Age and Renal Disease 251 



Renal growth and regeneration 



Both the tentative and the definitive foetal kidneys 

 develop from mesoderm, in intimate relation with the gonads. 

 So it is not altogether surprising that the adult kidney 

 resembles the other transient tissues, and its life cycle is not 

 completely synchronous with that of the rest of the body 

 (Kennedy, 1957). It would be disastrous to a species, of 

 course, if kidney and body got too far out of step, but any 

 tendency for this to happen during reproductive life would 

 be prevented by natural selection. There is some evidence, 

 however, that the kidney atrophies after the climacteric, and 

 in some species such as the rat this may limit life. 



Most mammals develop their full complement of nephrons 

 soon after birth, and postnatal growth of the kidney consists 

 chiefly of lengthening of its tubules, at first by the growth of 

 new cells and later by hypertrophy of existing ones. When a 

 rat is about six months old, or a man about 30 years, the 

 number of glomeruli in their kidneys begins to decrease, and 

 it may fall to half the young adult value, without pathological 

 change, by eighteen months old in the rat or seventy years in 

 the man (Arataki, 1926; Moore, 1931; Roessle and Roulet, 

 1932). Moore and Hellman (1930) showed that removing one 

 kidney from a rat did not slow down the loss of nephrons from 

 the other, so that involution of the kidney is an even more 

 relentless process than that of the ovary, where removal of one 

 gland does delay the loss of oocytes from the other (Mandl and 

 Zuckerman, 1951). 



Nowadays chemical analysis can be used to supplement 

 histology in determining the number and size of the cells in a 

 tissue. This is because one of the two forms of nucleic acids in 

 cells, deoxyribonucleic acid or DNA, is confined to the nuclei, 

 as the name suggests it ought to be, while the other, ribo- 

 nucleic acid or RNA, is distributed with the bulk of the 

 ordinary protein throughout the cytoplasm. So if DNA, 

 RNA and protein are determined at different stages during 

 the growth of a tissue, it is possible to distinguish between 



