Cellular senescence, explained in plain English

Some cells stop dividing yet refuse to die. They linger in our tissues, quietly altering the environment around them, and they accumulate as we grow older. This state is called cellular senescence, and over the past decade it has moved from a laboratory curiosity to one of the most studied processes in the biology of ageing. Here is what it is, why it matters, and what the evidence on clearing these cells actually shows.

What cellular senescence actually is

Cellular senescence describes a cell that has entered a stable, usually permanent, halt to division while remaining alive and metabolically active. It was first described in cultured human cells in 1961, when researchers noticed that normal cells could divide only a finite number of times before stopping. A senescent cell is neither dead nor dormant. It stays in the tissue, continues to consume resources, and behaves quite differently from its dividing neighbours. A 2021 review in Nature Reviews Molecular Cell Biology describes this durable arrest as the defining feature of the senescent state.

What pushes a cell into senescence

A cell can be driven into senescence by several kinds of stress. The best understood triggers are the shortening or dysfunction of telomeres, the protective caps on our chromosomes, persistent damage to DNA, the activation of cancer-promoting genes, and sustained oxidative stress. Many of these triggers share a theme. They signal that a cell has accumulated damage it cannot safely repair. Seen this way, senescence is partly a safeguard. By arresting a damaged cell, the body stops it from dividing and potentially turning cancerous, which is why senescence is often described as having evolved, in part, as a brake on tumour formation.

The problem is not the arrest, it is the secretion

If senescent cells simply sat quietly, they would matter far less. The complication is that many of them develop what researchers call the senescence-associated secretory phenotype, usually shortened to SASP. A cell with an active SASP secretes a mixture of signalling molecules, including pro-inflammatory cytokines, chemokines and tissue-remodelling enzymes. These signals do not stay local. They can promote chronic, low-grade inflammation, influence the behaviour of nearby healthy cells, and interfere with the normal repair and regeneration of tissue. The gradual build-up of senescent cells and their secretions is one proposed contributor to the slow decline in tissue function that accompanies ageing.

A double-edged process

It would be a mistake to read senescence as simply harmful. The same programme plays constructive roles. It contributes to wound healing, participates in normal embryonic development, and, as noted, helps suppress the growth of damaged cells. A 2024 article in Science emphasised that senescent cells have genuine physiological functions and are not merely a feature of decline. The current picture is of a process that is protective in the short term and in youth, but potentially troublesome when senescent cells accumulate and persist across a lifetime. Part of the reason they persist is that they switch on survival pathways, sometimes called senescent cell anti-apoptotic pathways, that make them unusually resistant to dying.

Clearing senescent cells: the research frontier

Because senescent cells resist death and drive inflammation, a natural question follows. What happens if you remove them, or quieten their secretions? Two broad strategies are under study. Senolytics are compounds intended to selectively kill senescent cells. Senomorphics aim instead to suppress the harmful SASP without killing the cell. The first senolytic combination to attract wide attention paired the cancer drug dasatinib with the plant compound quercetin, and such approaches have since moved into early clinical trials. In animal models, clearing senescent cells has improved a range of age-related measures, which is what has driven the intense interest.

What the evidence does and does not show

The mechanistic and animal evidence for senescence as a driver of ageing is substantial. The human evidence is younger and more cautious. Senescent cells are strikingly heterogeneous, which makes them hard to identify reliably and complicates any attempt to target them precisely. Their useful physiological roles also mean that blunt, body-wide clearance is not obviously desirable. The honest summary is that senescence is one of the most promising areas in the biology of ageing, that early human trials are under way, and that it is too soon to treat any specific intervention as established. This is a field to watch closely rather than a settled story.

Further reading

Continue reading from the journal: The hallmarks of cellular ageing, in plain English and Autophagy, explained without the jargon.

Sources

• Di Micco R, Krizhanovsky V, Baker D, d’Adda di Fagagna F. Cellular senescence in ageing: from mechanisms to therapeutic opportunities. Nature Reviews Molecular Cell Biology, 2021. doi:10.1038/s41580-020-00314-w
• Zhang L, Pitcher LE, Niedernhofer LJ, Robbins PD. Targeting cellular senescence with senotherapeutics: senolytics and senomorphics. The FEBS Journal, 2022. doi:10.1111/febs.16350
• de Magalhães JP. Cellular senescence in normal physiology. Science, 2024. doi:10.1126/science.adj7050
• Lucas V, Cavadas C, Aveleira CA. Cellular Senescence: From Mechanisms to Current Biomarkers and Senotherapies. Pharmacological Reviews, 2023. doi:10.1124/pharmrev.122.000622
• Palmer AK, Gustafson B, Kirkland JL, Smith U. Cellular senescence: at the nexus between ageing and diabetes. Diabetologia, 2019. doi:10.1007/s00125-019-4934-x