CHANGES IN SMALL INTESTINAL-MUCOSA MORPHOLOGY AND CELL RENEWAL IN SUCKLING, PROLONGED-SUCKLING, AND WEANED LAMBS

No information concerning the effect of weaning on intestinal cell proliferation is currently available in large species with early intestinal morphogenesis, a group including most domestic animals and humans. Changes in intestinal morphology and epithelial cell renewal were investigated in 1-, 5-, and 8-wk-old suckling and 8-wk-old weaned lambs after injection of [H-3]thymidine. In suckling lambs a gradual increase in crypt depth occurred with age, especially in the proximal intestine, whereas villus height was significantly reduced in the distal regions. At 8 wk of age weaned and prolonged-suckling lambs exhibited no significant differences in crypt depth throughout the intestine and in villus height proximally. However, weaned lambs had shorter villi in the jejunum and ileum. The highest enterocyte migration rates (4.4-9.7-mu-m/h) were observed in 1-wk-old lambs. In suckling animals, migration rates decreased with age by 60, 51, and 11% in the duodenum, jejunum, and ileum, respectively. Weaned and prolonged-suckling 8-wk-old lambs had a similar rate of enterocyte migration in the ileum. Furthermore, ruminating animals exhibited only slightly higher migration rates in the duodenum and the jejunum (53 and 15%, respectively). In suckling lambs, epithelial cell renewal required 2.1-4.0, 4.5-6.3, and 4.0-5.3 days at 1, 5, and 8 wk of age, respectively, whereas labeled cells reached the tips of the villi within 3.0-3.1 days in weaned animals. These data suggest that the suckling period corresponds to a gradual and important phase of postnatal intestinal adaptation in the sheep, a species with early patterns of intestinal cell replacement. By contrast, and although weaning is the major event in gastrointestinal adaptation in ruminants, the transition from milk to solid feeding, or temporally related events, is not associated with abrupt changes in intestinal epithelial morphology and cell kinetics as reported in altricial species.