THE TURNOVER OF CELLS 



thelial covering of surfaces exposed to any environment con- 

 taminated with micro-organisms. The epithehum of the skin 

 and of the Unings of respiratory and ahmentary tracts is being 

 constantly replaced, in part to repair minor lesions but 

 much more as a means of forestalling the necessity for repair. 

 (ii) Tissues which are liable to functional changes in 

 activity involving proliferation or regression of cells — 

 breast and uterus, and probably ovary and testis. In 

 a less obvious sense the endocrine organs may perhaps 

 be included here. 

 (iii) Tissues in which the average cell life is long but in 

 which, when demand arises, proliferative and repara- 

 tive processes take place readily and efficiently, for 

 example, liver and kidney. 

 (iv) Cells of the central nervous system in which no pro- 

 liferation or regeneration takes place. 

 In a discussion of the general problem of turnover based 

 mainly on studies of the rat made by counting the proportion 

 of cells in mitosis in different tissues, Leblond and Walker 

 (1956) made a number of interesting points. 



There are several tissues of the adult rat which show no 

 sign of renewal. These include central nervous system, liver, 

 pancreas, salivary glands, adrenal medulla. In the skin 

 epidermis multiplication takes place in the basal layer or the 

 immediately adjacent spinous cells. Here it is relatively easy 

 to conceive the possibility of a mutation in a cell of the basal 

 layer which can have a marked survival advantage until 

 eventually its descendants control the character of a few 

 square millimetres of surface. This is probably the nature of 

 the small hyperkeratoses of exposed skin that are very com- 

 mon in elderly men. In the alimentary canal the turnover 

 of cells can be very rapid. In the rat, and presumably also 

 in man, intestinal cell multiplication takes place in the crypts 

 of the mucosa with migration of chief and mucous cells toward 

 the tip of the villus, whence they are shed into the lumen. In 

 the stomach and large intestine, the same type of process, 



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