Amitosis meaning of the process. Mitosis, its biological significance, pathology
Amitosis (amitosis; Greek negative prefix a-, mitos - thread + -ōsis) direct nuclear division - division of the cell nucleus into two or more parts without the formation of chromosomes and achromatin spindle; During amitosis, the nuclear membrane and nucleolus are preserved and the nucleus continues to function actively.
Direct nuclear fission was first described by Remak (R. Bemak, 1841); the term “amitosis” was proposed by Flemming (W. Flemming, 1882).
Typically, amitosis begins with the division of the nucleolus, then the nucleus divides. Its division can proceed in different ways: either a partition appears in the nucleus - the so-called nuclear plate, or it is gradually laced together, forming two or more daughter nuclei. Using cytophotometric research methods, it was found that in approximately 50% of cases of amitosis, DNA is evenly distributed between the daughter nuclei. In other cases, division ends with the appearance of two unequal nuclei (meroamitosis) or many small unequal nuclei (fragmentation and budding). Following nuclear division, cytoplasmic division occurs (cytotomy) with the formation of daughter cells (Fig. 1); if the cytoplasm does not divide, one bi- or multinucleated cell appears (Fig. 2).
Amitosis is characteristic of a number of highly differentiated and specialized tissues (neurons of the autonomic ganglia, cartilage, glandular cells, blood leukocytes, endothelial cells of blood vessels, etc.), as well as for cells of malignant tumors.
Benninghoff (A. Benninghoff, 1922), based on the functional purpose, proposed to distinguish between three types of amitosis: generative, reactive and degenerative.
Generative amitosis- this is a complete division of nuclei, after which mitosis becomes possible (see). Generative amitosis is observed in some protozoa, in polyploid nuclei (see Chromosome set); in this case, a more or less orderly redistribution of the entire hereditary apparatus occurs (for example, the division of the macronucleus in ciliates).
A similar picture is observed during the division of some specialized cells (liver, epidermis, trophoblast, etc.), where amitosis is preceded by endomitosis - intranuclear doubling of the set of chromosomes (see Meiosis); The polyploid nuclei formed as a result of endomitosis then undergo amitosis.
Reactive amitosis caused by the influence of various damaging factors on the cell - radiation, chemicals, temperature, etc. It can be caused by disturbances in metabolic processes in the cell (during starvation, tissue denervation, etc.). This type of amitotic nuclear division, as a rule, does not end with cytotomy and leads to the appearance of multinucleated cells. Many researchers tend to consider reactive amitosis as an intracellular compensatory reaction that ensures the intensification of cell metabolism.
Degenerative amitosis- nuclear division associated with processes of degradation or irreversible differentiation of the cell. With this form of amitosis, fragmentation, or budding, of nuclei occurs, which is not associated with DNA synthesis, which in some cases is a sign of incipient tissue necrobiosis.
The question of the biological significance of amitosis has not been completely resolved. However, there is no doubt that amitosis is a secondary phenomenon compared to mitosis.
Bibliography: Klishov A. A. Histogenesis, regeneration and tumor growth of skeletal muscle tissue, p. 19, L., 1971; Knorre A. G. Embryonic histogenesis, p. 22, L., 1971; Mikhailov V.P. Introduction to cytology, p. 163, L., 1968; Guide to Cytology, ed. A. S. Troshina, vol. 2, p. 269, M. - L., 1966; Bucher O. Die Amitose der tierischen und menschlichen Zelle, Protoplasmalogia, Handb. Protoplasmaforsch., hrsg. v. L. V. Heilbrunn u. F. Weber, Bd 6, Wien, 1959, Bibliogr.
Yu. E. Ershikova.
