The Biology of Senescence 



1-2-1 MECHANICAL SENESCENCE 



A few precise analogies to the failure of a non-replaceable 

 part in a mechanical system are known to occur in organisms. 

 Deterioration of the waxy epicuticle in insect imagines and of 

 the teeth in the African elephant (Perry, 1953), the mongoose 

 (Pearson and Baldwin, 1953), the shrew (Pearson, 1945; Pruitt, 

 1954) and some large carnivores are examples of strictly 

 mechanical senescence. Such changes would ultimately kill the 

 animal. Similar, though less obvious, mechanical changes may 

 contribute to senescence in other forms. It is probable that the 

 gradual loss of nephra in the mammalian kidney is an example 

 of the incidental loss of essential structures, but one which 

 rarely reaches the point of causing death per se. On the other 

 hand, the differences between an old cart and an old horse are 

 sufficiently striking to make the extensive acceptance of 'wear* 

 as an explanation of senescence, and the resort to mechanical 

 analogies based on the 'spontaneous slow decomposition' of 

 explosives (Lepeschkin, 1931) or the behaviour of inanimate 

 colloids (Ruzicka, 1924; Dhar, 1932) largely irrelevant. 'The 

 old organism does not contain old colloids, it contains newly- 

 formed colloids of an old character' (Lansing) . The mean half- 

 life of human protein is 80 days, of liver and serum proteins 

 10 days, and that of the carcase proteins 158 days (Bender, 

 1953). The continuation of high protein turnover in adult life 

 has been demonstrated by isotope studies (Shemin and Ritten- 

 berg, 1944) though the turnover of materials such as collagen 

 decreases almost to zero with increasing age (Perrone and Slack, 

 1952; Neuberger and Slack, 1953). The weakness of the 'col- 

 loidal' concept of ageing had been pointed out even before the 

 discovery of colloids: 'Quoniam vero duplex est duratio cor- 

 porum: altera in identitate simplici, altera per reparationem: 

 quarum prima in inanimatis tantum obtinet, secunda in vege- 

 tabilibus et animalibus; et perficitur per alimentationem' {Hist. 

 Vitae et Mortis). 1 



A chemical extension of the idea of 'mechanical' senescence 



1 Since there are in fact two ways in which bodies maintain their identity, 

 the first, which applies only to inanimate objects, is simply by remaining 

 the same. The second which applies to plants and animals, is by renewing 

 themselves; and they do this by means of nourishment. 



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