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amitosis
Amitosis (''a-'' + ''mitosis'') is absence of mitosis, the usual form of cell division in the cells of eukaryotes. There are several senses in which eukaryotic cells can be amitotic. One refers to capability for non-mitotic division and the other refers to lack of capability for division.
In one sense of the word, which is now mostly obsolete, amitosis is cell division in eukaryotic cells that happens without the usual features of mitosis as seen on microscopy, namely, without nuclear envelope breakdown and without formation of mitotic spindle and condensed chromosomes as far as microscopy can detect. However, most examples of cell division formerly thought to belong to this supposedly "non-mitotic" class, such as the division of unicellular eukaryotes, are today recognized as belonging to a class of mitosis called closed mitosis. A spectrum of mitotic activity can be categorized as open, semi-closed, and closed mitosis, depending on the fate of the nuclear envelope. An exception is the division of ciliate macronucleus, which is not mitotic, and the reference to this process as amitosis may be the only legitimate use of the "non-mitotic division" sense of the term today.
In animals and plants which normally have open mitosis, the microscopic picture described in the 19th century as amitosis most likely corresponded to apoptosis, a process of programmed cell death associated with fragmentation of the nucleus and cytoplasm. Relatedly, even in the late 19th century cytologists mentioned that in larger life forms, amitosis is a "forerunner of degeneration".〔 〕
Another sense of ''amitotic'' refers to cells of certain tissues that are usually no longer capable of mitosis once the organism has matured into adulthood. In humans this is true of various muscle and nerve tissue types; if the existing ones are damaged, they cannot be replaced with new ones of equal capability. For example, cardiac muscle destroyed by heart attack and nerves destroyed by piercing trauma usually cannot regenerate. In contrast, skin cells are capable of mitosis throughout adulthood; old skin cells that die and slough off are replaced with new ones. Human liver tissue also has a sort of dormant regenerative ability; it is usually not needed or expressed but can be elicited if needed.
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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