GROWTH AND THE AGING PROCESS 963 



on the chemical composition of skeletal muscle gives an interesting comparison 

 with the effects of aging. They produced atrophy of the gastrocnemius muscle of 

 the ral by three methods: starvation, tenotomy, and denervation. The water con- 

 tent of the fibers appeared to remain relatively constant, the total amount of 

 protoplasm to decrease markedly, and the extracellular mass of tissue to decrease 

 only slightly (Table 5). 



There appears to be very little change with senescence in heart, liver, and brain 

 of the rat in relation either to the amovmt or composition of the intracellular and 

 extracellular phases. The heart shows a slight increase in amount of extracellular 

 fluid and an increase in lipid phosphorus svich as was found for skeletal muscle. 

 In both brain and heart there is a slight displacement of intracellular potassium 

 by sodium. 



On the other hand, data on total solids in the human brain do indicate a con- 

 sistent decline with senescence, amounting to from 8-15 per cent (Strobel, 1939). 

 Histological studies, as we shall see later, indicate a loss of cells in old age both 

 in the rat and man. 



The rat kidney shows an increase in extracellular fluid when the middle-aged 

 (603-day) is compared with the young mature animal (60-day) (Lowry and 

 Hastings, 1952); and in the senile animal (988 days) there was a slight further 

 increase. The data suggest to the authors a decrease in the proportionate cell mass 

 in this organ. 



The picture of intracellular and extracellular phases of fluid in many tissues, 

 then, is of an increasing "hydration" of the tissue with advancing age, a sort of 

 extracellular edema. On the other hand, the fluid content of the protoplasm 

 appears to remain essentially constant. As in almost all studies on the aging proc- 

 ess, it is a diflicult task to separate out the factor of "pure" senescence from patho- 

 logical conditions, including systemic disorders, which may operate to further 

 this type of change. 



Comparison of the biological characteristics of growth and of senescence in- 

 volves the study of the appearance and activities of the units of all living things, 

 the cells. That the majority of cell types show definite biological features during 

 periods of growth, particularly of rapid growth, is the opinion of Caspersson (1950). 

 In the process of synthesis of protein, the nucleolus and the nucleolus-associated 

 chromatin play a central role. The increase of nucleolar masses is a most conspic- 

 uous phenomenon during cytoplasmic protein synthesis. During periods of intense 

 growth the nucleolus often becomes very large and shows large quantities of 

 ribose nucleotides and of proteins rich in diamino acids. The great increase of 

 nucleotides in the cytoplasm is seen in a marked basophilia and can be detected 

 easily by means of the absorption of ultraviolet light at 2600 A. 



Caspersson presents figures of the cells of embryonic liver, kidney, and other 

 organs showing the large nucleoli and nucleotide-rich cytoplasm of these rapidly 

 growing cells. Cells of the regenerating liver, after partial hepatectomy, also 

 show the large nucleoli and the high concentration of nucleotides in the cytoplasm. 



In actively growing tissues another feature is, of course, the presence of large 

 numbers of mitotic figures, for with cell size as a fairly sharply limited character- 

 istic, the attainment of such a size is marked by cell division. 



Literature p. gyi 



