Amitosis. Its types and biological significance. Direct cell division is amitosis. Endomitosis, the importance of endomitosis and polythenia for the normal functioning of the body

Accent placement: AMITO`Z

AMITOSIS (amitosis; Greek, negative prefix a-, mitos - thread + -ōsis) direct nuclear fission- division of the cell nucleus into two or more parts without the formation of chromosomes and achromatin spindle; with A., the nuclear membrane and nucleolus are preserved and the nucleus continues to function actively.

direct division kernels are for the first time described by Remak (R. Bemak, 1841); the term "amitosis" was proposed by Flemming (W. Flemming, 1882).

Usually A. 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 gradually ligates, forming two or more daughter nuclei. With the help of cytophotometric research methods, it was found that in about 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 the division of the nucleus, the division of the cytoplasm (cytotomy) occurs with the formation daughter cells(Fig. 1); if the cytoplasm does not divide, one two- or multinuclear cell appears (Fig. 2).

A. is characteristic of a number of highly differentiated and specialized tissues (neurons of autonomic ganglia, cartilage, glandular cells, blood leukocytes, endothelial cells blood vessels etc.), as well as for cells of malignant tumors.

Benshshghoff (A. Benninghoff, 1922), based on the functional purpose, proposed to distinguish three types of A.: generative, reactive and degenerative.

Generative A. is a full-fledged nuclear fission, after which it becomes possible mitosis(cm.). Generative A. is observed in some protozoa, in polyploid nuclei (see. Chromosomal set); at the same time, 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 certain specialized cells (liver, epidermis, trophoblast, etc.), where A. is preceded by endomitosis - intranuclear doubling of the set of chromosomes (see. Meiosis); the resulting endomitosis and polyploid nuclei are then subjected to A.

Reactive A. due to the influence on the cell of various damaging factors - radiation, chemical. drugs, temperature, etc. It can be caused by violations 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 multinucleated cells. Many researchers tend to consider reactive A. as an intracellular compensatory reaction that ensures the intensification of cell metabolism.

Degenerative A. - nuclear division associated with the processes of degradation or irreversible cell differentiation. With this form of A., fragmentation, or budding, of nuclei occurs, which is not associated with DNA synthesis, which in some cases is a sign of incipient tissue necrobiosis.

Question about biol. the value of A. has not been finally resolved. However, there is no doubt that A. is a secondary phenomenon in comparison with mitosis.

Prerequisites preceding nuclear and cell division

The basis of reproduction is cell division. Nuclear division always precedes cell division. In the process historical development the nucleus, like other cell organelles, arose perhaps as a result of specialization and differentiation of individual sections of the cytoplasm. However, in the process individual development cell nucleus arises only from the nucleus as a result of division.

Growth plant organism(an increase in its size) occurs due to an increase in the number of cells by division. In unicellular organisms, cell division is both a way of their reproduction - an increase in their weight, and reproduction - an increase in the number of individuals of a given species.

Each cell grows over a given time, and in the process of its growth, the ratio between the growing volume of cells and its growing surface changes all the time.

The growth of the surface, of course, lags in its absolute terms behind the growth of the volume, since the surfaces increase quadratically, while the volume increases cubically.

Remark 2

As you know, the cell is fed through the surface. Therefore, in certain time the surface cannot "provide" the volume of the cell and it begins to divide intensively.

There are four ways of cell division:

amitosis,
mitosis,
endomitosis
meiosis.

Amitosis

Definition 1

Amitosis (from Greek a - negative particle and mitos - thread) - direct division of the nucleus, which occurs by restructuring the nuclear substance, without the formation of chromosomes.

The phenomenon of amitosis was first described by the German biologist R. Remarque (1841). The term "amitosis" was introduced by the German histologist W. Fleming (1882). Amitosis is much less common than mitosis. It occurs by constriction of the nucleolus, nucleus, and then the cytoplasm. Unlike mitosis, during amitosis, chromosomes do not condense in the nucleus, but only doubling, do not change physiochemical properties cytoplasm. By physiological significance There are three types of amitotic distribution:

generative amitosis is a full-fledged cell division, the daughter cells of which are capable of mitotic distribution and normal functioning.
reactive amitosis - caused by inadequate actions on the body.
degenerative amitosis - distribution associated with the processes of self-destruction and cell death.

