A state in which cells permanently stop dividing, often triggered by aging or stress.Cellular senescence is a state in which a cell stops dividing but does not die. It remains alive and metabolically active, sitting in the tissue like a retired worker who still occupies a desk. Senescence can be triggered by many things: the gradual shortening of chromosome ends after many divisions, damage to DNA from radiation or chemicals, oxidative stress, or the activation of certain cancer-causing genes. Whatever the trigger, the cell commits to a permanent exit from the cell cycle, and no amount of growth factor will coax it back.
This matters because senescence is a double-edged sword. On one hand it is a tumour-suppressing mechanism: a cell that might have accumulated dangerous mutations is taken out of circulation before it can divide into a cancer. On the other hand, senescent cells accumulate with age and secrete a mixture of inflammatory signals, growth factors and enzymes that degrade surrounding tissue. This so-called senescence-associated secretory phenotype can drive chronic low-grade inflammation, contribute to fibrosis, and even encourage neighbouring cells to become cancerous. Senescence is therefore one of the central mechanisms linking ageing to disease.
Senescent cells also explain part of why tissues lose their regenerative capacity. Stem cell pools become depleted or dysfunctional, and organs that once repaired themselves quickly do so slowly or not at all. Because the cells can be identified by specific markers, they have become a target for drugs designed to kill them selectively.
The phenomenon was first described by Leonard Hayflick and Paul Moorhead in 1961, who showed that normal human fibroblasts in culture divide only a limited number of times before stopping — the Hayflick limit. The term 'cellular senescence' was coined by Hayflick soon after.
The permanent exit from the cell cycle is enforced by two main molecular brakes. The p53 pathway responds to DNA damage and induces the cyclin-dependent kinase inhibitor p21, while the p16-Rb pathway provides a more durable block. Once Rb is kept in its active, hypophosphorylated form, the transcription factors needed for DNA replication cannot function, and the cell cannot enter S phase.
At the same time, senescent cells change their behaviour. They enlarge, flatten, and often accumulate a distinct form of chromatin called senescence-associated heterochromatin foci. They also switch on a secretory programme, releasing interleukins, chemokines and matrix metalloproteinases. This secretion is not random: it is driven by transcription factors such as NF-kB, and it allows the senescent cell to communicate with the immune system, which in young organisms clears it away. With age, that clearance becomes less efficient, and the cells persist.
Senescent cells are the target of a class of experimental drugs called senolytics, which selectively kill them, and are being investigated in clinical trials for osteoarthritis, idiopathic pulmonary fibrosis and age-related frailty.
Senescent cells are not simply inert: they can be up to twice the size of normal cells and often contain more than one nucleus, yet they remain alive and metabolically active for years.