Amitosis direct nuclear division, one of the methods of nuclear division in protozoa, plant and animal cells. A. was first described by the German biologist R. Remak (1841); the term was proposed by histologist W. Flemming (1882). With A., in contrast to Mitosis a ,
or indirect division of the nucleus, the nuclear membrane and nucleoli are not destroyed, the fission spindle is not formed in the nucleus, the chromosomes remain in a working (despiralized) state, the nucleus is either laced or a septum appears in it, which is apparently unchanged; division of the cell body - cytotomy (See Cytotomy) ,
as a rule, does not occur (Fig.); usually A. does not ensure uniform division of the nucleus and its individual components. The study of A. is complicated by the unreliability of its definition based on morphological characteristics, since not every constriction of the nucleus means A.; even pronounced “dumbbell-shaped” constrictions of the nucleus can be transient; nuclear constrictions can also be the result of incorrect previous mitosis (pseudoamitosis). Usually A. follows Endomitosis. In most cases, with A. only the nucleus divides and a binuclear cell appears; with repeated A. multinucleated cells can form. Many binucleate and multinucleate cells are the result of A. (a certain number of binucleate cells are formed during mitotic division of the nucleus without dividing the cell body); they contain (in total) polyploid chromosome sets (see Polyploidy).
In mammals, tissues with both mononuclear and binuclear polyploid cells (cells of the liver, pancreas and salivary glands, nervous system, bladder epithelium, epidermis) and only with binuclear polyploid cells (mesothelial cells, connective tissues) are known. Bi- and multinucleated cells differ from mononuclear diploid cells (see Diploid) in their larger sizes, more intense synthetic activity, and an increased number of various structural formations, including chromosomes. Bi- and multinucleated cells differ from mononuclear polyploid cells mainly in the larger nuclear surface. This is the basis for the idea of atomization as a method of normalizing nuclear-plasma ratios (see Nuclear-plasma ratio) in polyploid cells by increasing the ratio of the surface of the nucleus to its volume. During A., the cell retains its characteristic functional activity, which almost completely disappears during mitosis. In many cases, A. and binuclearity accompany compensatory processes occurring in tissues (for example, during functional overload, fasting, after poisoning or denervation). Usually A. is observed in tissues with reduced mitotic activity. This, apparently, explains the increase in the number of binucleate cells formed by A as the body ages. The idea of A as a form of cell degeneration is not supported by modern research. The view of A. as a form of cell division is also untenable; There are only isolated observations of amitotic division of the cell body, and not just its nucleus. It is more correct to consider A. as an intracellular regulatory reaction. Lit.: Wilson E. B., The cell and its role in development and heredity, trans. from English, vol. 1-2, M.-L., 1936-40; Baron M.A., Reactive structures of internal shells, [M.], 1949; Brodsky V. Ya., Cell trophism, M., 1966; Bucher O., Die Amitose der tierischen und menschlichen Zeile, W., 1959. V. Ya. Brodsky.
Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .
Synonyms:Mitosis–mitos (Greek - threads) – indirect cell division, a universal method of dividing eukaryotic cells.
Main events of the mitotic cycle consist in reduplication (self-duplication) hereditary material of the mother cell and in uniform distribution of this material between daughter cells. These events are accompanied by natural changes in the chemical and morphological organization chromosomes- nuclear structures in which more than 90% of the genetic material of a eukaryotic cell is concentrated (the main part of the extranuclear DNA of an animal cell is located in mitochondria).
Chromosomes, in interaction with extrachromosomal mechanisms, provide: a) storage of genetic information; b) using this information to create and maintain cellular organization; c) regulation of reading hereditary information; d) doubling of genetic material; d) transfer it from the mother cell to the daughter cells.
Mitosis is a continuous process that is divided into phases.
