Postnatal proximodistal development of the small bowel mucosal mass in growing rats.

The development of the duodenal, jejunal, and ileal mucosal mass was studied in five groups of rats killed at different ages during growth. After sacrifice, the small bowel was removed from the pylorus through the ileocecal valve, measured and divided into three equal segments (A, B, C). Gut weight, mucosal weight, DNA, protein, and sucrase activity were determined in each segment. For each segment, results of all mucosal mass parameters expressed per centimeter of gut length markedly increased during weaning (15-30 days), and at 40 days of age they were similar to those obtained in adult animals. A proximal to distal gradient of gut weight was present at birth and increased during lactation. However, the proximodistal gradients of mucosal weight and mucosal DNA per centimeter of length remained constant in all groups studied. The intestinal gradient of sucrase activity was absent at birth and in lactating rats (17 days), but was similar in rats of 30, 40, and 100 days of age. It was concluded that mucosal hyperplasia which occurs in rats at weaning equally develops in the proximal, mid, and distal small bowel.