The Nature and Criteria of Senescence 



could be based more plausibly on the existence of expendable 

 enzyme systems renewable only by cell division, to explain the 

 ultimate death of some fixed postmitotic cells (Cowdry, 1952); 

 this concept will be discussed later on. In all organisms except 

 those which are capable of total regeneration, mechanical 

 injury of a more general kind must accumulate with time, but 

 this process will vary greatly in rate under different environ- 

 mental conditions. The constancy of the specific age in forms 

 which senesce is a strong argument against the primacy of 

 'mechanical' ageing. 



1-2-2 'ACCUMULATION' AND 'DEPLETION' 



In addition to a limited number of cases in which mechanical 

 wear normally, or potentially, terminates an animal life-cycle, 

 most of the other postulated 'causes' of senescence such as the 

 accumulation of metabolites (MetchnikofT, 1915; Jickeli, 1902) 

 and the exhaustion of stored irreplenishable reserves, do very 

 probably contribute to senescence in specific instances. The 

 very large literature of calcium and pigment accumulation in 

 the cells of higher animals (reviewed by Lansing, 1951) deals 

 with changes which are probably reversible consequences, 

 rather than primary causes, of an underlying senile process. 

 Lansing (1942) found, however, that reduction in the calcium 

 content of the medium greatly increased the life-span of 

 rotifers. A similar increase was produced by a single immersion 

 in weak citrate solution. Accumulation of calcium with age was 

 demonstrated in the same organisms by microincineration. 

 Similar processes are described in plants (Molisch, 1938; 

 Ahrens, 1938; Lansing, 1942). The 'life' of spermatozoa, though 

 by no means analogous, has been shown to be prolonged by 

 chelating agents which bind Cu ++ and Zn ++ (Tyler, 1953). In 

 the case of the rotifer, at least, the evidence for an accumulative 

 element in senescence is fairly strong. 



Depletion certainly terminates the life-cycle of some non-feed- 

 ing insect imagines, especially among Lepidoptera (Norris, 

 1934; Waloff, Norris and Broadhead, 1947), and possibly other 

 types of imago (Krumbiegel, 1929a,b). Many animals die or 

 become more vulnerable, as a result of the depletion or physio- 

 logical derangement caused by spawning (Orton, 1929). The 

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