametes. There are no partitions separating the site of gamete formation. Such reproduction is called h o l o k a r p and e y. For the release of gametes to the surface of the thallus, rather long outgrowths are formed - papillae; gametes are released as a result of rupture of the shell at the tops of the papillae.[ ...]

When hybridizing animals, they face great difficulties. The main ones are as follows: 1) crossbreeding of species among themselves; 2) partial or complete sterility of hybrids. The main reasons for the non-crossing of distant species and the infertility of hybrids are genetic factors: a different set and structure of chromosomes in gametes, their inability to form a viable zygote, sperm, due to its morphological and biochemical characteristics, is not able to lyse the shell of a foreign egg, to penetrate into it. If a hybrid zygote is formed, then due to embryonic pathology, either the resorption of the fetus occurs in the early stages of formation, or its death. This is explained by the fact that the body's immune protective bodies fight the penetrating foreign protein, destroying it. Due to the genetic differences of the parents in hybrids, the formation of male and female gametes is disrupted and they become infertile. The sterility of hybrids is caused by abnormalities in the development of gonads and mitosis.[ ...]

Cells glabrous, mostly free-floating, sometimes attached by means of a trailing flagellum. One swimming and one trailing flagella; both with a basal grain connected by a thin rhizoplast to a kinetoplast. Contractile vacuoles more often 1-3, rarely absent. Reproduction by division. Copulation of gametes and autogamy are observed. Resting cysts are known. Animal nutrition: by suction of food with the tip of the anterior end, by direct swallowing and by the formation of food vacuoles (Lemm., 1914).[ ...]

The whole;> that process, as you know, it is customary to wash with double fertilization. After all, according to the definition generally accepted in the literature, fertilization (epgamin) is the process of fusion of male and female germ cells (gametes) with the formation of a zygote, from which a new organism is further separated. Such a definition of fertilization can be found in any textbook of biology and in any dictionary (including the Soviet scholarly books, Bolshoi and Malaya). And even in the famous Dictionary of the Russian Language by S.I. Ozhegov (1973) we read: “To fertilize. 1. Create an embryo in something. fusion of male and female sex cells. 2. Serve as a source of development, improvement. The fusion of one of the cells with the egg is undoubtedly fertilization, but the triple fusion is, strictly speaking, fertilization, since 1) the central cell is not a gamete, and 2) as a result of this fusion, a zygote is formed, from which it further developed a new organism. Obviously, the triple fusion is fertilization only in the second, figurative sense indicated by Ozhegov. In other words, in the expression "double fertilization" the term "fertilization" is used in two different senses - direct and figurative. Nevertheless, the expression "double onlodothioronie" has entered the literature so widely that it would be inappropriate to replace it (and attempts of this kind were made, including by a well-known German botanist. It is enough if we remember that we are talking here about two different biological processes conditionally united by a common.name.[ ...]

The biological role of sexual reproduction is exceptionally great. Undoubtedly, it has significant advantages over vegetative propagation and reproduction by spore formation. Even K. A. Timiryazev (1843-1920) repeatedly drew attention to sexual reproduction as an outstanding source of variability in organisms, since during meiosis gene recombination takes place, and when gametes are combined, new combinations of genes are formed. We can say that in nature sexual reproduction is dominant compared to other forms of reproduction. In animals that reproduce sexually, reproductive ability is maintained for a relatively long time. So in the case of a person, the ability to reproduce in women lasts mainly up to 40-45 years, and in men - almost all their life.[ ...]

Gymnosperms also differ from ferns in the development of the male gametophyte, in the structure and method of germination of microspores. In ferns, where the development of the gametophyte usually occurs only after sowing the spores, spore germination occurs through the so-called tetrad scar, located at the proximal pole of the spore. In gymnosperms, where the male gametophyte is greatly simplified and its development is accelerated, the first divisions of the microspore nucleus occur already inside the microsporangium. In connection with the early development of the male gametophyte and the formation of gametes even inside the spore membrane, there is a need for an adaptation by which the microspore can change its volume. Such an adaptation is a furrow at the distal pole of the microspore, which first appears in some seed ferns and is characteristic of the vast majority of gymnosperms. The furrow serves not only to regulate the volume of pollen grains. It becomes the site of exit from the microspore of the haustoria (in lower groups) or the pollen tube (in the oppressive and coniferous), which are also neoplasms. Thus, in gymnosperms, in contrast to ferns, the opening for the release of the contents of the microspore is formed at the distal pole. Gaustoria (sucker) of the cycad type grows horizontally and serves only to attach and feed the male gametophyte; the real pollen tube of conifers and gneaths grows vertically and serves mainly to conduct sperm to the eggs, that is, it is a conductor (vector), and not just a sucker. Although both of these formations are usually called pollen tubes, they are very different morphologically and functionally.[ ...]

