The Mechanisms of Senescence 



not appear to be the case in amphibian limbs (Borssuk, 1935; 

 Polezaiev and Ginsburg, 1939) but the physiological loss of 

 repair-power may be as complete, so far as the intact animal 

 is concerned, as is the loss of moulting-power in Rhodnius once 

 the evocator is lost. There is clearly here, as Minot recognized, 

 a possible mechanism for the induction of senile change. 



A certain amount of evidence is available concerning the 

 hormonal influences which affect protein anabolism, and regu- 

 late growth in mammals, especially in man. Where these factors 

 have been studied, they give little support to the idea of a 

 simple relation between senescence and growth-cessation, and 

 even less to the conception of a single, 'master', endocrine 

 inhibitor which can be detached from the general pattern of 

 progressing developmental change. The pattern which exists in 

 man has all the complication of a dynamic system where homo- 

 eostasis coexists with change. Much of the existing information 

 is provisional, and there are as yet no studies extending into 

 the period of senescence. It is plain, however, that in man, 

 and probably in some but not all other mammals, the 'anabolic' 

 stimulus to form new protein is not the same throughout life. 

 In adult life it is closely linked to the gonadal cycle. The 

 extent of the differences in endocrine control of growth between 

 determinate growers such as man and continual growers such 

 as the rat has not yet been mapped, and very little is known of 

 the hormonal control of growth in lower vertebrates. The exist- 

 ing evidence is quite enough, however, to render any static 

 conception of growth-cessation in terms of single-hormone 

 deficiencies untenable. A more accurate picture would perhaps 

 be obtained by treating pre-pubertal and pubertal life as separ- 

 ate instars separated by what amounts to a biochemical meta- 

 morphosis. 



The growth of human beings, like that of Daphnia (p. 93), 

 occurs in two overlapping cycles — one prepuberal, the other 

 coinciding with puberty. (See Figs. 42, 43.) The prepuberal 

 cycle has its most active phase during the first six months of 

 life. This cycle, according to Kinsell (1953), is almost wholly 

 controlled by the pituitary growth hormone. The puberal cycle 

 appears to be evoked directly by anabolic steroid hormones 

 derived from the gonad and adrenal cortex. During both cycles 



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