The Biology of Senescence 



incidence of parental mortality in molluscs is reviewed by 

 Pelseneer (1935). Attempts to explain the human menopause 

 in terms of exhaustion of the supply of ova will be discussed 

 later. There is no evidence of a 'depletive' senescence in 

 mammals, unless the decline of growth rate be taken as 

 evidence of the exhaustion of some hypothetical substance. 



1-2-3 MORPHOGENETIC SENESCENCE 



The accumulation of injuries presents no biological problem 

 — it is readily observable in structures such as skin, and the 

 only serious difficulty lies in assessing its contribution to ageing 

 in particular structures or animal species. 



But in addition to the processes of mechanical or metabolic 

 senescence, and sometimes affecting the same organisms if they 

 are protected from these, it is necessary to postulate a further, 

 morphogenetic senescence to explain the sequence of events 

 observed in many organisms. This senescence has been con- 

 sidered to arise directly from the operation of the processes of 

 cell development which determine the shape and size char- 

 acteristic of the species and of its organs either through changes 

 in cell behaviour, or through the effects of divergent processes 

 of heterogony; it expresses itself as a decline in the capacity to 

 regenerate or maintain structures or conditions which, during 

 growth and a post-growing period of variable duration, are nor- 

 mally regenerated and maintained. Morphogenetic senescence 

 is a cumulative failure of homoeostasis, affecting the body as a 

 whole, to which coincident or dependent mechanical failure or 

 accumulative processes may contribute, but which appears to 

 be continuous with the processes which control cell-differentia- 

 tion and regulation. More accurately, it appears to represent 

 the withdrawal of coordination between these processes, so that 

 physiological homoeostasis 'falls apart'. It is this form of sen- 

 escence which characterizes higher vertebrates and is par- 

 ticularly well seen in man. The chief evidence that this, 

 morphogenetic, senescence is more that the 'sum of environ- 

 mental insult' which was formerly invoked to explain it, is the 

 existence in many organisms of specific age, analogous to 

 specific size and possibly related to it, which displays little en- 

 vironmental, but marked inter-race and interspecific, variation. 



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