In mitosis we can distinguish four phases. The main events for individual phases are presented below.
| Mitosis phase | Contents of changes | |
| Prophase (0.60 time from total mitosis, 2n4c) | The volume of the core increases. Chromosomes spiral, become visible, shorten, thicken, and take on the appearance of threads. In the cytoplasm, the number of rough network structures decreases. The number of policies is sharply reduced. The centrioles of the cell center diverge to the cell poles, between them microtubules form a fission spindle. The nucleolus is destroyed. The nuclear membrane dissolves, chromosomes appear in the cytoplasm | |
| Metaphase (0.05 time) | Spiralization reaches its maximum. Chromosomes line up in the equatorial plane of the cell (metaphase plate). Spindle microtubules are associated with chromosome kinetochores. The mitotic spindle is fully formed and consists of nets connecting the poles to the centromeres of the chromosomes. Each chromosome is longitudinally split into two chromatids (daughter chromosomes), connected at the kinetochore region. | |
| Anaphase (0.05 time) | Centromeres are separated, the connection between chromatids is broken, and they, as independent chromosomes, move to the poles of the cell at a speed of 0.2-5 μm/min. The movement of chromosomes is ensured by the interaction of the centromeric regions of the chromosomes with the microtubules of the spindle. Upon completion of the movement, two equal complete sets of chromosomes are assembled at the poles. | |
| Telophase (0.3 time) | The interphase nuclei of daughter cells are reconstructed. Chromosomes, consisting of one chromatid, are located at the poles of the cell. They despiral and become invisible. The nuclear envelope is formed, the filaments of the achromatin spindle disintegrate. The nucleolus is formed in the nucleus. The cytoplasm divides (cytotomy and cytokinesis) and the formation of two daughter cells. In animal cells, the cytoplasm is divided by constriction, invagination of the cytoplasmic membrane from the edges to the center. In plant cells, a membrane septum is formed in the center, which grows towards the cell walls. After the formation of a transverse cytoplasmic membrane in plants, a cellular wall is formed. |
Biological significance of mitosis: the formation of cells with hereditary information that is qualitatively and quantitatively identical to the information of the mother cell. Ensuring the constancy of the karyotype over a number of cell generations. Mitosis serves as a cellular mechanism for the processes of growth and development of the body, its regeneration and asexual reproduction. Thus, mitosis is a universal mechanism for reproducing the cellular organization of the eukaryotic type in individual development.
Pathology of mitosis
Disturbances in one or another phase of mitosis lead to pathological changes in cells. Deviation from the normal course of the spiralization process can lead to swelling and sticking together of chromosomes. Sometimes a fragment of a chromosome section is observed, which, if it is deprived of a centromere, does not participate in anaphase movement to the poles and is lost. Individual chromatids may lag behind during movement, which leads to the formation of daughter nuclei with unbalanced chromosome sets. Damage to the spindle leads to a delay in mitosis in metaphase and chromosome scattering. When the number of centrioles changes, multipolar or asymmetric mitoses occur. Violation of cytotomy leads to the appearance of bi- and multinucleated cells.
Based on the mitotic cycle, a number of mechanisms have emerged by which in a particular organ the amount of genetic material and, consequently, the intensity of metabolism can be increased while maintaining a constant number of cells.
Endomitosis. The doubling of a cell's DNA is not always accompanied by its division into two. Since the mechanism of such doubling coincides with premitotic DNA reduplication and it is accompanied by a multiple increase in the number of chromosomes, this phenomenon is called endomitosis. When cells are exposed to substances that destroy spindle microtubules, division stops, and chromosomes will continue the cycle of their transformations: replicate, which will lead to the gradual formation of polyploid cells - 4n, 8n, etc. This transformation process is otherwise called endoreproduction. From a genetic point of view, endomitosis is a genomic somatic mutation. The ability of cells to undergo endomitosis is used in plant breeding to obtain cells with a multiple set of chromosomes. For this purpose, colchicine and vinblastine are used, which destroy the filaments of the achromatin spindle. Polyploid cells (and then adult plants) are large in size; the vegetative organs from such cells are large, with a large supply of nutrients. In humans, endoreproduction occurs in some hepatocytes and cardiomyocytes.
