Plan 2

1. Amitosis 3

1.1. Concept of amitosis 3

1.2. Features of amitotic division of the cell nucleus 4

1.3. Amitosis value 6

2. Endomitosis 7

2.1. Concept of endomitosis 7

2.2. Examples of endomitosis 8

2.3. Endomitosis meaning 8

3. References 10

1.1. The concept of amitosis

Amitosis (from the Greek a - negative particle and mitosis)-direct division of the interphase nucleus by ligation without transformation of chromosomes.

During amitosis, uniform divergence of chromatids to the poles does not occur. And this division does not ensure the formation of genetically equivalent nuclei and cells.

Compared to mitosis, amitosis is a shorter and more economical process. Amitotic division can occur in several ways.

The most common type of amitosis is the lacing of the nucleus into two parts. This process begins with the division of the nucleolus. The constriction deepens and the core splits in two.

After this, the separation of the cytoplasm begins, but this does not always happen. If amitosis is limited only to nuclear division, then this leads to the formation of bi- and multinucleated cells. During amitosis, budding and fragmentation of nuclei can also occur.

A cell that has undergone amitosis is subsequently unable to enter the normal mitotic cycle.

Amitosis occurs in cells of various tissues of plants and animals. In plants, amitotic division occurs quite often in the endosperm, in specialized root cells and in storage tissue cells.

Amitosis is also observed in highly specialized cells with weakened viability or degenerating, during various pathological processes such as malignant growth, inflammation, etc.

1.2. Features of amitotic division of the cell nucleus

It is known that the formation of polynuclear cells occurs due to four mechanisms: as a result of the fusion of mononuclear cells, in the case of blockade of cytokinesis, as a result of multipolar mitoses and during amitotic division of the nucleus.

Unlike the first three, well-studied mechanisms, amitosis rarely appears as an object of study, and the amount of information on this issue is extremely limited.

Amitosis is important in the formation of multinucleated cells and is a staged process, during which the following sequentially occur: stretching of the nucleus, invagination of the karyolemma, and constriction of the nucleus into parts.

Although the amount of reliable information about the molecular and subcellular mechanisms of amitosis is insufficient, there is information about the participation of the cell center in the implementation of this process. It is also known that if nuclei are segmented due to the action of microfilaments and microtubules, then the role of cytoskeletal elements in amitotic division is not excluded.

Direct division, accompanied by the formation of nuclei that differ in volume, may indicate an unbalanced distribution of chromosomal material, which is refuted by data obtained from studies carried out using light and electron microscopy methods. These contradictions may indicate the use of different methods of morphometric analysis and evaluation of the results obtained, which underlie certain conclusions.

Regeneration in pathological and physiological conditions is carried out by amitosis, which also occurs with an increase in the functional activity of the tissue, for example, amitosis is responsible for the increase in the number of binuclear cells that make up the glandular epithelium of the mammary glands during lactation. Therefore, to consider amitotic nuclear division only as a sign of a pathological nature should be recognized as a one-sided approach to the study of this issue, and to reject the facts confirming the compensatory significance of this phenomenon.

Amitosis has been observed in cells of various origins, including cells of some tumors, so its participation in oncogenesis cannot be denied. An opinion has been expressed about the presence of amitosis in intact cells cultured in vitro, although it is possible to classify them as such only conditionally, since incubation in itself is an influencing factor that changes the morphological and functional characteristics of cells extracted from the body.

The fundamental importance of amitosis in the implementation of intracellular processes is evidenced by the fact of its existence in many types of cells and under different conditions.

Since the role of amitotic division of polyploid nuclei in the formation of polynuclear cells is considered proven, in this case the main meaning of amitosis is to establish optimal nuclear-cytoplasmic relationships that allow cells to adequately carry out various functions.

The existence of amitosis in multinucleated cells of various origins and their formation due to several mechanisms, including due to amitotic division of the nucleus, has been demonstrated.

Summarizing the information presented, we can conclude that amitosis, as a result of which polynuclear cells are formed, has a staged nature and takes part in ensuring the adequate functioning of cells and tissues of the body under physiological and pathological conditions.

However, the amount of information about the peculiarities of the formation of multinuclear fibroblasts as a result of the amitotic division of their nuclei, depending on the influence of various factors, probably cannot be considered sufficient. At the same time, obtaining such data is necessary to understand many aspects of the functioning and morphogenesis of these cells.

Familiarization with the information contained in this article will allow the reader to learn about one of the methods of cell division - amitosis. We will find out the features of this process, consider the differences from other types of division and much more.

What is amitosis

Amitosis is direct cell division. This process occurs thanks to the usual two parts. However, it may miss the spindle formation phase for division. And ligation occurs without chromatin condensation. Amitosis is a process characteristic of animal and plant cells, as well as simple organisms.

