The Biology of Senescence 



precocious puberty' — Novak, 1944) and pregnancy has actually 

 been reported in a child of five (Escomel, 1939). In albino rats, 

 according to Mandl and Zuckerman (1952) genetic factors 

 seem to play the major part in determining the age of puberty. 

 Lorenz and Lerner (1946) likewise found clear evidence that 

 age of sexual maturation in turkeys is heritable. 'The reactivity 

 of the gonads may be the most important factor in determining 

 the time at which sexual maturity actually occurs, but the 

 factors which affect this reactivity are largely unknown' (Rob- 

 son, 1947). Change also takes place at puberty in the specificity 

 of pituitary response — while oestradiol induces pituitary and 

 adrenal hypertrophy in rats castrated after puberty, it reduces 

 pituitary and adrenal weight in animals castrated while still 

 immature (Selye and Albert, 1942). 



The end of the reproductive period, as well as the beginning, 

 is marked by changes in gonadal reactivity. These have led 

 some writers to regard the human menopause as a form of 

 depletion-senescence (Swyer, 1954): Hertig (1944) describes the 

 exhaustion of a 'capital' of ova, which is not increased during 

 post-natal life, but his findings suggest that the actual meno- 

 pause precedes the end of all follicular activity. In man and 

 many other mammals (the only admitted exceptions occur in 

 Lemuroidea, the galago and the loris), the occurrence of oogen- 

 esis during post-pubertal life has been doubted — the case 

 against it has been persuasively put by Zuckerman (1951): 

 this 'perennial controversy' has been continued by Parkes and 

 Smith (1953), who found evidence of oocyte regeneration in 

 rat ovaries grafted after freezing. But it is in any case most 

 unlikely on existing evidence that the menopause occurs because 

 the supply of ova is exhausted. Engle (1944) mentions the find- 

 ing of apparently normal corpora lutea in women of 50: at the 

 menopause most if not all ova and follicles have normally dis- 

 appeared. Kurzrok and Smith (1938) found that in the human 

 ovary, in contrast to the senile ovary of some other mammals, 

 ova cease to be found, and that this change occurs at or soon 

 after the menopause. They consider that the postmenopausal 

 ovary can no longer respond to pituitary gonadotrophin. 

 Gardner (1952) transplanted ovaries between old and young 

 rats, and apparently found a greater tendency to malignant 



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