The Biology of Senescence 



(5) What are the limits of the power of enzyme-renewal and 

 physicochemical self-maintenance in fixed post- mitotic 

 cells? 



(6) In the case of single organs such as the mammalian ovary, 

 how far is the expectation of life of the organ intrinsic in 

 its stage of development, and what is its life-span in pas- 

 sage through successive young hosts? What, in more 

 general terms, is the relation between the physiological 

 age of individual tissues and the chronological age of the 

 host animal? 



I have singled out these specific questions for attention as 

 being rather unlikely to be solved incidentally, in the course of 

 general biological research upon other topics. The great bulk 

 of the information which is missing on other specific points is 

 likely to be derived eventually from studies in endocrinology 

 or morphogenetics which are not undertaken ad hoc; this type 

 of background research cannot be hurried on, ahead of the 

 general progress of knowledge, except by the cultivation of 

 interest in ageing among biologists of all kinds. 



Three main types of research are involved in the investiga- 

 tion of our six preliminary questions — study of the phylogeny 

 of senescence in vertebrates, study of the correlations and the 

 experimental modification of growth and development in popu- 

 lations where the life-span can be concurrently measured, and 

 study of tissue-environment relationships through the creation 

 of age chimaeras. The problems of the first of these studies have 

 already been mentioned. A reliable test of 'senescence' which 

 correlates with the decline of resistance, does not kill the indi- 

 vidual animal, and can be related to actuarial senescence by an 

 intelligible process of reasoning, might offer some solution. The 

 development of such a test would probably depend, however, 

 upon the establishment of the part which declining growth- 

 energy plays in the process of ageing. The time-lag in explant 

 growth might conceivably give a basis for some such attempt. 

 Any method of marking tissue cells in situ, to enable their life- 

 span to be determined like that of red blood corpuscles, would 

 be a highly desirable advance, and a key to many doors. The 

 study of growth and development relations, and the whole 

 group of studies which require to be undertaken in determining 



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