From history and research

Robert Remak described the process of amitosis for the first time in 1841, but the term itself arose much later. Already in 1882, histologist and biologist of German origin Walter Flemming proposed the modern name for the process itself. Amitosis of a cell in nature is a relatively rare phenomenon, but it can often occur because it is necessary.

Process Features

How does cell division occur? Amitosis most often occurs in cells that have reduced mitotic activity. Thus, many cells that should die as a result of old age or pathological changes can delay their death for some time.

Amitosis is a process in which the state of the nucleus during interphase retains its morphological characteristics: the nucleolus is clearly visible, as is its shell, DNA is not replicated, chromatin is proteinaceous, DNA and RNA are not spiralized, and there is no detection of chromosomes in the nucleus of a eukaryotic cell.

There is indirect cell division - mitosis. Amitosis, in contrast, allows the cell after division to maintain its activity as a functioning element. The spindle (a structure designed for chromosomal segregation) is not formed during amitosis, but the nucleus still divides, and the consequence of this process is the random distribution of hereditary information. The absence of the cytokinetic process results in the reproduction of cells with two nuclei, which in the future will not be able to enter into the typical mitotic cycle. Repeated repetition of amitosis can lead to the formation of cells with many nuclei.

Current situation

Amitosis as a concept began to appear in many textbooks back in the 80s of the twentieth century. Today, there are assumptions that all the processes that were previously subsumed under this concept are in fact incorrectly interpreted results of studies on poorly prepared microslides. Scientists believe that the phenomenon of cell division, accompanied by the destruction of the latter, could lead to the same misunderstood and interpreted data. However, some processes of eukaryotic cell division cannot be classified as either mitosis or meiosis. A striking example and confirmation of this is the process of division of the macronucleus (the nucleus of a ciliate cell, large in size), during which the segregation of some chromosome sections occurs, despite the fact that the spindle for division is not formed.

What causes the complication of studying the processes of amitosis? The fact is that this phenomenon is difficult to determine by its morphological characteristics. This definition is unreliable. The inability to clearly determine the process of amitosis by morphological signs is based on the fact that not every nuclear constriction is a sign of amitosis itself. And even its dumbbell-shaped form, which is clearly expressed in the core, can only belong to the transitional type. Also, nuclear constrictions can be a consequence of errors in the phenomenon of previous division by mitosis. Most often, amitosis occurs immediately after endomitosis (a method of doubling the chromosome number without dividing both the cell and its nucleus). Usually the process of amitosis leads to doubling. The repetition of this phenomenon creates a cell with many nuclei. Thus, amitosis creates cells with a polyploid chromosome set.

Conclusion

To summarize, we can say that amitosis is a process during which the cell divides in a direct manner, that is, the nucleus divides into two parts. The process itself is not capable of ensuring cell division into equal, identical halves. This also applies to information about the heredity of the cell.

This process has a number of sharp differences from the gradual division by mitosis. The main difference in the processes of amitosis and mitosis is the absence of destruction of the nuclear membrane and nucleolus during amitosis, as well as the occurrence of the process without the formation of a spindle, which ensures the division of information. Cytotomy in most cases does not divide.

Currently, there are no studies of the modern era that could clearly identify amitosis as a form of cell degeneration. The same applies to the perception of amitosis as a method of cell division due to the presence of a very small amount of division of the entire cell body. Therefore, amitosis may be better attributed to a regulatory process occurring inside cells.

Atypical forms of mitosis include amitosis, endomitosis, and polyteny.

Amitosis sometimes also called simple division. Amitosis is direct cell division by constriction or intussusception. During amitosis, condensation of chromosomes does not occur and the division apparatus is not formed. Amitosis does not ensure uniform distribution of chromosomes between daughter cells. Amitosis is usually characteristic of aging cells. During amitosis, the cell nucleus retains the structure of the interphase nucleus, and complex rearrangement of the entire cell, chromosome spiralization, does not occur, as during mitosis. There is no evidence that DNA is evenly distributed between two cells during amitotic division, so it is believed that DNA during such division may be distributed unevenly between two cells. Amitosis occurs quite rarely in nature, mainly in unicellular organisms and in some cells of multicellular animals and plants. There are several forms of amitosis:

• uniform, when two equal nuclei are formed;
• uneven – unequal nuclei are formed;
• fragmentation - the nucleus breaks up into many small nuclei, of the same size or not.

The first two types of division cause the formation of two cells from one. In the cells of cartilage, loose connective tissue and some other tissues, division of the nucleoli occurs, followed by division of the nucleus by constriction. In a binuclear cell, a circular constriction of cytoplasm appears, which, when deepened, causes complete division of the cell into two. Example. Isogenic groups appear in cartilage, i.e. groups originating from the same cell. Such cells are specialized to perform certain functions in the body, but do not have the ability to divide mitotically. During the process of amitosis, division of the nucleoli occurs in the nucleus, followed by division of the nucleus by a constriction; the cytoplasm is also divided by a constriction.