Amitosis process

With amitotic type of cell division, the splitting of the nucleus is accompanied by cytoplasmic narrowing. During amitosis, the nucleus first lengthens and then acquires dumbbells. The depression or narrowing increases in size and eventually divides the nucleus into two nuclei; nuclear division is followed by a narrowing of the cytoplasm, which divides the cell into two identical or approximately identical halves. Without the occurrence of any nuclear event, two daughter cells are formed. Due to auxetic growth, the cell enlarges. The nucleus expands and eventually forms a dumbbell-shaped structure with the appearance of a median constriction.

On the middle part cell membrane two constrictions appear. The constriction of the nucleus gradually deepens and divides the nucleus into two daughter nuclei without the formation of any spindle fiber. The invaginated cells also move inward, and the parent cell divides into two equal-sized daughter cells.

Amitosis is observed in young, completely normally developed cells (in the daughter of the bulb, root tissues). But more often it is inherent in highly differentiated and older cells. Amitosis is also inherent in low-level organisms - yeast, bacteria, etc. The disadvantage of amitosis is that in this process of cell division there is no possibility of genetic recombination and there is the possibility of expression of unwanted recessive genes.

The meaning of amitosis

Remark 3

The essence of amitosis is that the nucleus, followed by the contents of the cell, is divided into two parts - daughter cells without any preliminary changes in the structure of organelles, including the nucleus.

Moreover, the nucleus is divided into two parts, even without prior dissolution of the nuclear envelope. There is no fission spindle formation, which is characteristic of other types of fission.

After the division of the nucleus, the protoplast and the entire cell begin to divide into two parts, but in cases where the nucleus is fragmented into several parts, multinucleated cells are formed. During amitosis, there is no uniform distribution of the substance of the nucleus between the daughter nuclei, that is, their biological uniformity is not ensured. However, the formed cells do not lose their structural organization and vital activity.

For a long time, there was an opinion in science that amitosis is a pathological phenomenon inherent only in pathologically altered cells. However latest research do not support this view. Many studies (Karolinskaya, 1951 and others) have shown that amitosis is also observed in young, normally developed cells. This type of cell and nucleus division was observed in the cells of the internodes of Chara algae, in the cells of onion, tradescantia. In addition, amitosis also occurs in specialized tissues with high activity. metabolic processes, namely: in the cells of the tapetum of microsporangia, in the endosperm of the seeds of some plants, and the like.

However, this type of separation does not occur in cells where complete genetic information must be preserved, such as eggs and embryonic cells. Therefore, according to a number of scientists, amitosis cannot be considered a full-fledged way of cell reproduction.

Amitosis is sometimes also called simple division.

Definition 1

Amitosis - direct cell division by constriction or invagination. During amitosis, there is no condensation of chromosomes and no division apparatus is formed.

Amitosis does not provide an even distribution of chromosomes between daughter cells.

Usually amitosis is characteristic of senescent cells.

During amitosis, the cell nucleus retains the structure of the interphase nucleus, and complex restructuring of the entire cell, chromosome spiralization, as during mitosis, does not occur.

There is no evidence for an even distribution of DNA between two cells during amitotic division, so it is believed that DNA during this division can be distributed unevenly between two cells.

Amitosis is quite rare in nature, mainly in unicellular organisms and in some cells of multicellular animals and plants.

Types of amitosis

There are several forms of amitosis:

uniform when two equal nuclei are formed;
uneven- different 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 cartilage cells, loose connective and some other tissues, nucleolus division occurs, followed by nuclear division by constriction. In a binuclear cell, a circular constriction of the cytoplasm appears, which, when deepened, causes a complete division of the cell into two.

In the process of amitosis in the nucleus, the division of the nucleoli occurs, 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 cells of the epithelium, the liver, the process of division of the nucleoli in the nucleus is observed, after which the entire nucleus is laced with an annular constriction. This process ends with the formation of two nuclei. Such a binuclear or multinuclear cell no longer divides mitotically, after a while it ages or dies.

Remark 1

Thus, amitosis is a division that occurs without the spiralization of chromosomes and without the formation of a division spindle. It is also unknown whether DNA synthesis is synthesized before the onset of amitosis and how DNA is distributed between the daughter nuclei. Whether the previous DNA synthesis occurs before the onset of amitosis and how it is distributed between the daughter nuclei is unknown. When certain cells divide, sometimes mitosis alternates with amitosis.

The biological significance of amitosis

Some scientists consider this method of cell division to be primitive, while others consider it to be a secondary phenomenon.

