Enteroendocrine cells in the developing opossum small intestine and colon.

Enteroendocrine cells immunoreactive for gastrin, bovine pancreatic polypeptide (BPP), glucagon (glicentin), 5-hydroxytryptamine (5-HT), somatostatin, secretin, motilin, gastric inhibitory peptide (GIP) and cholecystokinin (CCK) are scattered throughout the small intestinal epithelium of the newborn opossum and in all later postnatal stages examined. The number of BPP- and glucagon-immunoreactive cells is relatively high in the newborn and rapidly decreases until only occasional cells are present after the first postnatal week. Cells immunoreactive for GIP, CCK, 5-HT, motilin, gastrin and secretin increase in number with development. Secretin-, motilin-, CCK- and GIP-immunoreactive cells generally are concentrated proximally in the small intestine and as they increase in number, differentiate in more distal regions. The number of gastrin-immunoreactive cells actually decreases just prior to weaning but then increases at and after, weaning. Neurotensin-immunoreactive cells are unusual in that they do not appear until about the 74th postnatal day and then are first encountered in the distal small intestine. As development progresses they increase in number and appear in the more proximal regions. Cells immunoreactive for 5-HT at first increase but then decrease sharply at weaning only to increase markedly again after this time. In contrast, somatostatin-immunoreactive cells gradually decrease in number until weaning then dramatically increase. If the total number of enteroendocrine cells in the small intestine is considered, there is a gradual decrease from birth until weaning when a dramatic increase occurs. Cells immunoreactive for neurotensin, 5-HT and somatostatin are also found in the intestinal epithelium of the developing colon and caecum. Somatostatin- and 5-HT-immunoreactive cells are found throughout the colon in the newborn whereas neurotensin-immunoreactive cells, although observed initially in the proximal colon, do not form a significant population until weaning and then are concentrated distally.