From Big Medical Encyclopedia

MITOSIS (mitosis; grech, mitos thread + - osis; synonym: karyokinesis, indirect cell division) — the most widespread way of a reproduction of a cell inherent to animal, plant cells, and also the elementary organisms. The m provides a possibility of formation of genetically equivalent cells and keeps succession chromosomes (see) among cellular generations (see. Cell division ). It is reached by a combination of two basic processes — identical reduplication of chromosomes and their hypodispersion between daughter cells. All these features of M. promoted that M. was fixed as one of the central mechanisms of an ontogeny of almost all representatives of an animal and flora. The m is only a part of a life (cellular) cycle, i.e. the period of existence from education before own division or death. A life cycle of cells of different type is not identical. For the cells which lost ability to self-reproduction in development it comes to the end with aging and death. The cells which kept ability to M. have the cellular cycle limited to the sequence of division. One after another M. in such cells call the period between the following interphase. In interphase distinguish the period of heterosynthetic interphase connected with processes of a differentiation, growth and functioning of a cell and the period of autosintetichesky interphase, in time to-rogo there are processes of preparation of a cell for a reproduction.

The interconnected events which are taking place during autosintetichesky interphase and actually M. designate as a mitotic cycle.

Actually M. precedes the beginning the period of preparation occurring in autosintetichesky interphase. It consists in accumulation of the power and structural components necessary for the subsequent division: it, in particular, accumulation of the makroergichesky connections capable in the bonds to store a large amount of energy (see. Vysokoergichesky connections ), synthesis four nucleoside triphosphates, chromosomes, necessary for reduplication, and reproduction of the cellular (mitotic) center.

Sometimes allocate one more stage which is directly preceding the beginning professional elements — a so-called preprofaza, or an antefaza, edges corresponds to the end of interphase.

In a preprofaza there is a doubling of DNA and synthesis of material of a spindle of division.

Fig. 1. Diagrammatic representation of stages of a mitosis: 1 and 2 — a pro-phase (a stage of formation of chromosomes and discrepancy of centrioles, disappearance of a kernel; the nuclear envelope still remains); 3 and 4 — metaphase (movement of chromosomes to the equatorial plane of the sharing cell, formation of the equatorial plate, or «maternal star», a nuclear envelope breaks up): 5 — an anaphase (a stage of discrepancy of chromosomes to poles and formations of a figure of «affiliated stars»; the interpolar threads connecting poles of a cell are visible; the dotted line designated the threads connecting poles with centromeres of chromosomes); 6 and 7 — telophase (a final stage of a mitosis, formation of affiliated kernels from groups of the chromosomes which gathered at poles, recovery of nuclear envelopes and division of a body of a cell into two parts).

M.'s process is usually subdivided into 4 stages: professional azu, metaphase, anaphase and telophase (fig. 1).

By the beginning professional elements the cell is usually rounded, viscosity of its cytoplasm increases. There is a destruction of a kernel, nuclear envelope and reorganization of a kernel to condensation and spiralling of chromosomes, to-rye were already reproduced in interphase or at the end of the previous M. Centrioles of the cellular center disperse to poles. Between them the spindle of division forms, a cut consists of microtubules to dia. 14 — 25 nanometers. The fibrilla which is radially located around centrioles forms radiant shine (astro-sphere). All these structures in total form the mitotic device of a cell necessary for implementation of discrepancy of chromosomes.

In metaphase there is a movement of chromosomes to the equatorial plane (a so-called metakinesis, or prometaphase), formation of the equatorial plate («a maternal star») and separation of sisterly chromosomes.

Fig. 2. An electronic and microscopic picture of a bunch of microtubules of the spindle going from kinetochores (K) to centrioles (D) in a metaphase cell of the Chinese hamster; x 36 000.

Anaphase — a stage of discrepancy of chromosomes to poles when two groups of chromosomes turned by centromeres to a pole are formed, and shoulders — to the equator (a figure of «affiliated stars»). By this time in a spindle of division distinguish interpolar threads — microtubules (fig. 2), to-rye the alny threads connecting poles with centromeres of chromosomes connect both poles of a cell and chromosomes. The Anafazny movement providing hypodispersion of chromosomes between daughter cells is total expression of two types of the movement: drawing apart poles as a result of lengthening of interpolar microtubules of a spindle of division and movement of chromosomes to opposite poles owing to shortening its hromosomalny mikrotrubochek.

The mechanism of the anafazny movement is studied not enough. Two most probable hypotheses explaining this mechanism are widespread: a hypothesis of the sliding threads and a hypothesis of a dynamic equilibrium of polymerization processes and a depolymerization. It agrees the first of them at anafazny discrepancy of chromosomes of a microtubule, like thick and thin myofilaments, decrease due to shortening in the course of their sliding relatively each other. The hypothesis of a dynamic equilibrium is most accepted, according to a cut shortening hromosomalny and lengthening of interpolar microtubules is connected with a partial depolymerization of the first and polymerization of the second. As a basis of polymerization processes and a depolymerization serves the dynamic equilibrium between microtubules and a pool of free subunits, from to-rykh protofibrils of these structures are constructed. The possibility of participation in the movement of chromosomes of the octynic microtubules found in the field of a spindle of division remains disputable so far. The movement of chromosomes in a metakinesis and in an anaphase is broken after damage of kinetochores, the mitotic centers and microtubules of a spindle.

In a final stage of M. — telophase — there is a reconstruction of affiliated kernels from the chromosomes which gathered at poles and division of a cellular body (process of a tsitotomiya, or a cytokinesis). Reconstruction of affiliated kernels is connected with a despiralization of chromosomes, recovery of a kernel and nuclear envelope. Tsitotomiya is carried out by formation of an intracellular plate at plant cells or by formation of a furrow of division at zooblasts. Mechanisms of a tsitotomiya usually connect or with reduction of the gelated ring of cytoplasm surrounding the equator (a hypothesis of a reducible ring) or with expansion of a surface of a cell owing to straightening of loop-shaped proteinaceous chains (a hypothesis of expansion of membranes).