Polythenia. During polyteny in the S-period, as a result of replication and non-disjunction of chromosomal strands, a multi-stranded, polytene structure is formed. They differ from mitotic chromosomes in their larger sizes (200 times longer). Such cells are found in the salivary glands of dipteran insects and in the macronuclei of ciliates. On polytene chromosomes, swellings and puffs (transcription sites) are visible - an expression of gene activity. These chromosomes are the most important object of genetic research. Endomitosis and polyteny lead to the formation polyploid cells, characterized by a multiple increase in the volume of hereditary material. In such cells, unlike diploid cells, genes are repeated more than twice. In proportion to the increase in the number of genes, the cell mass increases, which increases its functionality. In the mammalian body, polyploidization with age is characteristic of liver cells.
Mitotic cycle abnormalities. The mitotic rhythm, usually adequate to the need for restoration of aging, dead cells, can be changed under pathological conditions. A slowdown of the rhythm is observed in aging or poorly vascularized tissues, an increase in the rhythm is observed in tissues under various types of inflammation, hormonal influences, in tumors, etc.
Anomalies in the development of mitoses. Some aggressive agents, acting on the S phase, slow down DNA synthesis and duplication. These include ionizing radiation, various antimetabolites (metatrexate, mercapto-6-purine, fluoro-5-uracil, procarbozine, etc.). They are used for antitumor chemotherapy. Other aggressive agents act on the phases of mitosis and interfere with the formation of the achromatic spindle. They change the viscosity of the plasma without splitting the chromosome strands. Such a cytophysiological change can lead to a blockade of mitosis in metaphase, and then acute cell death, or mitonecrosis. Mitonecrosis is often observed, in particular, in tumor tissue, in the foci of certain inflammations with necrosis. They can be caused with the help of podophyllin, which is used in the treatment of malignant neoplasms.
Abnormalities in mitotic morphology. During inflammation, the action of ionizing radiation, chemical agents, and especially in malignant tumors, morphological abnormalities of mitoses are found. They are associated with severe metabolic changes in cells and can be referred to as “abortive mitoses.” An example of such an abnormality is mitosis with an abnormal number and shape of chromosomes; three-, four- and multipolar mitoses.
Multinucleate cells. Cells containing many nuclei are also found in the normal state, for example: osteoclasts, megakaryocytes, syncytiotrophoblasts. But they are often used in pathological conditions - for example: Langhans cells in tuberculosis, giant cells of foreign bodies, many tumor cells. The cytoplasm of such cells contains granules or vacuoles; the number of nuclei can vary from a few to several hundred, and the volume is reflected in the name - giant cells. Their origin is variable: epithelial, mesenchymal, histiocytic. The mechanism of formation of giant multinucleated cells is different. In some cases, their formation is due to the fusion of mononuclear cells, in others it is carried out due to the division of nuclei without division of the cytoplasm. It is also believed that their formation may be a consequence of certain mitotic abnormalities after irradiation or the administration of cytostatics, as well as during malignant growth.
Amitosis
Direct fission or amitosis- This is the division of a cell in which the nucleus is in an interphase state. In this case, chromosome condensation and spindle formation do not occur. Formally, amitosis should lead to the appearance of two cells, but most often it leads to the division of the nucleus and the appearance of bi- or multinucleated cells.
Amitotic division begins with fragmentation of the nucleoli, followed by division of the nucleus by constriction (or invagination). There may be multiple divisions of the nucleus, usually of unequal size (in pathological processes). Numerous observations have shown that amitosis almost always occurs in cells that are obsolete, degenerating and unable to produce full-fledged elements in the future. Normally, amitotic division occurs in the embryonic membranes of animals, in the follicular cells of the ovary, and in giant trophoblast cells. Amitosis has a positive meaning in the process of tissue or organ regeneration (regenerative amitosis). Amitosis in aging cells is accompanied by disturbances in biosynthetic processes, including replication, DNA repair, as well as transcription and translation. The physicochemical properties of chromatin proteins in cell nuclei, the composition of the cytoplasm, the structure and functions of organelles change, which entails functional disorders at all subsequent levels - cellular, tissue, organ and organismal. As destruction increases and restoration fades, natural cell death occurs. Amitosis often occurs during inflammatory processes and malignant neoplasms (induced amitosis).
amitosis (amitosis; a- + mitosis; synonym: amitotic division, direct division)
cell division without the formation of a spindle and chromosome spiralization; A. is characteristic of cells of some specialized tissues (leukocytes, endothelial cells, neurons of the autonomic ganglia, etc.), as well as malignant tumors.
