Review Lectures on Senescence - I. The causes of ageing

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Abstract
Ageing processes are defined as those which increase the susceptibility of individuals as they grow older to the factors which may cause death. Various possible theories of ageing are considered, and evidence which may help to decide between them discussed. Changes in different organ systems may be merely symptoms of some single ageing process, or they may be largely independent, and synchronized by natural selection. Even if different organ systems age independently, they may do so as a result of similar changes at a cellular level. Cellular theories of ageing may have to take into account the effects of selection between the cells in a tissue. The effects of radiation, and somatic mutation theories of ageing, are discussed. It is suggested that radiation shortens life by inducing somatic mutations, but that normal ageing is not to any important extent caused by somatic mutations, although it may result from changes in cells which have effects on the physiology of the individual similar to those of somatic mutations. Evidence is presented that in Drosophila and in mice there are two phases in the life span. In Drosophila there is an initial 'ageing' phase which is irreversible and occurs at a rate approximately independent of temperature, and a second 'dying' phase, temperature-dependent in rate and reversible at lower temperatures.

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