What is ageing?

It's both. Cells in the body are pre-programmed with a finite lifespan and a finite number of times that they can divide (split) to form daughter cells. For example, the red blood cells that carry oxygen to our tissues have a lifetime of 120 days after which they wear out and are broken down so that their constituents can be recycled. At the same time other cell types such as muscle and nerves can survive for much longer, even a lifetime, whilst the cells lining the mouth and intestines might last less than a day before being scraped off during a meal.

These cells are replaced by stem cells that divide (split) through the process of mitosis. However, scientists have found that every cell has a built-in limit on the number of times it can divide like this, known as the Hayflick number. This occurs because the ends of the DNA chromosomes contain structures called telomeres, which are similar in some ways to the piece of tape that protects the free ends of a shoelace. Whenever a cell divides a small amount of material is lost from each telomere. Eventually, after about thirty to forty cell divisions, the telomeres become too short and the cell can no longer divide. At this stage it's said to be senescent (aged). Stem cells can also be forced into early senescence if their DNA becomes damaged. Switching the cell off like this is a protective mechanism to guard against cancer, which can occur when genetic changes (mutations) inappropriately activate a cell's growth systems.

This DNA damage is usually caused by reactive chemicals called free radicals that are produced by metabolism, by toxins we take into the body in things like cigarette smoke and by exposure to sunlight. Thankfully, vitamins and other anti-oxidants help to protect cells from the effects of radicals, which is why eating a healthy diet rich in antioxidants can help to slow down the ageing process.

Nevertheless, as we age the number of stem cells capable of producing new cells to repair damage, replace lost cells and react to infections steadily declines. This means that fewer resources are devoted to repair and growth causing tissues to steadily deteriorate, and that's why skin goes wrinkly!