In addition to crosses, the results of which are given above, in order to prove the gene hypothesis, G. Mendel also turned to backcrosses, which later received the name analyzing (test crosses) in the literature. The meaning of these crosses is that heterozygous E hybrids, which produced, for example, round seeds and originated from crosses between parent plants producing round (1R) and rough (rg) seeds, were again crossed with the original (parental) homozygous recessive plants producing rough seeds. Since the gametes projected by the heterozygous (Ig) E1 hybrid are always pure and can only carry either the I allele or the r allele, and half of the gametes must be K gametes, half must be r gametes, and all gametes produced by the original homozygous recessive (gg) plant, should be only gametes r, in the case of the validity of the gene hypothesis, it would be expected that the backcrossing of such plants should lead to the formation of zygotes of half Ir and half r. In other words, the resulting offspring from such backcrosses should be half heterozygous organisms producing round seeds (Rg) and half homozygous recessive organisms producing rough seeds (Rg). Having carried out back crossings and analyzing the properties of the plants that appeared in these crossings, G. Mendel discovered that they really are half heterozygous organisms and half homozygous, that is, the ratio between them was 1: 1.[ ...]

Carrying out hybridization is associated with a number of difficulties arising from the species characteristics of hybridized animals. The most important of them are: 1) the difference in the structure of the genital organs, which makes mating difficult, 2) the absence of a sexual reflex in a male to a female of another species, 3) the mismatch of mating seasons in animals of different species (especially in wild ones), 4) poor viability or the death of spermatozoa of animals of one species in the genital tract of females of another species, 5) the absence of a reaction of spermatozoa to the egg of a female of another species and the impossibility of fertilization, 6) the death of the zygote (if it is formed) at the very beginning of development, 7) the infertility of many hybrids, complete or partial (males are sterile in mammalian hybrids). Complete infertility is associated with the lack of conjugation of chromosomes during reduction division (due to their great dissimilarity - non-homology) and with the formation of non-viable gametes; partial (infertility of hybrid males), - probably with a violation of the hormonal regulation of spermatogenesis. Some of these difficulties can be overcome by human intervention, but there are others that are still insurmountable.[ ...]

Most seed-borne viruses appear to also be transmitted through the pollen of infected plants, but not all have been adequately studied. In other words, there does not seem to be a single example of any nyl-borne virus that is also seed-borne. There are few studies on the relative efficiency of virus transmission through an infected egg versus pollen. With regard to bean mosaic virus found in bean plants, Nelson and Down found that the percentage of infected seeds resulting from the pollination of flowers of healthy plants with the pollen of infected plants was about the same as in the case of the formation of such seeds from the flowers of infected plants, pollinated healthy pollen. However, Kraspin Medina and Groga it have shown that transmission of this virus by pollen is somewhat more affective. In contrast, lettuce mosaic virus is transmitted through the ovules of this plant; less than 0.5% of infected seeds result from pollen transfer. Self-pollination of infected plants can apparently lead to the production of more infected seeds than if only one of the infected plant's gametes is involved.

The process of formation of germ cells in plants is divided into two stages: 1st stage - sporogenesis- ends with the formation of haploid cells - spores, during the 2nd stage - gametogenesis- a series of divisions of haploid cells occurs before mature gametes are formed.

The process of formation of microspores, or pollen grains, in plants is called microsporogenesis, and the process of formation of megaspores (or macrospores) - mega- or macrosporogenesis. Microsporogenesis proceeds similarly to the division of maturation in animals of male germ cells to the stage of spermatids, and megasporogenesis, respectively, to the stage of an immature egg cell - oocyte II.

The process of gametogenesis in plants is in principle similar to that in animals, but proceeds in a slightly different way. In animals, after two meiotic divisions, gametes are formed, and no additional cell divisions occur. In plants, as a result of two meiotic divisions, a haploid spore arises, from which a gametophyte develops, which in lower plants (fungi, liverworts, mosses, a number of algae) is a whole organism and the longest stage of the life cycle. In higher plants, the haploid phase is reduced, but the nuclei of male and female spores undergo a series of mitotic divisions before gametes are formed.

