Cells become senescent in response to a toxic environment, or during regeneration, or when damaged in ways that may increase cancer risk, but the vast majority are created when cells reach the end of their replicative life span, the Hayflick limit. Senescence is irreversible, and a senescent cell is blocked from further replication. In all these cases, near all newly senescent cells are soon destroyed, either by their own programmed cell death mechanisms, or by the immune system. A tiny fraction lingers, however. Senescent cells are very metabolically active, secreting a potent mix of molecules that disrupts tissue structure, produces chronic inflammation, and encourages nearby cells to also become senescent. This is just fine in the short-term context in which a cell becomes senescent: it assists regeneration, or helps protect against cancer, and so forth, and then it is gone when the senescent cells are destroyed. But when a small but growing number of senescent cells remain alive, and their secretions continue, day in and day out, their presence becomes very harmful. In fact, long-lasting senescent cells are one of the causes of aging and age-related disease.
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