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Negligible senescence is the lack of symptoms of ageing in a few select organisms. Negligibly senescent organisms do not have measurable reductions in their reproductive capability with age, or measurable functional decline with age. Death rates in negligibly senescent organisms do not increase with age as they do in senescent organisms. Negligibly senescent organisms have no "post-mitotic" cells; they reduce the effect of damaging free radicals by cell division and dilution. Another related mechanism is that of planarian flatworms, which have “apparently limitless telomere regenerative capacity fueled by a population of highly proliferative adult stem cells.” There are many examples of species for whose organisms scientists have not detected an increase in mortality rate after maturity. In other words, they are equally likely to die at any given age after maturity; or, alternatively, it could be that the mean lifespan of the organisms is so long—multiple millennia—that researchers' subjects have not yet had the chance to live up to the time when a measure of the species' longevity can be made. Study of negligibly senescent animals may provide clues that lead to better understanding of the ageing process and influence theories of ageing. The phenomenon of negligible senescence in some animals is a traditional argument for attempting to achieve similar negligible senescence in humans by technological means. There are also organisms that exhibit negative senescence, whereby mortality chronologically decreases as the organism ages, for all or part of the life cycle, in disagreement with the Gompertz–Makeham law of mortality (see also Late-life mortality deceleration). Furthermore, there are even more peculiar examples, those of species that have been observed to regress to a larval state and regrow into adults multiple times; e.g., ''Turritopsis dohrnii''.〔(【引用サイトリンク】title=Cheating Death: The Immortal Life Cycle of ''Turritopsis'' )〕 ==In vertebrates== Some fish, such as some varieties of sturgeon and rougheye rockfish, and some tortoises and turtles are thought to be negligibly senescent. The age of a captured fish specimen can be measured by examining growth patterns similar to tree rings on the otoliths (parts of motion-sensing organs). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Negligible senescence」の詳細全文を読む スポンサード リンク
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