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The cellular basis of aging.

Biological aging involves physiological decline and the emergence of pathology that are increasingly understandable in cellular and molecular terms. Studies on human cells grown in vitro support the concept of a replicative `clock' that counts the number of cell divisions to a maximum limit. Under normal conditions, there is excess capacity for cell division but accelerated turnover following chronic trauma may lead to replicative exhaustion, first focally and then more generally. DNA undergoes rearrangements that are evident in older cells in vitro and in vivo. This may contribute to the loss of replicative capacity which simultaneously leads to physiological decline and the rising likelihood of malignancy and other age-dependent disorders. Application of such research data to medical practice should help to promote vigorous longevity and improve the quality of life in elderly people.