Amitosis, compared with mitosis, is much less common in multicellular organisms and can be attributed to an inferior method of cell division that has lost the ability to divide.

The biological significance of the processes of amitotic division:

there are no processes that ensure the uniform distribution of the material of each chromosome between two cells;
the formation of multinucleated cells or an increase in the number of cells.

Definition 2

Amitosis- this is a peculiar type of division, which can sometimes be observed during normal cell activity, and in most cases, when functions are impaired: the influence of radiation or the action of other harmful factors.

Amitosis is characteristic of highly differentiated cells. Compared to mitosis, it is less common and plays a minor role in cell division in most living organisms.

amitosis (amitosis; a- + mitosis; synonym: amitotic division, direct division)

cell division without the formation of a division spindle and spiralization of chromosomes; A. is characteristic of cells of some specialized tissues (leukocytes, endothelial cells, neurons of autonomic ganglia, etc.), as well as malignant tumors.

Amitosis

direct nuclear fission, one of the methods of nuclear division in protozoa, in plant and animal cells. A. was first described by the German biologist R. Remak (184

; the term was proposed by the histologist W. Flemming (188

During A., in contrast to mitosis, or indirect nuclear division, the nuclear envelope and nucleoli are not destroyed, the spindle of division in the nucleus is not formed, the chromosomes remain in a working (despiralized) state, the nucleus either ligates or a septum appears in it, externally unchanged; division of the cell body - cytotomy, as a rule, does not occur (Fig.); usually A. does not provide uniform division of the nucleus and its individual components.

The study of A. is complicated by the unreliability of its definition by morphological features, since not every constriction of the nucleus means A.; even pronounced "dumbbell" constrictions of the nucleus can be transient; nuclear constrictions can also be the result of an incorrect previous mitosis (pseudoamitosis). Usually A. follows endomitosis. In most cases, with A. only the nucleus is divided and a binuclear cell appears; at repeated And. multinuclear cells can be formed. Very many binuclear and multinuclear cells are the result of A. (a certain number of binuclear 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 are known both with mononuclear and binuclear polyploid cells (cells of the liver, pancreas and salivary glands, nervous system, epithelium Bladder, epidermis), and only with binuclear polyploid cells (mesothelial cells, connective tissues). Bi- and multinucleated cells differ from single-nuclear diploid cells (see Diploid) large sizes, more intense synthetic activity, an increased number of different structural formations including chromosomes. Binuclear and multinuclear cells differ from mononuclear polyploid cells mainly in the larger surface area of the nucleus. This is the basis for the concept of A. as a method of normalizing nuclear-plasma relations in polyploid cells by increasing the ratio of the nuclear surface to its volume. During A., the cell retains its characteristic functional activity, which almost completely disappears during mitosis. In many cases, A. and binuclearity are accompanied by compensatory processes occurring in tissues (for example, during functional overload, starvation, after poisoning or denervation). Usually A. is observed in tissues with reduced mitotic activity. This, apparently, explains the increase with the aging of the body in the number of binuclear cells formed by A. Ideas about A. as a form of cell degeneration are not supported. modern research. The view of A. as a form of cell division is also untenable; there are only single observations of amitotic division of the cell body, and not just its nucleus. It is more correct to consider And. as 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 in two.

It was first described by the German biologist Robert Remak in 1841, and the term was proposed by the histologist Walter Flemming in 1882. Amitosis is a rare but sometimes necessary occurrence. In most cases, amitosis is observed in cells with reduced mitotic activity: these are aging or pathologically altered cells, often doomed to death (cells of the embryonic membranes of mammals, tumor cells, etc.).

During amitosis, the interphase state of the nucleus is morphologically preserved, the nucleolus and the nuclear membrane are clearly visible. DNA replication is absent. Spiralization of chromatin does not occur, chromosomes are not detected. The cell retains its inherent functional activity, which almost completely disappears during mitosis. During amitosis, only the nucleus divides, and without the formation of a fission spindle, therefore, the hereditary material is distributed randomly. The absence of cytokinesis leads to the formation of binuclear cells, which are subsequently unable to enter into a 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 inadequately prepared microscopic preparations, or of the interpretation of phenomena accompanying cell destruction as cell division or other pathological processes. At the same time, some variants of eukaryotic nuclear fission cannot be called mitosis or meiosis. Such, for example, is the division of the macronuclei of many ciliates, where, without the formation of a spindle, segregation of short fragments of chromosomes occurs.

