Age and Renal Disease 253 



kidneys of the six-month-old rats. It may be emphasized 

 that these findings accord very well with Minot's definition 

 of ageing. As will be shown later, hyperplasia can and does 

 occur in the tubules of older rats, but it does not then repre- 

 sent the normal primary response to loss of moderate amounts 

 of renal tissue, and some additional stimulus, possibly endo- 

 crine in nature, is probably involved. 



Renal Senescence 



The compensatory changes that we have been considering 

 are self-limiting, and once they have been achieved, the kidney 

 undergoes no further changes for many months. A different 

 sort of tubular change will now be considered. In rats killed 

 after 18 months of age very active hyperplasia has been found 

 in occasional tubules, at first widely scattered, affecting 

 principally the proximal convolutions, and quite unlike the 

 regular, orderly growth of cells in young rats' kidneys. At 

 this age a lot of nephrons have already disappeared, but one 

 would expect the surviving tubules to compensate for their 

 loss by hypertrophy rather than hyperplasia. Further, this 

 hyperplasia in ageing kidneys appears to be destructive rather 

 than helpful, because the tubules are often blocked and 

 functionless and eventually become dilated by hyaline casts. 

 As age increases still further the kidneys become greatly 

 enlarged and granular in appearance, and microscopically 

 they show chronic interstitial fibrosis, generalized tubular 

 dilatation, and hyaline or fibrotic changes in the glomeruli 

 and smaller vessels. These histological changes have been 

 described and illustrated more fully elsewhere (Kennedy, 

 1951, 1957). The terminal appearance has been studied by 

 numerous pathologists, but since no two agree on a morbid 

 anatomical diagnosis, there is no need to add to the confusion 

 here. The terms chronic glomerulonephritis (Wilens and 

 Sproul, 1938), nephrosis (Saxton and Kimball, 1941), pyelo- 

 nephritis (Goldblatt, 1947) and senile nephrosclerosis (Oliver, 

 1942) have all been used. 



