NON-NEOPLASTIC LATE EFFECTS 181 



tiatioii to a point where the interdependence of tissue and cells is 

 incompatible with the indefinite life of the total organism. The deaths of 

 individuals covdd, however, contribute to the survival of the species by 

 insuring a progression of generations and reducing competition for the 

 food supply between young and old, Dobzhansky (1958) has ably pre- 

 sented aging as an adaption of evolution. 



A second group hold the view that aging arises from genetic noise or 

 random somatic mutations. Henshaw et al. (1957), Failla (1958), Szilard 

 (1959) and Strehler (1959) have all discussed theories of aging based 

 on somatic mutation. These are reviewed by Glass (1960) in the recent 

 AAAS publication on aging. 



The rate constant for somatic mutations viewed as chemical reactions 

 would be very small, possibly of the order of 10~^^. If genetic material 

 had the thermal stability of purified DNA (Doty et al., 1960), there 

 would be little probability of the thermal mutation, because of the great 

 stability of the hydrogen-bonded DNA helix. On the other hand, in 

 certain cells, genes may be considerably less stable than purified DNA.The 

 rate constant for thermal mutation of Escherichia coli and Bacillus subtilis 

 is of the order of 1 x 10~^ at temperatures between 55° and 60°C 

 (Zamenhof and Greer, 1958). Human genes of this order of stability 

 might undergo considerable spontaneous somatic change at 38°C. Such 

 deductions, however, must remain speculative until they can be made to 

 rest on firmer experimental evidence. It will be difficult to demonstrate 

 that random somatic mutations do occur in aging tissue, particularly if 

 such mutations are truly random. However, an eifort should be made to 

 ascertain whether clones of cells from aging individuals have altered 

 biochemical properties. The greying of hair might be an example of a 

 somatic mutation in aging melanocytes (Fitzpatrick et al., 1958). 



A particular aspect of genetic interest concerns the instructive theory 

 of antibody formation. Is there an impairment in self recognition in 

 aging animals due to alteration in either antigen or antibody? 



Somatic theories of aging have appeal to those who feel that ionizing 

 radiation also produces somatic mutations, for such theories would 

 explain the similarity between aging and radiation injury. 



For the extracellular processes of ageing, much attention has been paid to 

 the intercellular medium in its composition of fibres (coUagenic, reticular and 

 elastic) and amorphous ground substance, soft and firm, giving structure to 

 the extracellular liquids. 



As expressed in Ham and Leeson's textbook, it forms the edifice in which 

 the cells are housed. A continuous renewal during life of this structure seems 

 to be beyond the capacity of the organism and it is first the amorphous 

 substance which gradually changes and disappears. Also the fibres deteriorate 



