Why do some cells stop dividing as you age?

Every time a cell divides, the protective tips of the chromosomes become a little shorter. Think of them like the plastic caps on shoelaces: without those caps, the ends fray. After enough divisions, those tips are so short that the cell triggers its alarm system and stops multiplying. Besides this wear and tear, DNA damage and the activation of genes that can cause cancer can give the same stop signal. The process is called cellular senescence.

That stopping is not a flaw in the body. It is a protective mechanism with two purposes. First, it prevents damaged cells from continuing to copy themselves and potentially becoming cancer cells. Senescence acts as an emergency brake on cancer development. Second, senescent cells temporarily play a useful role in tissue repair and in embryonic development.

The problem arises when those stopped cells accumulate over the years. They do not disappear on their own, but secrete a mix of pro-inflammatory substances that damage surrounding healthy cells. This contributes to chronic low-grade inflammation and to age-related diseases such as atherosclerosis and osteoarthritis. In the brain, similar stopped cells accumulate during normal ageing and in neurodegenerative diseases, although it is not yet clear whether they are the cause or the consequence.

There is extensive research into whether drugs can address this accumulation. Senolytics selectively clear senescent cells; another category only suppresses their harmful secretions without removing the cells. In animal research the results are promising. In humans, however, these agents are still in early testing phases and their safety and effectiveness have not been established. A real risk is that senescent cells also act as a brake on cancer development: clearing them indiscriminately could undermine that protective effect.