Amitosis-fragmentation causes the formation of multinucleated cells. In some epithelial and liver cells, the process of division of nucleoli in the nucleus is observed, after which the entire nucleus is laced with a ring constriction. This process ends with the formation of two nuclei. Such a binuclear or multinucleate cell no longer divides mitotically; after some time it ages or dies. Thus, amitosis is a division that occurs without chromosome spiralization and without spindle formation. It is also unknown whether DNA synthesis occurs before the onset of amitosis and how DNA is distributed between daughter nuclei. Whether previous DNA synthesis occurs before the onset of amitosis and how it is distributed between daughter nuclei is unknown. When certain cells divide, mitosis sometimes alternates with amitosis.

Biological significance of amitosis Some scientists consider this method of cell division to be primitive, others attribute it to secondary phenomena. Amitosis, compared to mitosis, is much less common in multicellular organisms and can be attributed to an inferior method of dividing cells that have lost the ability to divide. Biological significance of amitotic division processes:

• processes ensuring uniform distribution of the material of each chromosome between two cells are absent;
• the formation of multinucleated cells or an increase in the number of cells.

Endomitosis. With this type of division, after DNA replication, the chromosomes do not separate into two daughter chromatids. This leads to an increase in the number of chromosomes in a cell, sometimes tens of times compared to the diploid set. This is how polyploid cells arise. Normally, this process takes place in intensively functioning tissues, for example, in the liver, where polyploid cells are very common. However, from a genetic point of view, endomitosis is a genomic somatic mutation.

Polythenia. There is a multiple increase in the DNA content (chromonemas) in the chromosomes without an increase in the content of the chromosomes themselves. In this case, the number of chromonemas can reach 1000 or more, and the chromosomes acquire gigantic sizes. With polythenia, all phases of the mitotic cycle are lost, except for the reproduction of the primary DNA strands. This type of division is observed in some highly specialized tissues (liver cells, salivary gland cells of dipteran insects). Drosophila polytene chromosomes are used to construct cytological maps of genes in chromosomes.

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 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 ordered 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.

The process of direct division without cell preparation is called amitosis. First discovered in 1841 by biologist Robert Remak. The term was introduced by histologist Walter Flemming in 1882.

Peculiarities

Amitosis is a simpler process than mitosis or meiosis. Amitosis in eukaryotes is quite rare and is more common in prokaryotes. It is a faster and more economical process than mitosis. Observed during rapid tissue restoration. Amitosis divides aging cells and tissue cells that will not further divide mitotically. Most often, this is a group of cells that perform strictly defined functions.

Amitosis is observed:

• with an increase in the root cap;
• in epithelial cells;
• when growing onions;
• in loose connective tissue;
• in cartilage tissue;
• in the muscles;
• in the cells of the germinal membranes;
• with an increase in algae tissue;
• in endosperm cells.

The main features of amitosis, compared to mitosis:

• not accompanied by restructuring of the entire cell;
• the spindle is missing;
• chromatin spiralization does not occur;
• chromosomes are not detected;
• lack of DNA replication (doubling);
• genetic material is distributed unevenly;
• the resulting cell is not capable of mitosis.

Rice. 1. Mitosis and amitosis.

Amitosis can occur in tumor tissues. With an uneven distribution of genetic material, defective eukaryotic cells with disrupted intracellular processes are formed.

Mechanism

Amitosis is a simple and rare method of cell division that has been little studied. It is known that amitosis occurs due to simple constriction (invagination) of the karyolemma - the nuclear membrane, which leads to the division of the parent cell into two parts. During division, the cell is in interphase, i.e. in a state of growth and development, in no way preparing for division. The process of amitosis is described in the table.

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Cytokinesis does not always occur during amitosis, i.e. division of the cell body - the cytoplasm with all its contents. In this case, two or more nuclei are formed under one shell (multinuclear cell), which can lead to the formation of colonies (yeast).

Rice. 2. Yeast budding.

Meaning

Amitosis has biological significance for the rapid restoration of tissues and the reproduction of unicellular eukaryotic and prokaryotic organisms. Amitosis is characteristic of yeasts that reproduce asexually (by budding, division), bacteria, and leukocytes.

Bacteria and other prokaryotes do not have a nucleus. Therefore, amitosis occurs somewhat differently. First, circular DNA attached to a fold of the cytoplasmic membrane (mesosome) doubles. Then, a constriction forms between the two DNAs attached to the mesosomes, dividing the cell in half.

Rice. 3. Division of prokaryotes.

What have we learned?

We found out how mitosis differs from amitosis, how direct cell division occurs, and what role it plays in nature. Amitosis is the fastest method of division, which helps restore damaged tissue in a short period of time. Characteristic of eukaryotes (rare) and prokaryotes. Direct cell division does not require preparation: chromosome spiralization, DNA doubling, or creation of a division spindle. With this method, the cell divides unevenly: daughter cells may differ in size and amount of genetic information.

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