Amitosis
direct nuclear division, one of the methods of nuclear division in protozoa, plant and animal cells. A. was first described by the German biologist R. Remak (184
; the term was proposed by histologist W. Flemming (188
During A., in contrast to mitosis, or indirect nuclear division, the nuclear membrane and nucleoli are not destroyed, a fission spindle is not formed in the nucleus, the chromosomes remain in a working (despiralized) state, the nucleus is either laced or a septum appears in it, which is apparently unchanged; division of the cell body ≈ cytotomy, as a rule, does not occur (Fig.); usually A. does not ensure uniform division of the nucleus and its individual components.
The study of A. is complicated by the unreliability of its definition based on morphological characteristics, since not every constriction of the nucleus means A.; even pronounced “dumbbell-shaped” constrictions of the nucleus can be transient; nuclear constrictions can also be the result of incorrect previous mitosis (pseudoamitosis). Usually A. follows endomitosis. In most cases, with A. only the nucleus divides and a binuclear cell appears; with repeated A. multinucleated cells can form. Very many binucleate and multinucleate cells are the result of A. (a certain number of binucleate cells are formed during mitotic division of the nucleus without dividing the cell body); they contain (in total) polyploid chromosome sets (see Polyploidy).
In mammals, tissues with both mononuclear and binuclear polyploid cells (cells of the liver, pancreas and salivary glands, nervous system, bladder epithelium, epidermis) and only with binuclear polyploid cells (mesothelial cells, connective tissues) are known. Bi- and multinucleated cells differ from mononuclear diploid cells (see Diploid) in their larger sizes, more intense synthetic activity, and an increased number of various structural formations, including chromosomes. Bi- and multinucleated cells differ from mononuclear polyploid cells mainly in the larger nuclear surface. This is the basis for the idea of atomization as a method of normalizing nuclear-plasma relations in polyploid cells by increasing the ratio of the surface of the nucleus to its volume. During A., the cell retains its characteristic functional activity, which almost completely disappears during mitosis. In many cases, A. and binuclearity accompany compensatory processes occurring in tissues (for example, during functional overload, fasting, after poisoning or denervation). Usually A. is observed in tissues with reduced mitotic activity. This, apparently, explains the increase in the number of binucleate cells formed by A as the body ages. The idea of A as a form of cell degeneration is not supported by modern research. The view of A. as a form of cell division is also untenable; There are only isolated observations of amitotic division of the cell body, and not just its nucleus. It is more correct to consider A. as an intracellular regulatory reaction.
Lit.: Wilson E. B., The cell and its role in development and heredity, trans. from English, vol. 1≈2, M.≈L., 1936≈40; Baron M.A., Reactive structures of internal shells, [M.], 1949; Brodsky V. Ya., Cell trophism, M., 1966; Bucher O., Die Amitose der tierischen und menschlichen Zeile, W., 1959.
V. Ya. Brodsky.
Wikipedia
Amitosis
Amitosis, or direct cell division- cell division by simple division of the nucleus into two.
It was first described by the German biologist Robert Remak in 1841, and the term was coined by histologist Walter Flemming in 1882. Amitosis is a rare but sometimes necessary phenomenon. In most cases, amitosis is observed in cells with reduced mitotic activity: these are aging or pathologically altered cells, often doomed to death (mammalian embryonic membrane cells, tumor cells, etc.).