Microsporogenesis and microgametogenesis

We will consider microsporogenesis and microgametogenesis on the example of angiosperms as the most general. In the subepidermal tissue of a young anther, a special sporogenous tissue is isolated, called archesporium. Each primary archesporial cell, after a series of divisions, becomes a pollen mother cell (microsporocyte), which goes through all the phases of meiosis.

As a result of two meiotic divisions, four haploid microspores arise. The latter lie in fours and are called cell tetrads.

In monocot plants, each nuclear division in meiosis is usually accompanied by cytokinesis; in dicots, both cell divisions occur simultaneously at the end of meiosis.

When maturing, cell tetrads break up into separate microspores with the formation of the inner (intin) and outer (exine) membranes. The outer shell, as a rule, is rough, cutinized, its surface is either smooth or rough; adapted to carry pollen and stick it to the stigma of the pistil. This ends microsporogenesis, after the formation of a single-nuclear microspore, microgametogenesis begins. The first mitotic division of the microspore leads to the formation of vegetative and generative cells. In the future, the vegetative cell and its nucleus do not divide. It accumulates reserve nutrients, which subsequently ensure the division of the generative cell and the growth of the pollen tube in the style of the pistil.

The generative cell, containing a smaller amount of cytoplasm, divides again. This division can take place while still in the pollen grain or during its germination in the pollen tube. As a result, two male germ cells are formed, which, unlike animal spermatozoa, are called sperm cells, or sperm.

Thus, from one spore with a haploid set of chromosomes, as a result of two mitotic divisions, three nuclei are formed: Two of them are sperm and one is vegetative. With the formation of a pollen tube, this vegetative nucleus in a semi-liquid diffuse state passes into the pollen tube.

The process of division of the generative cell and the formation of spermatozoa in the pollen tube were first studied in detail by S. G. Navashin in 1910 on lily plants.

Megasporogenesis and megagametogenesis

In angiosperms, the female gametophyte is the embryo sac that begins and develops inside the ovule.

The development of the female gametophyte in higher angiosperms is preceded by megasporogenesis. In the subepidermal layer of a young ovule, an archesporial cell separates, more often it is only one. The archesporium cell grows to become a megaspore mother cell. As a result of two divisions of meiosis of the mother cell of the megaspore, a tetrad of megaspores is formed. Each of the tetrad cells is haploid in terms of the number of chromosomes. However, only one of them continues to develop, the remaining three degenerate (monosporic type of development), the fate of these cells resembles the fate of reduction bodies during the maturation of eggs in animals.

The next step is megagametogenesis. The remaining functioning megaspore continues to grow and then its core undergoes a series of equational divisions. In this case, the cell itself does not divide, only the nucleus divides.

In different systematic groups of plants, the number of equational divisions of the megaspore nucleus can vary from one to three. In most plants (70% of angiosperm species), these divisions, as a rule, result in eight hereditarily identical nuclei, during these divisions the nuclei occupy a polar position, four of them are closer to the micropyle (the site of penetration of spermatozoa), and four others - in opposite end of the embryo sac, called the chalazal. Further, these nuclei separate into independent cells with significant amounts of cytoplasm.

Of the four cells located at the micropyle, three cells are the egg, and two so-called synergids form the egg apparatus. However, only one of these three cells develops after fertilization, while the other two are destroyed. The fourth nucleus departs to the center of the embryo sac, where it merges with one of the nuclei that has departed from the chalazal end. Two haploid nuclei merged in the central part form one diploid - secondary or central - nucleus of the embryo sac. This nucleus with the cytoplasm of the embryo sac is usually called the central cell of the embryo sac. However, often the polar nuclei that have moved to the center do not merge until fertilization. The three nuclei remaining at the chalazal end of the embryo sac also separate into cells; they're called antipodes.

Thus, as a result of three mitotic divisions in the embryo sac, 8 hereditarily identical haploid nuclei are formed, of which only one produces an egg.

The considered scheme for the formation of an eight-nuclear embryo sac from one megaspore is the most typical. However, this process proceeds very differently in different groups of plants. In some cases, as we have just considered, the development of the embryo sac begins from one haploid spore (monosporic type of development), in others - from two (bisporic type) and four spores (tetrasporic type).