Amitosis - what is it and what does it consist of fundamental difference from mitosis itself? The solution of these issues has been relevant for the past two or three decades. A review of the obtained literature not only confirms the involvement of amitosis in cell proliferation, this process implies the existence of more than one amitotic mechanism capable of producing new nuclei without the participation of mitotic chromosomes.

Amitosis (biology): it all starts with the cell

It's hard to imagine, but the cells present in the tiny fetus eventually give rise to all the cells that make up the body of an adult. Bone and flesh, organs and tissues are the products of thousands of generations of cell division. Most plant and animal cells replicate by separating into two identical daughter cells. Simple division, which is the means of asexual reproduction of single-celled organisms such as bacteria and protozoa, is called amitosis. It is also a way of reproduction or growth in the fetal membranes of some vertebrates.

The splitting of the nucleus is accompanied by cytoplasmic narrowing. In the process of division, the nucleus elongates and then takes on an elongated shape, then it increases in size and, ultimately, is divided into two halves. This process is accompanied by a narrowing of the cytoplasm, which divides the cell into two equal or approximately identical parts. Thus, two daughter cells are formed.

Discovery of cell division

In the 19th century, Flemming, a professor at the Institute of Anatomy in Kiel (Germany), first documented the details of cell division. He was highly regarded as an innovator in this field, largely due to technology such as the use of microscopes to study biological tissues. Flemming experimented with the technique of using dyes to stain specimens that he wanted to examine under a microscope. He discovered some positive properties aniline dyes and came to the conclusion that different types fabrics absorb from different intensity depending on their chemical composition. This made it possible to reveal structures and processes that were invisible before.

Fleming was interested in the process of cell division. He began a series of live observations under a microscope using stained animal tissue samples and found that a certain mass of material inside the nucleus absorbs the dye quite well. After a while, it began to be called "chromatin" (from the Greek saturated). Today, the process of splitting one nucleus into two is called mitosis, and the division itself is called cytokinesis. But what is amitosis? Scientists began to think about this issue only in the 20th century.

Key Difference Between Mitosis and Amitosis

Mitosis is the process in which cells arrange their chromosomes into two identical sets. Amitosis is a process that occurs in the absence of mitosis in cells. Life is beautiful and complex. It's amazing how everything around grows, changes and develops. Mitosis is an integral part of the cell cycle, which basically involves a series of events leading a cell to divide and create two daughter cells. So there are exact copies of the parent cell. This is followed by cytokinesis, which separates the cytoplasm, organelles, and membrane.

Another way of division is amitosis. This concept can be classified as a form of closed mitosis. During this process, the mother cell also produces two daughter cells, but they are not identical to each other or parent cell. Amitosis is also sometimes called direct cell division during which the cell and its nucleus split into two halves. However, unlike mitosis, no complex changes occur in the nucleus.

Amitosis to the rescue

In 1882, the scientific term amitosis appeared in medicine. Where it has already been observed, a normal mitotic cycle is no longer possible. Previously called the primitive form, amitosis in modern understanding is a qualitatively peculiar process of nuclear fission, which appeared on the basis of mitotic transformations. Sometimes amitosis is observed in various pathological phenomena, for example, inflammatory processes or malignant tumors.

Amitosis is also discussed when the cell has lost the ability to mitosis. Most often this happens in adulthood. An example is the human body. Cells of cardio-vascular system lose the ability to mitosis, therefore, when they are damaged (for example, heart attack) they cannot recreate or replace themselves. Remarkably, skin cells continue to replicate and replace themselves throughout their lives and ours. Amitosis may be accompanied by cell division, or it may be limited to nuclear division without division of the cytoplasm, which leads to the formation of multinucleated cells. Basically, this process occurs in degenerating cells that are doomed to death, especially in the embryonic membranes of mammals.

Main features of amitosis

The activity of the cell is preserved, but the hereditary material is distributed in a chaotic manner.
Lack of cytokinesis, this can lead to the formation of cells with multiple nuclei.
The resulting cells are no longer capable of mitosis.
Difficulties in identification, sometimes amitosis can be the result of incorrectly proceeding mitosis.
Most often found in unicellular organisms, as well as in plant and animal cells with weakened physiological activity and other deviations from the norm.

The question of what exactly is amitosis is still controversial. A large number of scientists and biologists dispute the fact that this is simply a form of cell division, calling it an internal regulatory response of the cell.