amitosis(redirected from direct nuclear division)
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, process of nuclear division in a living cell by which the carriers of hereditary information, or the chromosomes, are exactly replicated and the two copies distributed to identical daughter nuclei.
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(direct nuclear division), one of the methods of nuclear division in protozoans and in plant and animal cells. Amitosis was first described by the German biologist R. Remak in 1841. The term was proposed by the his-tologist W. Flemming in 1882. With amitosis, in contrast to mitosis, or indirect nuclear division, the nuclear membrane and nucleoli are not destroyed, a cleavage spindle is not formed in the nucleus, and the chromosomes remain in a functioning (despiralized) state. Either the nucleus undergoes segmentation, or a septum appears in the externally unchanged nucleus. The division of the cell body—cytoto-my—as a rule, does not occur. Ordinarily, amitosis does not provide even division of the nucleus and its individual components.
The study of amitosis is complicated by the unreliability of determining it from morphologic features, since not every constriction of the nucleus means amitosis. Even expressed “dumbbell-like” constrictions of the nucleus can be transitory. The nuclear constrictions can also be the result of incorrect preceding mitosis (pseudoamitosis). Ordinarily, amitosis follows endomitosis. In most cases, with amitosis, only the nucleus divides and a binucleate cell occurs. With repeated amitoses, polynucleate cells can form. Many of the binucleate and polynucleate cells are the result of amitosis; a certain number of binucleate cells are formed with the mitotic division of the nucleus without the division of the cell body. They contain (totally) the polyploid chromosome sets.
Mammals are known to have tissues with both uninucleate and binucleate polyploid cells (cells of the liver, pancreas, salivary glands, nervous system, the bladder epithelium, and the epidermis), as well as tissues with only binucleate polyploid cells (cells from the mesothelium and connective tissue). The binucleate and polynucleate cells differ from the uninucleate diploid ones in their greater dimensions, more intensive synthetic activity, and the increased quantity of various structural formations, including chromosomes. The binucleate and polynucleate cells differ from the uninucleate polyploid ones mainly in the greater nucleus surface. This is the basis for the concept of amitosis as a method of normalizing the nucleus-plasma relationships in polyploid cells by increasing the ratio of the nucleus surface to its volume. During amitosis, the cell maintains the inherent functional activity, which disappears almost completely with mitosis. In many instances, amitosis and binuclearity accompany the compensatory processes occurring in the tissues—for example, with functional overloads or starvation and after poisoning or denervation. Usually, amitosis is observed in tissues with reduced mitotic activity. This, evidently, explains the increase in the number of binucleate cells formed by amitosis which accompanies aging of the organism. The notions of amitosis as a form of cell degeneration are not substantiated by modern research. Also invalid is the view of amitosis as a form of cell division. There are only isolated observations of the amitotic division of an entire cell body but not of its nucleus alone. It is more correct to view amitosis as an intracellular regulatory reaction.
REFERENCEWilson, E. B. Kletka i ee rol’ v razvitii i nasledstvennosti, vols. 1–2. Moscow-Leningrad, 1936–40. (Translated from English.)
Baron, M. A. Reaktivnye struktury vnutrennikh obolochek. [Moscow,] 1949.
Brodskii, V. Ia. Trofika kletki. Moscow, 1966.
Bucher, O. Die Amitose der tierischen und menschlichen Zelle. Vienna, 1959.
V. IA. BRODSKII