As we have pointed out, in the monosporic type, only one of the four megaspores develops, while the remaining three are destroyed, similar to what happens with reduction bodies in animals. In other types of development of the embryo sac, a different number of megaspores are preserved, which arose as a result of meiosis and are ready for further mitotic divisions.

Studying gametogenesis, one cannot but be amazed at the parallelism that is observed during the maturation of germ cells in animals and plants, despite the fact that their divergence (divergence) in phylogenesis occurred at a very early stage in the emergence of cellular organization. This indicates the uniformity of the principles of constructing a number of adaptive mechanisms in both the plant and animal worlds.

So, the study of the development of germ cells in animals and plants has shown that the formation of gametes is a complex process. Before the egg and sperm combine in the process of fertilization, they undergo a series of transformations. However, germ cells, like cells of any other tissue, originate from somatic cells. Therefore, they cannot be considered as something isolated from the body of the organism. However, germ cells have their own characteristics. The main characteristic points that distinguish them from somatic cells are the following:

1. In different animals and plants, at different stages of embryonic tissue differentiation, the germ cells separate. The process of laying and differentiation, germ cells in animals is called rudimentary way.

2. In the development of germ cells, meiosis with its characteristic stages of nuclear division, namely prophase I, during which homologous chromosomes are conjugated, metaphase I and anaphase I, is of particular importance, when the number of chromosomes is reduced and homologous chromosomes diverge to different poles.

3. The main property of germ cells is their ability to merge into one during fertilization with the formation of a zygote, which then undergoes fragmentation and development. Somatic cells, as a rule, do not possess this ability.

Gamete (gamete): a germ cell (sperm or egg) containing a haploid set of chromosomes, that is, having one copy of each of the chromosomes.

With sexual reproduction, offspring usually have two parents. Each parent produces sex cells. Sex cells, or gametes, have a half or haploid set of chromosomes and result from meiosis. Thus, a gamete (from the Greek gamete - wife, gametes - husband) is a mature reproductive cell containing a haploid set of chromosomes and capable of merging with a similar cell of the opposite sex to form a zygote, while the number of chromosomes becomes diploid. In a diploid set, each chromosome has a paired (homologous) chromosome. One of the homologous chromosomes comes from the father, the other from the mother. The female gamete is called egg, the male is sperm. The process of gamete formation is collectively called gametogenesis.

In embryos of all vertebrates, at an early stage of development, certain cells are isolated as precursors of future gametes. Such primary germ cells migrate to the developing gonads (ovaries in females, testis and males), where, after a period of mitotic reproduction, they undergo meiosis and differentiate into mature gametes. In germ cells, before meiosis, additional genes are activated that regulate the pairing of homologous chromosomes, recombination and separation of recombined homologous chromosomes in the anaphase of the first division.

Oocytes develop from primary germ cells, which at an early stage of development of the organism migrate to the ovary and turn into oogonia there. After a period of mitotic reproduction, oogonia become first order oocytes, which, having entered the first division of meiosis, linger in prophase I for a time measured in days or years, depending on the type of organism. During this delay, the oocyte grows and accumulates ribosomes, mRNA, and proteins, often using other cells, including surrounding helper cells. Further development (egg maturation) depends on polypeptide hormones (gonadotropins), which, acting on the supporting cells surrounding each oocyte, induce them to induce the maturation of a small part of the oocytes. These oocytes complete the first meiotic division, producing a small polar body and a large second-order oocyte, which later enters metaphase of the second meiotic division. In many species, the oocyte remains at this stage until fertilization initiates the completion of meiosis and the start of embryonic development.

The sperm is usually a small and compact cell that is highly specialized for the function of bringing its DNA into the egg. While in many organisms the entire pool of oocytes is formed at an early stage of female development, in males, after the onset of puberty, more and more germ cells enter meiosis, with each first-order spermatocyte giving rise to four mature spermatozoa. Sperm differentiation occurs after meiosis, when the nuclei are haploid. However, since cytokinesis is not completed during mitotic division of mature spermatogonia and spermatocytes, the descendants of one spermatogonium develop as

In nature, there are two types of reproduction of living organisms - asexual and sexual.

Asexual reproduction is characterized by the fact that the daughter cells are completely identical to the parent cells in terms of the content of hereditary information, morphological, anatomical and physiological features (see sections "Botany" and "Zoology").

