GROWTH AND THE AGING PROCESS 965 



and have confirmed the observation by a number of earher workers that there is 

 a considerable loss of nerve cells in old age. 



While the nature of the changes in old age is generally "degenerative", with 

 loss of the essential tissues of the organ, there are a number of instances where 



Fig. 6. Hypertrophied liver cell nucleus, with inclusion bodies, from a mouse of 81 1 days. 



X 1500. (After Andrew, 1956b). 



proliferation of cells occurs (Andrew, 1944; Korenchevsky, Paris and Benjamin, 

 1950). The proliferation, however, is that of duct cells and tends to hasten rather 

 than retard the degeneration of the functioning cells. 



In a number of organs aberrant forms of cells, which in general have been 

 called "oncocytes", have been described in old age (Hamperl, 1937) (Fig. 7B). 

 These cells frequently show an amitotic division of the nucleus. This process 

 we have interpreted (Andrew, 1955a) as indicating a "defensive" reaction of the 

 cell by which the surface area of the interface between nucleus and cytoplasm is 

 increased and the cell is enabled to survive. Recently Rudzinska ( 1 955) has shown 

 such amitotic division in senile individuals of a protozoan, Tokophyra infiisionum 



(Fig-ii)- 



These senile changes in tissues and cells are varied and certainly are not the 

 reverse of the changes occurring during growth. Still, in the complicated biological 

 features of the two processes, we would not expect a reversal of growth changes. 

 We do see, however, that the stabilization of the tissues, the cessation of growth, 

 leads sooner or later to the onset of senile change. Senescence is not synonymous 

 with the cessation of growth but appears to be an inevitable result of such 

 cessation. 



We have dealt with the chemical, physical, and biological characteristics of 



Literature p. gyi 



