Extract: At 15 days after birth, the small intestine of the rat showed a striking increase in relative weight, an increased depth of intestinal crypts, an elevation of the mitotic and the labeling index of crypt cells, and a decrease in the ratio of height of the villi to depth of the crypts.Metabolically, the biological half-life of 14C phenylalanine incorporated into protein was decreased in the intestine of 21-day-old rats compared with that of the 32-day-old rat. With autoradiography, a decreased half-life of 3H-leucine in crypt cells was noted in rats 5 and 21 days of age, compared with fully weaned rats 32 days of age. These observations indicated that protein turnover is not completely dependent on cellular turnover in suckling rats.Artificial feeding of suckling rats from 9 to 15 days of age produced an increase in the relative weight, mitotic index, depth of crypts, and activity of invertase in the jejunum. Prolonged administration of hydrocortisone to newborn rats resulted in similar changes in the duodenum and in the jejunum, but not in the ileum. These changes could be related to an increased rate of cellular migration along the villi of the duodenum and the jejunum. It was concluded that both diet and hormones were important factors in the marked changes in intestinal cell proliferation and differentiation observed at weaning.Speculation: The usual changes in cellular proliferation and differentiation of the intestinal mucosa of the weanling rat and the observation that these changes can be stimulated precociously suggest that the intestinal tract of suckling rats could be used to investigate further those factors important to the control of cell differentiation. Since cell turnover increases to adult levels after 15 days of age, the observed increase in protein turnover in suckling and in weanling rats, compared with that of adult rats, suggests that at these ages protein turnover is not coordinated with cell turnover.
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