Quantitative histological changes in the small intestine of rats artificially reared on different milk substitutes

Growth of the small intestine was studied in rats reared normally by their mothers (MR) or artificially reared (AR) by intragastric infusion of milk substitutes from postnatal day 5. Two milk substitutes were used: one high in carbohydrate and low in protein as compared with rats' milk (Messer) and the other closely resembling rats' milk in its composition (Auestad). Pups reared on these formulae are termed ARM and ARA, respectively. Pups were killed at 7, 12, and 20 days for quantitative histological measurements on transverse sections of duodenum and ileum. They included cross-sectional areas of muscle and of mucosa and submucosa combined (other tissue), internal and external perimeters, and length of longest villus profile. Artificial rearing affected mucosal and submucosal measures, but did not affect muscle. The effects depended on age, type of milk substitute, and site within the small intestine. There was a tendency for deficient growth of small intestine in the early stages of artificial rearing, especially in the ileum of ARA pups. Enhancement of growth as shown by cross-sectional area of mucosa and submucosa and by longest villus profile occurred later--by 12 days in the duodenum and by 20 days in the ileum of ARM pups. Enhanced growth was less evident in ARA rats, being apparent only in the duodenum, and not until 20 days were the cross-sectional area of mucosa and submucosa and the longest villus profile increased.