Ageing, it’s one of those things we’ve simply grown accustomed too: we’re born, we live, we get old and we cease to live. Ageing is a complex process that involves every cell and organ in the body and that leads to the deterioration of many body functions over the lifespan of an individual. With age, for example, the skin loses its elasticity and injuries heal more slowly than in childhood. The same holds true for bones, which turn brittle with age and take much longer to heal when fractured. Although the vulnerability to infectious disease and cancer is caused by a decline of the immune system, the latter is in turn a product of interactions among haematopoietic stem cells and the microenvironments in the bone marrow and the thymus, as well as in the mucous lining of the bronchus and gut systems. Hence, all ageing phenomena—tissue deterioration, cancer and propensity to infections—can be interpreted as signs of ageing at the level of somatic stem cells. As the regenerative prowess of a living organism is determined by the ability and potential of its stem cells to replace damaged tissue or worn-out cells, a living organism is therefore as old as its stem cells.
Mammals, and especially humans, have paid a high price for climbing up the evolutionary ladder: they have lost much of the regenerative power found in lower animals. Whereas humans have only limited potential to rejuvenate their ailing tissues, other organisms show amazing regenerative abilities. On decapitation, planaria will regenerate a new head within five days. Hydra, a small tubular freshwater animal that spends its life clinging to rocks, is able to produce two new organisms within 7–10 days when its body is halved. After losing a leg to a predator, salamanders recover with a new limb within a matter of days. Animals with staggering regenerative potential either have an abundance of stem cells or can de-differentiate specialized tissue cells into stem cells. It has been estimated that about 20% of a flatworm consists of stem cells, and hydra is a “kind of permanent embryo”. Salamanders use a completely different mechanism. When they are in urgent need of a new limb, they convert adult differentiated cells back to an embryonic undifferentiated state. These cells then migrate to the site of injury where they regenerate the missing part. Read the full article »»»»