With amitosis, the interphase state of the nucleus is morphologically preserved, the nucleolus and nuclear envelope are clearly visible. There is no DNA replication. Chromatin spiralization does not occur, chromosomes are not detected. The cell retains its characteristic functional activity, which almost completely disappears during mitosis. During amitosis, only the nucleus divides, without the formation of a fission spindle, so the hereditary material is distributed randomly. The absence of cytokinesis leads to the formation of binucleate cells, which are subsequently unable to enter into the normal mitotic cycle. With repeated amitoses, multinucleated cells can form.
This concept still appeared in some textbooks until the 1980s. It is currently believed that all phenomena attributed to amitosis are the result of an incorrect interpretation of insufficiently well-prepared microscopic preparations, or interpretation of phenomena accompanying cell destruction or other pathological processes as cell division. At the same time, some variants of nuclear division in eukaryotes cannot be called mitosis or meiosis. This is, for example, the division of the macronuclei of many ciliates, where the segregation of short fragments of chromosomes occurs without the formation of a spindle.
Mitosis(from the Greek mitos - thread), or karyokinesis (Greek karyon - core, kinesis - movement), or indirect division. This is a process during which chromosome condensation occurs and daughter chromosomes are evenly distributed between daughter cells. Mitosis includes five phases: prophase, prometaphase, metaphase, anaphase and telophase. IN prophase chromosomes condense (twist), become visible and are arranged in the form of a ball. The centrioles divide into two and begin to move towards the cell poles. Between the centrioles, filaments consisting of the protein tubulin appear. The formation of a mitotic spindle occurs. IN prometaphase the nuclear membrane disintegrates into small fragments, and the chromosomes immersed in the cytoplasm begin to move towards the equator of the cell. In metaphase chromosomes are installed at the equator of the spindle and become maximally compacted. Each chromosome consists of two chromatids connected to each other by centromeres, and the ends of the chromatids diverge, and the chromosomes take an X-shape. In anaphase daughter chromosomes (former sister chromatids) move to opposite poles. The assumption that this is achieved by contraction of the spindle filaments has not been confirmed.
Many researchers support the sliding filament hypothesis, according to which neighboring spindle microtubules, interacting with each other and contractile proteins, pull chromosomes towards the poles. In telophase daughter chromosomes reach the poles, despiral, a nuclear envelope is formed, and the interphase structure of the nuclei is restored. Then comes the division of the cytoplasm - cytokinesis. In animal cells, this process manifests itself in the constriction of the cytoplasm due to the retraction of the plasmalemma between two daughter nuclei, and in plant cells, small EPS vesicles merge to form a cell membrane from within the cytoplasm. The cellulose cell wall is formed due to the secretion that accumulates in dictyosomes.
The duration of each phase of mitosis is different - from several minutes to hundreds of hours, which depends on both external and internal factors and the type of tissue.
Violation of cytotomy leads to the formation of multinucleated cells. If the reproduction of centrioles is disrupted, multipolar mitoses can occur.
Amitosis
This is a direct division of the cell nucleus, which maintains the interphase structure. In this case, chromosomes are not detected, spindle formation and their uniform distribution do not occur. The core is divided by constriction into relatively equal parts. The cytoplasm can divide by a constriction, and then two daughter cells are formed, but it may not divide, and then binucleate or multinucleated cells are formed.
Amitosis as a method of cell division can occur in differentiated tissues, such as skeletal muscle, skin cells, and also in pathological tissue changes. However, it is never found in cells that need to preserve complete genetic information.
11. Meiosis. Stages, biological significance.
Meiosis(Greek meiosis - reduction) - a method of dividing diploid cells with the formation of four daughter haploid cells from one maternal diploid cell. Meiosis consists of two successive nuclear divisions and a short interphase in between. The first division consists of prophase I, metaphase I, anaphase I and telophase I.
In prophase I paired chromosomes, each of which consists of two chromatids, approach each other (this process is called conjugation of homologous chromosomes), cross over (crossing over), forming bridges (chiasmata), and then exchange sections. Crossing over involves recombination of genes. After crossing over, the chromosomes are separated.
In metaphase I paired chromosomes are located along the equator of the cell; spindle strands are attached to each chromosome.
