Why do some cells stop dividing as you age?
Cells stop dividing because their chromosome tips wear down and they accumulate DNA damage: a protective mechanism that in later life becomes harmful precisely because the stalled cells build up. Treatments that address this are still under active investigation and are not ready for self-application.
Every time a cell divides, the protective tips of the chromosomes get a little shorter. Think of the plastic caps on a shoelace: once they come off, everything frays. When those tips become too short, the cell sends an internal stop signal and no longer divides. This is called replicative senescence, and it is one of the main reasons cells 'lock up' as you get older.
Alongside the wearing down of chromosome tips, there are two other triggers: DNA damage caused by radiation, toxic substances or errors in the copying process, and the activation of genes that would otherwise lead to cancer. The cell then quite literally chooses to freeze itself rather than keep dividing and risk becoming cancerous. Two molecular brake switches lock the division cycle in place and make that halt irreversible.
These frozen cells accumulate over the course of your life. They are not dead, but they do secrete a mixture of signalling molecules that can damage surrounding tissue and are thought to contribute to chronic age-related diseases, from joint inflammation to cardiovascular problems. It is therefore not the case that stopping division is purely harmful: in younger people, that very same block protects against cancer and assists with wound healing. The problem is the build-up in later life, when the body becomes less effective at clearing away these stalled cells.
Researchers are investigating whether drugs can selectively clear senescent cells or suppress their harmful secretions. Animal experiments are promising, but in humans this research is still in its early stages. A small study using hyperbaric oxygen therapy (60 sessions, 35 participants, no control group) found an increase in telomere length of more than 20% and a reduction in the number of senescent immune cells of an average of 37%. Caution is warranted here: the group was small, there was no comparison group, and several authors have financial ties to a company that commercially offers this treatment.
This answer draws on several broad review studies on cellular senescence, which describe the mechanism as well established. The clinical indications regarding treatments (hyperbaric oxygen, senotherapies) come from a single small human study and predominantly animal research; that evidence is preliminary and thin. The review studies describe the molecular pathway as causal, but the link between the accumulation of senescent cells and specific age-related diseases in humans is probable but not yet definitively proven.
Why do damaged proteins accumulate in your cells as you age?
As you age, several systems that clear away damaged proteins weaken simultaneously and amplify each other's failure. This is one of the central mechanisms behind age-related diseases, but how to stop it in a targeted way is still actively being researched.
Do my stem cells run out as I get older?
Your stem cells do indeed decline with age: they repair tissue less effectively and become partly exhausted. Regular exercise, good sleep and not overeating are the best-supported ways to slow this process.
Why do cells lose their ability to repair themselves over time?
Cells lose their repair capacity through a combination of declining energy stores, impaired DNA repair and the accumulation of 'burned-out' cells that secrete inflammatory substances. These are well-supported mechanisms, although it is still too early to act on them through supplements or therapies.
What exactly does senescence do to your cells as you age?
Senescent cells stop dividing, accumulate with age and cause damage throughout the body through a constant stream of inflammatory substances. They are both a protector (a brake on cancer) and a cause of damage, and that makes targeted intervention more complex than it appears.
What do zombie cells do to your ageing?
Zombie cells accumulate as you age and accelerate ageing through inflammatory signals; how to tackle them is promising but still very much under investigation.
Does caloric restriction actually make your cells younger?
Caloric restriction slows ageing processes in cells, but whether your cells actually become younger as a result has not yet been proven in humans. The metabolic benefits, such as improved insulin sensitivity, have been demonstrated in human studies.