Sexual reproduction is characterized by the exchange of genetic information between females and males. Such an exchange is carried out in several ways:

1. the formation of cytoplasmic bridges along which the male chromosome moves into the female cell (typical for prokaryotes);
2. conjugation - temporary connection and exchange of parts of chromosomes (found in viruses, bacteria, ciliates, etc.);
3. the formation of special haploid germ cells - gametes (typical for most eukaryotes). In different species of animals and plants, germ cells have different sizes, shapes, structures and development.

The development of germ cells and fertilization in animals. The eggs (female germ cells) in animals are immobile, have a rounded shape, are covered with two membranes - yolk and protein and contain reserve nutrients necessary for the development of the embryo. Spermatozoa (male sex cells) are much smaller than eggs, they are mobile, have the form of a long thread, consisting of a head, neck and tail. The head is somewhat expanded, the nucleus is located in it, its anterior end is pointed.

There is a centriole in the neck, and the tail resembles a flagellum in structure and is an organelle of movement. Sex cells develop in the sex glands - the ovaries and testes. They distinguish 3 zones:

1. in the breeding area primary cells repeatedly divide by mitosis;
2. in the growth zone, the original cells grow intensively, especially during the formation of eggs;
3. in ripening zone two peculiar divisions occur, as a result of which four haploid (n) cells of equal size are formed in the testes, each of which turns into a spermatozoon, and four haploid cells are also formed in the ovaries, of which only one, large, turns into an egg and three are small (guide bodies) die.

Meiosis is a division in the zone of maturation of germ cells, accompanied by a halving of the number of chromosomes (color table XIII). It consists of two specific, consecutive divisions that have the same phases as mitosis. However, as shown in the table, the duration of the individual phases and the processes occurring in them differ significantly from mitosis.

These differences are mainly as follows. In meiosis, prophase I is longer. In it, the conjugation of homologous chromosomes and the exchange of genetic information take place. In anaphase I, the centromeres that hold the chromatids together do not divide, and one of the homologous chromosomes moves to the poles. The interphase before the second division is very short; DNA is not synthesized in it. Cells (gametes) formed as a result of two meiotic divisions contain a haploid (single) set of chromosomes.

		I division	II. division
Interphase	Set of chromosomes (2 n). there is an intensive synthesis of proteins, ATP and other organic substances. Chromosomes are duplicated, each turns out to consist of two sister chromatids held together by a common centromere	Set of chromosomes 2 n. The same processes are observed as in mitosis; but longer, especially during the formation of eggs	The set of chromosomes is haploid (n). Synthesis of organic substances is absent
	Not long, chromosomes spiralize, the nuclear envelope and nucleolus disappear, a division spindle is formed	More lengthy. At the beginning of the phase, the same processes as in mitosis. In addition, it happens conjugation of chromosomes in which homologous chromosomes converge along their entire length and twist. This can involve the exchange of genetic information. (crossover of chromosomes). Then the chromosomes separate	Short, same processes as in mitosis, but with n chromosomes
metaphase	Further spiralization of chromosomes occurs, their centromeres are located along the equator	There are processes similar to those in mitosis	The same happens as in mitosis, but with n chromosomes
	The centromeres holding sister chromatids together divide; each of them becomes a new chromosome and moves to opposite poles	Centromeres do not divide. One of the homologous chromosomes departs to opposite poles, consisting of two chromatids held together by a common centromere	It happens the same as in mitosis, but with n chromosomes
Telophase	The cytoplasm divides, 2 daughter cells are formed, each with a diploid set of chromosomes. Spindle of division disappears, nucleoli form	Doesn't last long. Homologous chromosomes end up in different cells with a haploid set. The cytoplasm does not always divide	The cytoplasm is divided. After two meiotic divisions, 4 cells are formed with a haploid set of chromosomes

Fertilization is the process of fusion of the egg and sperm, in which the diploid set of chromosomes is restored. A fertilized egg is called a zygote.

The development of germ cells and fertilization in flowering plants occurs in the flower. Male sex cells mature in the anther. It contains many diploid cells, each of which divides by meiosis and forms 4 haploid pollen grains. Each pollen grain divides by mitosis and forms 2 cells - vegetative and generative. The generative cell divides again by mitosis and forms 2 spermatozoa. Thus, a mature pollen grain contains three cells - 1 vegetative and 2 spermatozoa.

female sex cells develop in the ovule. One of its cells divides by meiosis and forms 4 haploid cells. Of these, one divides three more times by mitosis and forms 8 haploid nuclei. embryo sac, in which 4 cores are located at one end, and 4 at the other. Then, one nucleus migrates from each end to the center of the embryo sac; merging, they form diploid nucleus of the embryo sac. One of the 3 haploid cells located at the pollen entrance is an ovum.