In anaphase I bichromatid chromosomes diverge to the cell poles; in this case, the number of chromosomes at each pole becomes half that in the mother cell.
Then comes telophase I– two cells with a haploid number of bichromatic chromosomes are formed; Therefore, the first division of meiosis is called reduction.
Telophase I is followed by a short interphase(in some cases, telophase I and interphase are absent). In the interphase between two divisions of meiosis, chromosome duplication does not occur, because each chromosome already consists of two chromatids.
The second division of meiosis differs from mitosis only in that it is passed through cells with a haploid set of chromosomes; in the second division, prophase II is sometimes absent.
In metaphase II bichromatid chromosomes are located along the equator; the process occurs in two daughter cells at once.
In anaphase II Single-chromatid chromosomes move towards the poles.
In telophase II in four daughter cells, nuclei and partitions (in plant cells) or constrictions (in animal cells) are formed. As a result of the second division of meiosis, four cells with a haploid set of chromosomes (1n1c) are formed; the second division is called equational (equalization) (Fig. 18). These are gametes in animals and humans or spores in plants.
The significance of meiosis is that it creates a haploid set of chromosomes and conditions for hereditary variability due to crossing over and probabilistic divergence of chromosomes
12.Gametogenesis: ovo - and spermatogenesis.
Gametogenesis- process of formation of eggs and sperm.
Spermatogenesis- from Greek sperma, gen. n. spermatos - seed and...genesis), the formation of differentiated male germ cells - sperm; in humans and animals - in the testes, in lower plants - in antheridia.
In most higher plants, spermatozoa are formed in the pollen tube, more often called sperm. Spermatogenesis begins simultaneously with the activity of the testicle under the influence of sex hormones during puberty in adolescence and then proceeds continuously (in most men almost until the end of life), has a clear rhythm and uniform intensity. Spermatogonia, containing a double set of chromosomes, divide by mitosis, leading to the emergence of subsequent cells - first-order spermatocytes. Further, as a result of two successive divisions (meiotic divisions), 2nd order spermatocytes are formed, and then spermatids (spermatogenesis cells immediately preceding the sperm). During these divisions, the number of chromosomes is halved. Spermatids do not divide, enter the final period of spermatogenesis (the period of sperm formation) and, after a long differentiation phase, turn into sperm. This occurs through gradual elongation of the cell, changes, and elongation of its shape, as a result of which the cell nucleus of the spermatid forms the head of the sperm, and the membrane and cytoplasm form the neck and tail. In the last phase of development, the sperm heads are closely adjacent to the Sertoli cells, receiving nutrition from them until full maturation. After this, the sperm, already mature, enter the lumen of the testicular tubule and then into the epididymis, where they accumulate and are excreted from the body during ejaculation.
Oogenesis- the process of development of female gametes, ending with the formation of eggs. During a woman's menstrual cycle, only one egg matures. The process of oogenesis is fundamentally similar to spermatogenesis and also goes through a number of stages: reproduction, growth and maturation. Eggs are formed in the ovary, developing from immature germ cells - oogonia, containing a diploid number of chromosomes. Oogonia, like spermatogonia, undergo successive mitotic
divisions that are completed by the time the fetus is born. Then comes the period of growth of oogonia, when they are called first-order oocytes. They are surrounded by a single layer of cells - the granulosa membrane - and form the so-called primordial follicles. A female fetus on the eve of birth contains about 2 million of these follicles, but only about 450 of them reach the stage of second-order oocytes and leave the ovary during ovulation. The maturation of the oocyte is accompanied by two successive divisions leading to
reducing the number of chromosomes in a cell by half. As a result of the first division of meiosis, a large oocyte of the second order and the first polar body are formed, and after the second division - a mature one, capable of fertilization and further
development of an egg with a haploid set of chromosomes and a second polar body. Polar bodies are small cells that play no role in oogenesis and are eventually destroyed.
13.Chromosomes. Their chemical composition, supramolecular organization (DNA packaging levels).
