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 (Clark, 1999). 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. Lung tissue also loses its elasticity and the muscles of the rib cage shrink. Blood vessels accumulate fatty deposits and become less flexible, which results in arteriosclerosis. The reduced capacity to regenerate injured tissues or organs and an increased propensity to infections and cancers are probably the most prominent hallmarks of senescence (Hayflick, 1994). 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.
Stem cells and ageing
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