sex cells - gametes(from the Greek. gametes - "husband") can be found already in a two-week-old human embryo. They are called primary germ cells. At this time, they are not at all like sperm or eggs and look exactly the same. At this stage of development of the embryo, no differences inherent in mature gametes can be found in primary germ cells. This is not their only feature. Firstly, the primary germ cells appear in the embryo much earlier than the gonad itself (gonads), and secondly, they arise at a considerable distance from the place where these glands will form later. At a certain moment, an absolutely amazing process takes place - the primary germ cells together rush to the gonad and populate, "colonize" it.

After the future gametes have entered the sex glands, they begin to divide intensively, and their number increases. At this stage, the germ cells still contain the same number of chromosomes as the "corporeal" ( somatic) cells - 46. However, for the successful implementation of their mission, germ cells must have 2 times fewer chromosomes. Otherwise, after fertilization, that is, the fusion of gametes, the cells of the embryo will contain not 46, as established by nature, but 92 chromosomes. It is not difficult to guess that in the next generations their number would progressively increase. To avoid such a situation, the emerging germ cells undergo a special division, which in embryology is called meiosis(Greek meiosis - "reduction"). As a result of this amazing process diploid(from the Greek diploos - “double”), the set of chromosomes is, as it were, “pulled apart” into its single constituents, haploid sets (from Greek haploos - single). As a result, from a diploid cell with 46 chromosomes, 2 haploid cells with 23 chromosomes are obtained. This is followed by the final stage of the formation of mature germ cells. Now the existing 23 chromosomes are copied in the haploid cell and these copies are used to form a new cell. Thus, as a result of the described two divisions, 4 new ones are formed from one primary germ cell.

Moreover, in spermatogenesis(Greek genesis - origin, development) as a result of meiosis, 4 mature spermatozoa with a haploid set of chromosomes appear, and in the process of egg formation - in oogenesis (from the Greek oon - "egg") only one. This is because the second haploid set of chromosomes formed as a result of meiosis, the egg does not use to form a new mature germ cell - the oocyte, but “throws” them out as “extra” out in a kind of “garbage container”, which is called the polar body. The first division of the chromosome set ends in oogenesis with the release of the first polar body just before ovulation. The second replication division occurs only after the penetration of the sperm into the egg and is accompanied by the release of the second polar body. For embryologists, polar bodies are very important diagnostic indicators. There is the first polar body, which means the egg is mature, the second polar body has appeared - fertilization has occurred.

The primary germ cells that are in the male gonad do not divide for the time being. Their division begins only during puberty and leads to the formation of a cohort of so-called diploid stem cells, from which spermatozoa are formed. The stock of stem cells in the testicles is constantly replenished. Here it is appropriate to recall the feature of spermatogenesis described above - 4 mature spermatozoa are formed from one cell. Thus, after puberty, hundreds of billions of new spermatozoa are formed in a man throughout his life.

The formation of eggs proceeds differently. As soon as they populate the sex gland, the primary germ cells begin to divide intensively. By the 5th month of intrauterine development, their number reaches 6-7 million, but then there is a massive death of these cells. In the ovaries of a newborn girl, there are no more than 1-2 million of them, by the age of 7 - only about 300 thousand, and during puberty 30-50 thousand. The total number of eggs that reach maturity during puberty will be even less. It is well known that during one menstrual cycle only one follicle usually matures in the ovary. It is easy to calculate that during the reproductive period, which lasts for women 30-35 years, about 400 mature eggs are formed.

If meiosis in spermatogenesis begins during puberty and repeats billions of times during the life of a man, in oogenesis, the forming female gametes enter meiosis even in the period of intrauterine development. Moreover, this process begins almost simultaneously in all future eggs. It starts but doesn't end! Future eggs reach only the middle of the first phase of meiosis, and then the division process is blocked for 12 - 50 years! Only with the advent of puberty, meiosis in oogenesis will continue, and not all cells at once, but only for 1-2 eggs per month. The full process of meiotic division of the egg will be completed, as already mentioned above, only after its fertilization! Thus, the sperm enters the egg, which has not yet completed division, which has a diploid set of chromosomes!

spermatogenesis And oogenesis- very complex and in many ways mysterious processes. At the same time, their subordination to the laws of interconnection and conditionality of natural phenomena is obvious. For the fertilization of one egg in vivo(lat. in a living organism) tens of millions of spermatozoa are needed. The male body produces them in gigantic quantities for almost a lifetime.

