The Biology of Senescence 



interpreted immunologically, or, more probably, morphogenet- 

 ically, cannot be ruled out. If cells in general or certain cells 

 could be shown to acquire adaptive resistance to physiological 

 regulators, as do bacteria to unfamiliar metabolites, from gener- 

 ation to generation, interesting possibilities would certainly be 

 opened. 



Connection between the processes of senescence and those of 

 immunology was suggested long since by Metchnikoff, though 

 in rather a different context (1907). He attributed the senile 

 atrophy of differentiated tissues, especially neurones, to over- 

 active phagocytosis, brought on by constant exposure to the 

 toxic (and antigenic) products of symbiotic and endoparasitic 

 bacteria: the second part of this theory has received more 

 publicity than the first. Metchnikoff was also responsible for the 

 suggestion that many morphogenetic processes in the embryo 

 are 'immunological' in character, within the wide definition of 

 immunology implied in his theory of phagocytosis, and that the 

 defence mechanisms of the adult animal are directly derived 

 from mechanisms which, in embryonic life, have been primarily 

 concerned with morphogenesis. The embryo is not exposed as 

 a rule to exogenous antigens, though its own chemical com- 

 position is changing. More recent work, in fact, has shown that 

 exposure to an antigen during embryonic life can lead to a 

 lifelong inability to react against that antigen (Billingham, 

 Brent and Medawar, 1953). The possibility that morphogenetic 

 mechanisms give rise in ontogeny to the defence mechanisms of 

 the adult, which is evidently true in the case of some mech- 

 anisms of chemical homoeostasis, is still popular in Russian 

 research. Whether such effects modify cell responses during the 

 later phase of development, senescence, is unknown. 



Gaillard (1942) has carried out a long series of studies relating 

 the degree of differentiation which can be produced and main- 

 tained in tissue explants to the age-status of the press juice in 

 which they are grown. According to these results, functional 

 differentiation in endocrine and other explants can be obtained 

 if they are grown successively in press juices from embryos of 

 increasing age, while some degree of regression of structure 

 occurs if the series is reversed, and explants are grown in juices 

 of decreasing age-order. This process has not so far been followed 



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