Perinatal changes in bovine SEGI's caps with special reference to their endocrine cells migrating into the lamina propria.

Large aggregations of endocrine cells at the top of villi in the upper small intestine, discovered by SEGI (1935) in human fetuses and hence called SEGI's caps, have been known to occur also in bovine and porcine fetuses. Using bovine fetuses and newborns, this study aimed to clarify the fate of and changes in the SEGI's cap during the perinatal period. Immunocytochemistry was performed for CgA and PGP 9.5 as markers of the endocrine and nervous elements concerned. In the second trimester in bovine fetuses, numerous endocrine cells were confirmed, by CgA immunocytochemistry, to gather in the epithelium on the top of almost every duodenal villus, either in a single layer or accumulating in a rounded cell mass. They persisted until birth, though decreasing in number during the third trimester. In these fetal periods the endocrine cells came to be invaginated into the lamina propria, and to be separated from the epithelium. They were attached by nerve fibers and shifted to the base of the villi and to the intercryptal tissue. The endocrine cell aggregations at the villous tips rapidly disappeared within three days after birth. The intraepithelial endocrine cells (paraneurons) decreased in their immunoreactivity for CgA after they shifted into the lamina propria, whereas they increased in their immunoreactivity for PGP 9.5 while descending the villous interstitium. Using immunocytochemistry for PGP 9.5, we were able to trace the descent of the endocrine cells in association with nerve fibers. Microscopic images were obtained supporting the possibility that the endocrine-type cells eventually might be taken up by the intramural plexuses of Meissner and Auerbach. Furthermore, the fetal and perinatal intestine revealed enlarged solitary endocrine cells and authentic neurons, as well as their intermediate types, in the villous and intercryptal interstitium. The present results indicate that the SEGI's caps in cattle are best developed in the second trimester of the fetal life, and do not support a hypothesis that the endocrine sensor cells in the caps principally function at the time of birth, especially for responding to the advent of colostrum. The endocrine or paraneuronal elements of the SEGI's cap appear to migrate into the interstitium to obtain more neuron-like features. A possibility that they may be incorporated in the intramural ganglia and further be transformed into neurons awaits further investigation.