The Biology of Senescence 



There is no evidence that this staggering increase, maintained 

 over 40-50 years, has any observable effect on the rate of ageing. 



6-2-3 HORMONAL REGULATION OF GROWTH IN VERTEBRATES 



In mammals, where growth and differentiation are difficult 

 to dissociate experimentally, we have abundant evidence of 

 senescence even in the longest-lived forms. In amphibia, where 

 there is a clear-cut metamorphosis, and where growth and 

 differentiation can be manipulated with relative ease, we have 

 so far no evidence of senescence. We cannot directly find out 

 whether the life of intact amphibia, the neoteny of the axolotl, 

 or the gigantism of athyroid tadpoles, ends in senescence, for 

 the practical reason that axolotls may well be capable of living 

 for 50, and normal frogs for twelve, fifteen or twenty years. 

 This conspiracy of circumstances perpetually recurs in the study 

 of ageing. The large literature of lower vertebrate endocrin- 

 ology and morphogenesis cannot be brought to bear on the 

 problem, for lack of actuarial data. 



Both homoeotherms and poikilotherms, whether they meta- 

 morphose or not, tend to pass through an earlier phase of active 

 growth and a later phase of active reproduction, each char- 

 acterized by a separate type of endocrine control, and the 

 second by a relative loss of regenerative in favour of reproduc- 

 tive capacity. These phases are separated by the operation of a 

 timing-mechanism which is linked to processes in the juvenile 

 phase. In mammals, these phases are apparently controlled by 

 the pituitary growth and gonad-regulating mechanisms suc- 

 cessively. In lower vertebrates the differentiation-process and 

 the transition to adult function appears to depend on a pituitary- 

 thyroid balance. Pituitary growth hormone of mammalian 

 origin is able to promote the growth offish (Swift, 1954). 



The relation between morphogenesis under the influence of 

 gonadal hormones and loss of regenerative power has special 

 interest in gerontology. Grobstein (1947) found that when the 

 gonopodium of poeciliid fish differentiates, under the influence 

 of androgens, regenerative power is lost: he stresses the analogy 

 between this process and the loss of regenerative capacity in 

 the developing anuran limb. Such a change need not depend 

 upon irreversible loss of cellular capacity to grow — this does 



182 



