Sprague-Dawley rats subjected 2 months previously to partial resection (10 per cent) of the small intestine and their controls were injected with tritiated thymidine and sacrificed at 2 and 23 hours. Segments of the duodenum, jejunum, and ileum were autoradiographed, and the migration of the labelled cells during the period between 2 and 23 hours was measured with an eyepiece micrometer. The cells had migrated 35, 42, and 34 per cent of the total distance from thecrypts to the tips of the villi in the control segments of duodenum, ileum, and jejunum respectively, and 43, 90, and 82 per cent, respectively, in similar segments from resected animals. The rate of migration in the portion of the intestine remaining after resection was approximately three times the normal rate in the ileum, twice the normal rate in the jejunum, and showed an increase of one-third in the duodenum. These results demonstrate that the rate of cell renewal is considerably greater in the remaining portion of the intestine of resected animals than in normal intestine. The increased rate of migration after resection, together with the increase in the height of the villi, resulted in an increase in the rate of cell renewal amounting to 141 per cent in the ileum, l l 4 p e r cent in the jejunum, and 23 per cent in the duodenum when compared with control segments.This study is the fourth in a series of reports on the effects of "partial" (10 per cent) resection of the small intestine. The first three dealt with intestinal absorption, enzymatic activity, hypertrophy, and metabolism in the resected rat (1 3); the present study is concerned with the effects of resection on cellular dynamics. As Leblond (4) has stated: "Many tissues are in a state of flux, their (cell) population size varying chiefly with the rates of reproduction, on the one hand, and with the rates of conversion to non-reproductive units or of destruction, on the other hand. The state of a tissue fluctuates with its physical and *

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The transport of vitamin A across normal and resected intestine was studied in vitro by the method of Darlington and Quastel. The results showed that under aerobic conditions the rate of transport of vitamin A across normal and resected intestine was identical. In contrast, under anaerobic conditions transport was reduced 50% for resected intestine and totally inhibited for control intestine. These results were duplicated under aerobic conditions when 2, 4, dinitrophenol was added to the mucosal solution. Thus for the control intestine, transport is entirely dependent on DNP-sensitive phosphorylation mechanisms which require a wetting agent (polyoxyethylene sorbitan monooleate, ‘Tween 80’) as a solubilizer on the serosal side for the transport of vitamin A across the intestine. The DNP-insensitive mechanisms responsible for 50% of the total transport across the resected intestine were inhibited by iodoacetic acid and sodium fluoride. Chromatographic analysis of intestinal homogenates and mucosal solution at the end of the perfusion demonstrated that vitamin A palmitate is converted to the alcohol either in the lumen or on the surface of the epithelial cells and is then re-esterified in the intestinal mucosa. Transport of this vitamin is via ‘active’ energy-requiring mechanisms. In the normal intestine energy is supplied by oxidative phosphorylation, and in the resected intestine by oxidative as well as anaerobic glycolytic phosphorylation.

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