Differences in the development of jejunum and ileum as observed in fetal rat isografts. Possible implications related to the villus size gradient.

Previous studies have shown that villus size in the small intestine decreases from the duodenum toward the ileum. Villus-enlarging and villus-reducing factors have been found in the gastric, pancreatic, and duodenal secretions, and in bile (Altmann and Leblond, '70; Altmann, '74). We here explore the role of intrinsic factors present in the individual sections of the small intestine by following the development of villus height in fetal (21-day old) jejunal and ileal segments implanted under the kidney capsule of syngeneic adult rats. Twenty-eight and 45 days after implantation, the jejunal villi contained significantly more columnar epithelial cells than did the ileal villi, but the jejunoileal gradient was even greater in small intestine of corresponding ages, developing in situ. Our experiments, thus, show that the jejunoileal gradient is already programmed in rats 1 day before birth, while other factors contribute postnatally to regulation of its magnitude.