The Biology of Senescence 



Lowry and Hastings (1952) find that increased hydration, due 

 perhaps to extracellular oedema, loss of cells, and even gross 

 pathological causes such as heart failure, is the most consistent 

 finding in senile mammalian tissue. There is at present no bio- 

 chemical sign characteristic of 'oldness' in tissues or in cells, 

 and the search for one may well reflect a fundamental mis- 

 conception. It is in assessing the relevance of all such criteria to 

 the main phenomenon of senescence, the decline in resistance to 

 random stresses, that the statistical approach is essential. All 

 assertions about senescence based upon pathological anatomy, 

 or upon general theories which treat it as a single process, are 

 open to question. 



The decline of growth rate throughout life in some or all tissues 

 appears to be a near-universal feature of metazoa. 



'The specific growth rate always falls: living tissue progres- 

 y sively loses the power to reproduce itself at the rate at which it 

 was formed. Minot arrived at this generalization, which should 

 rightly be known as "Minot's Law", from the collation of his 

 percentage growth-rate curves; and it was he who first recog- 

 nized that the point of inflection of the integral curve of growth, 

 and the division it makes between a period of positive and 

 negative acceleration, is not of critical importance. The pro- 

 gressive dissipation of "growth energy" which this first law 

 affirms was thought by Minot to be an expression of the 

 phenomenon of senescence — "ageing" with its everyday impli- 

 cations. Senescence is not, in this view, a process which sets in 

 after a preliminary period of maturation has run its course: 

 senescence is development, looked at from the other end of life' 

 (Medawar, 1945b). 



The use of this criterion, which is a readily measurable one, 

 and can be applied to smallish groups of animals with suitable 

 precautions, as well as to single tissues or organs, implies the 

 acceptance of Minot's definition of senescence. The definition 

 is defensible. On the other hand, one of the important points at 

 issue at the present moment is precisely whether all animals 

 which show a decline in the specific growth rate — all verte- 

 brates, and almost all invertebrates — display pari passu an ulti- 

 mate increase in mortality either as a result of this process, or 



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