Cellular senescence is one of the various processes that cause us to age. As you grow older, an increasing number of your cells enter into a state known as senescence. These senescent cells no longer divide or support the tissue they are part of, instead they spew out a cocktail of harmful chemical signals. This causes inflammation and drives the ageing process.
Senescence is a safety measure in-built in cells to prevent the propagation of damaged cells. This allows them to enter apoptosis (cell death) and have the immune system dispose them off. So generally speaking, it is a good thing as it keeps us safe from damaged cells causing cancer and promotes healthy tissue repair including wound healing.
However, there is a tipping point in this safety checkpoint. Clearing away unwanted senescent cells is the job of the immune system and replacing the lost cells requires stem cells to repopulate the tissue. As we age, the ability of the immune system to clear these cells begins to fail and they begin to accumulate.This in turn increases inflammation reducing the ability of stem cells to replace losses. Thus begins the downward spiral.
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