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The effect of growth and of fasting on the number of villi and crypts in the small intestine of the albino rat.

Recent investigations of epithelial cell turnover and three-dimensional structure of the mucosa of the small intestine were based on the intestinal villus as a unit of architecture (Clarke, 1970a, b). For long-term studies it would be useful to know whether a single villus is a constant unit of mucosal architecture: does the number of villi change as the animal grows or undergoes experimental manipulations? Clarke (1967) showed that the number of villi in the duodenum of the chicken was the same three days before hatching as in the adult, and Forrester (1972) has independently concluded that in the rat the number of villi does not alter substantially as the animal grows. The number of villi and of crypts per unit of 'serosal area' can be counted in the longitudinally opened intestine; the present paper describes how the 'serosal area' may be measured, and multiplied by the population density of villi and of crypts to give estimates of the total number of villi and of crypts in the small intestine. The effect of age and of five days' deprivation of food upon these estimates will be described and discussed. The preliminary findings of this investigation have already been published (Clarke, 1971).