M.'s duration is not identical in different types of cells. In zooblasts mean time of M. makes 30 — 60 min., in vegetable it reaches 2 — 3 hours of Neodinakov and duration of different stages M. Boley the stages connected with processes of synthesis (a pro-phase and telophase) are long. Metaphase and anaphase are less long. M.'s time depends on the sizes of cells, their ploidy, number of kernels, and also on conditions of the environment, in particular temperature.

Comparative tsitol, researches convince that during evolution process of distribution of chromosomes significantly changed. As showed Pikkitt-Hips (G. Pickett-Heaps, 1975), A. I. Alov and sotr. (1977) at early stages of phylogenesis distribution of chromosomes was carried out by means of a plasma membrane and a nuclear envelope and only later the special structure — the mitotic device was differentiated.

In an organism of metazoans of M. it is controlled by a nervous system, hormones of an adrenal gland, hypophysis, thyroid gland, gonads and chalones (see). Along with the general system of neurohumoral regulation processes of cell fission are controlled also by the local factors which are formed at functional activity of cells. Disturbance of normal interaction of various regulatory mechanisms provides as the general, and local changes of mitotic activity. In particular, in tumor cells of M. gets out of the control of neurohumoral regulation.

As expression of regulation of M. in connection with interaction of an organism and Wednesday serves the day-night (circadian) rhythm of proliferation of cells (mitotic activity). In most bodies at animals, active at night, M.'s maximum is noted in the morning, and at least — at night. At animals, active in the afternoon, and at the person the return parameters of a day-night rhythm are noted. At the same time within a day not only mitotic activity, but also many metabolic processes, functions and reactivity of cells fluctuates. The day-night rhythm of M. is a consequence of a chain of reaction, in to-ruyu rhythmic changes of the environment (illumination, temperature, a diet, etc.), a rhythm of functional activity of cells and change of a course of processes of metabolism are involved. Day-night rhythms differ in plasticity and stability. The last is connected with hereditary fixedness of circadian rhythms (see. Biological rhythms ).

Except M. and meiosis, also other ways of a reproduction of a cell — an endomitosis and a politeniya are known. At these types of a reproduction there is neither formation of new kernels, nor division of a cellular body. The endomitosis and a politeniya do not lead to increase in number of cells though owing to reduplication of chromosomes in both cases observe increase in cellular weight. At an endomitosis reduplication of chromosomes without destruction of a nuclear envelope is noted. In a kernel there is a spiralling of chromosomes (endopro-phase) and endometaphase, discrepancy of chromatids and their despiralization are consistently carried out. As a result chromosome number increases, there are polyploid cells and the mass of cytoplasm increases. Politeniya consists in increase in chromonemas in chromosomes that also leads to polyploidization and increase in nuclear mass and cytoplasm.

At various patol, processes the normal current of M. is broken. Allocate three main types of pathology of M.: 1) damages of chromosomes (their swelling, pasting, fragmentation, damages centromeres, lag of separate chromosomes at the movement, formation of microkernels, disturbances of spiralling and a despiralization, early separation of chromatids); 2) damages of the mitotic device (M.'s delay in metaphase, multipole, monocentric N asymmetric M., three-group and hollow metaphases). Special value in this group of pathology of M. has kolkhitsinopodobny M., or the K-mitosis, to-ry is caused by alkaloid colchicine, and also Colcemidum, vinblastine, Vincristinum, acenaphthene and other so-called statokinetic poisons stopping M. in metaphase (antimitotic agents, antitubulina). K-mitoses arise and is spontaneous in culture of fabric and in tumors. At a K-mitosis discrepancy of centrioles is broken, the spindle of division will be disorganized, there is no separation of chromatids, spiralling of chromosomes amplifies, and sometimes there is also their damage (swelling, fragmentation, a translocation, etc.); 3) disturbances of a tsitotomiya therefore arise two - or multinucleate cells — so-called simplasta, plasmodiums.

Pathological M. can be a consequence of influence of mitotic poisons, toxins, various extreme factors (ionizing radiation, an anoxia, a hypothermia), and also viral infections. Sharp increase in number of pathological M. is typical for a row patol, processes, in particular for a progression of malignant tumors (see. Progression of a tumor ).

See also Cell .

Bibliography: Alov I. A. Cytophysiology and pathology of a mitosis, M., 1972, bibliogr.; it, the Anafazny movement of chromosomes in a mitosis, Usp. sovr, biol., t. 83, century 2, page 213, 1977, bibliogr.; Albertson P. O. Division of cellular particles and macromolecules, the lane with English, M., 1974; Kiri of l-l about in O. I. Processes of cellular updating and growth in the conditions of a stress, M., 1977; bibliogr.; Laguchev S. S. Hormones and mitotic cycle of a cell, M., 1975, bibliogr.; Moesia D. Mitoz and physiology of cellular division, the lane with English, M., 1963, bibliogr.; Tsanev R. G. and Markov G. G. Biochemistry of cellular division, the lane with bolg., M., 1964, bibliogr.; Cande W. Z., Laz arides E. McIntosh J. K. A comparison of the distribution of actin and tubulin in the mammalian mitotic spindle as seen by indirect immunofluorescence, J. cell. Biol., v. 72, p. 552, 1977, bibliogr.; G r u n DM a n n E. Der mitotische Zellcyclus, Handb, allg. Path., hrsg. v. H. W. Altmann, Bd 2, T. 2, S. 282, B., 1971, Bibliogr.

I. A. Alov.