Injury and recovery of intestinal function following 1000 rads whole-body X-irradiation of female ICR/JCL mice was investigated by assaying activities of three enzymes (invertase, L-leucine aminopeptidase, alkaline phosphatase). By comparing changes in these enzyme activities with the epithelial cell kinetics of total DNA synthesizing cell number and total cell number, the causal relationships between functional changes in the small intestine and the epithelial cell kinetics involved in the recovery process were studied. After cell injury from irradiation, first the number of DNA synthesizing cells, second the number of total cells, and last the enzyme activities decreased to the minimum value. During recovery, the order was the same: first the DNA synthesizing cell number, then the total cell number, and finally the enzyme activities. This injury process was induced by epithelial cell death in the crypt, and the subsequent recovery was the result of epithelial cell proliferation and differentiation to functional cells of the villi.

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