Carrying and giving birth to a child is an extremely heavy burden on the body. Doctors say that pregnancy is a test of health. How a child will be born depends on the state of health of the mother. Health, as you know, is not eternal. Old age and disease, unfortunately, are inevitable. Nature gives a woman a strictly limited irreplaceable number of germ cells. The decline in the ability to bear children develops slowly, but gradually along an oblique path. We get clear evidence that this is indeed the case by daily evaluating the results of ovarian stimulation in ART programs. Most of the eggs are usually used up by the age of 40, and by the age of 50 their entire supply is completely exhausted. Often the so-called ovarian exhaustion comes much earlier. It should also be said that the egg is subject to “aging”, over the years its ability to fertilize decreases, the process of chromosome division is increasingly disrupted. Engaging in childbearing at a late reproductive age is risky due to the increasing risk of having a child with a chromosomal pathology. A typical example is Down's syndrome, which occurs due to the third extra chromosome 21 remaining during division. Thus, by limiting the reproductive period, nature protects the woman and takes care of healthy offspring.

What are the rules for division of chromosomes? How is hereditary information transmitted? In order to deal with this issue, we can draw a simple analogy with cards. Imagine a young married couple. Let's call them conditionally - He and She. In each of its somatic cells there are chromosomes of black suit - clubs and spades. He received a set of clubs from six to ace from his mother. A set of spades from my dad. In each of its somatic cells, the chromosomes of the red suit are diamonds and hearts. She received a set of tambourines from six to ace from her mother. A set of worms - from my dad.

In order to obtain a germ cell from a diploid somatic cell, the number of chromosomes must be halved. In this case, the germ cell must necessarily contain a complete single (haploid) set of chromosomes. None should be lost! In the case of maps, such a set can be obtained as follows. Take one at random from each pair of black suit cards and thus form two single sets. Each set will include all cards of the black suit from six to ace, however, which cards these will be (clubs or spades) is determined by chance. For example, in one such set, the six may be a spade, and in another, a club. It is easy to estimate that in the example with cards, with such a choice of a single set from a double set, we can get 2 to the ninth combinations - more than 500 options!

In the same way, we will make a single set of her red suit cards. We will get more than 500 different options. From his single and her single set of cards, we make a double set. It will turn out to be, to put it mildly, “motley”: in each pair of cards, one will be red and the other black. The total number of such possible sets is 500 x 500, that is, 250 thousand options.

Approximately the same, according to the law of random sampling, nature also acts with chromosomes in the process of meiosis. As a result, from cells with a double, diploid set of chromosomes, cells are obtained, each of which contains a single, haploid complete set of chromosomes. Suppose, as a result of meiosis, a germ cell was formed in your body. Sperm or egg - in this case it does not matter. It will necessarily contain a haploid set of chromosomes - exactly 23 pieces. What exactly are these chromosomes? Consider, for example, chromosome number 7. This could be the chromosome you received from your father. With equal probability, it can be a chromosome that you received from your mother. The same is true for chromosome number 8, and for any other.

Since in humans the number of chromosomes of the haploid set is 23, the number of possible variants of germ haploid cells formed from diploid somatic cells is 2 to the power of 23. More than 8 million variants are obtained! In the process of fertilization, two sex cells are connected to each other. Therefore, the total number of such combinations will be equal to 8 million x 8 million = 64,000 billion options! At the level of a pair of homologous chromosomes, the basis of this diversity looks like this. Take any pair of homologous chromosomes in your diploid set. You got one of these chromosomes from your mother, but it could be either your grandmother or your maternal grandfather. You received the second homologous chromosome from your father. However, it again can be, regardless of the first, either your grandmother's chromosome or your grandfather's already on the paternal side. And you have 23 pairs of such homologous chromosomes! There are an incredible number of possible combinations. It is not surprising that at the same time, children are born to one pair of parents who differ from each other both in appearance and character.

By the way, a simple but important conclusion follows from the above calculations. Each person, now alive, or who ever lived in the past on Earth, is absolutely unique. The chances of a second one appearing are practically zero. Therefore, there is no need to compare yourself with anyone. Each of you is unique, and that is already interesting!

