Senescence

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Cellular senescence
(upper) Primary mouse embryonic fibroblast cells (MEFs) before senescence. Spindle-shaped. (lower) MEFs became senescent after passages. Cells grow larger, flatten shape and expressed senescence-associated β-galactosidase (SABG, blue areas), a marker of cellular senescence.

Biological aging or senescence (from the meaning “to grow old,” from “senex”) is the collection of accumulated changes to molecular and cellular structure of an adult organism that disrupt the organism's biology with the passage of time, resulting in deterioration and death. The science of senescence is biogerontology. Senescence occurs both on the level of the whole organism (organismal senescence) as well as on the level of its individual cells (cellular senescence). Albeit indirectly, senescence is by far the leading cause of death. Of the roughly 150,000 people who die each day across the globe, about two thirds—100,000 per day—die of age-related causes; in industrialized nations, moreover, the proportion is much higher, reaching 90%. Senescence is not the inevitable fate of all organisms, and some animal organisms even experience chronological decrease in mortality, for all or part of their life cycle. On the other extreme are accelerated aging diseases, rare in humans. There are a number of hypotheses as to why senescence occurs; for example, some posit it is programmed by gene expression changes, others that it is the cumulative damage caused by biological processes. Whether senescence as a biological process itself can be slowed down, halted or even reversed, is a subject of current scientific speculation and research.