What is the difference between cell division and cellular ageing?
Cell division and cellular ageing are two sides of the same biological story: every division costs a small piece of telomere, and once those are used up, the cell stops dividing permanently. That distinction is well established and forms the basis of much research into ageing.
Cell division is the process by which a cell copies itself, producing two daughter cells. Normal human cells can do this only a limited number of times, after which they stop permanently. That limit is called the Hayflick limit.
The main cause of that limit is telomere shortening. Telomeres are protective caps at the beginning and end of each chromosome, comparable to the plastic tips on a shoelace. With every division they become a little shorter. Once they reach a critical length, the cell stops dividing. This is no coincidence but a causal relationship: it has been shown that cells artificially given the enzyme telomerase, which can replenish their telomeres, continued to divide healthily for at least twenty times beyond their normal limit, without chromosomal damage.
Cellular ageing, or senescence, is the state that follows. The cell is still alive and active, but no longer divides. That is the fundamental difference: cell division is an active, repeating process; senescence is a permanent brake on that process. Senescent cells accumulate in tissues as we grow older, as has been found in the skin of older people.
Senescence can also set in earlier than the telomere limit requires. Substantial oxidative damage, for example from hydrogen peroxide, drives cells along the same pathway into senescence, even without the telomeres being fully used up. The same applies to severe DNA damage caused by radiation or chemotherapy: a cell then opts for permanent arrest as an alternative to eliminating itself. The distinction from the latter is important: a senescent cell remains present in the tissue and can influence surrounding cells.
For a long time senescence was regarded as a fixed, irreversible state. More recent research adds nuance to this: senescence proceeds more as a gradual process than as a sharp on/off switch. Under certain circumstances cells can start dividing again, but they are fundamentally changed when they do. Whether that turns out to be beneficial or harmful remains unclear.
Based on reviews, mechanistic studies and studies in humans (cell culture and skin tissue). No randomised trials; the relationships described are partly demonstrated as causal in cell culture experiments, and partly associations found in humans through tissue research.
How does a cell know when to stop dividing?
Cells stop dividing through built-in emergency brakes that respond to damage, stress, or viruses. This system is protective in the short term, but when chronically activated it can cause damage itself.
Why do some cells stop dividing as you age?
Cells stop dividing as a protection against damage and cancer, but the accumulation of such stopped cells contributes over the years to inflammation and age-related diseases. Drugs that target this are promising in animal research, but are not yet ready for use outside clinical studies.
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.
Can meditation do anything about ageing at the cellular level?
Meditation appears to activate an enzyme that protects chromosome caps from ageing, although the studies are still too small to draw firm conclusions. Meditation is in any case safe and has positive effects beyond cell biology, so the barrier to trying it is low.
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 are the hallmarks of aging, exactly?
Aging follows nine recurring biological patterns, from telomere shortening to disrupted cell communication, which together drive the major age-related diseases. Which hallmark has the greatest influence and how to intervene in it effectively remains unresolved.