But back to our sex cells. Each diploid human cell contains 23 pairs of chromosomes. Chromosomes from 1 to 22 pairs are called somatic and they are the same in shape. The chromosomes of the 23rd pair (sex chromosomes) are the same only in women. They are denoted by the Latin letters XX. In males, the chromosomes of this pair are different and are designated XY. In the haploid set of the egg, the sex chromosome is always only X, while the spermatozoon can carry either the X or Y chromosome. If the X sperm fertilizes the egg, the baby will be a girl, if the Y sperm is a boy. Everything is simple!

Why is meiosis in the egg so long stretched in time? How is the selection of a cohort of follicles that begin their development every month, and how is the leading, dominant, ovulatory follicle selected from them, in which the egg will mature? Biologists do not yet have unambiguous answers to all these difficult questions. The process of formation of mature eggs in humans is waiting for new researchers!

The formation and maturation of spermatozoa, as already mentioned, occurs in the seminiferous tubules of the male sex gland - testicles. The formed spermatozoon has a length of about 50-60 microns. The nucleus of the spermatozoon is located in its head. It contains paternal hereditary material. Behind the head is the neck, in which there is a large convoluted mitochondrion- an organelle that provides movement of the tail. In other words, it is a kind of "energy station". There is a cap on the head of the spermatozoon. Thanks to her, the shape of the head is oval. But, the point is not in the form, but in what is contained under the "cap". This "hat" is actually a container and is called acrosome, but it contains enzymes that are able to dissolve the shell of the egg, which is necessary for the penetration of the sperm inside - into the cytoplasm of the egg. If the spermatozoon does not have an acrosome, its head is not oval, but round. This pathology of spermatozoa is called globulospermia(round-headed spermatozoa). But, the trouble is again not in shape, but in the fact that such a sperm cannot fertilize an egg, and a man with such a violation of spermatogenesis was doomed to childlessness until the last decade. Today, thanks to ART, infertility in these men can be overcome, but we will talk about this later in the chapter on micromanipulation, in particular ICSI.

The movement of the spermatozoon is carried out due to the movement of its tail. The speed of movement of the sperm does not exceed 2-3 mm per minute. It would seem a little, however, in 2-3 hours in the female genital tract, spermatozoa travel a path that is 80,000 times their own size! If a person were in the place of the sperm in this situation, he would have to move forward at a speed of 60-70 km / h - that is, at the speed of a car!

The spermatozoa in the testicle are immobile. They acquire the ability to move only by passing through the vas deferens under the influence of the fluids of the vas deferens and seminal vesicles, the secretion of the prostate gland. In the genital tract of a woman, spermatozoa remain motile for 3-4 days, but they must fertilize the egg within 24 hours. The entire process of development from a stem cell to a mature sperm lasts approximately 72 days. However, since spermatogenesis occurs continuously and a huge number of cells enter it at once, there are always a large number of spermatozoa in the testicles that are at different stages of spermatogenesis, and the stock of mature spermatozoa is constantly replenished. The activity of spermatogenesis is individual, but decreases with age.

As we have already said, the eggs are in follicles ovary. As a result of ovulation, the egg enters the abdominal cavity, from where it is "caught" by the fimbriae of the fallopian tube and transferred to the lumen of its ampullary section. This is where the egg meets the sperm.

What is the structure of a mature egg? It is quite large and reaches 0.11-0.14 mm in diameter. Immediately after ovulation, the egg is surrounded by a cluster of small cells and a gelatinous mass (called radiant crown). Apparently, in this form, it is more convenient for the fimbriae of the fallopian tube to capture the egg. In the lumen of the fallopian tube, with the help of enzymes and mechanical action (beating of the cilia of the epithelium), the egg is “cleansed” from the radiant crown. The final release of the egg from the radiant crown occurs after its meeting with spermatozoa, which literally stick around the egg. Each spermatozoon secretes an enzyme from the acrosome that dissolves not only the radiant crown, but also acts on the membrane of the egg itself. This shell is called shiny, as it looks under a microscope. By releasing the enzyme, all spermatozoa tend to fertilize the egg, but the zona pellucida will only let one of them through. It turns out that, rushing to the egg, acting on it collectively, the spermatozoa “clear the way” for only one lucky person. The role of the zona pellucida is not limited to the selection of the spermatozoon; at the early stages of embryo development, it maintains an ordered arrangement of its cells (blastomeres). At some point, the zona pellucida becomes tight, it breaks and occurs hatching(from English hatching - "hatching") - hatching of the embryo.