Morphogenesis of gut and distribution of the progenitors of gut endocrine cells at cranial somite levels of the chick embryo.

The primary objective of this study was to establish the distribution of the progenitors of selected gut endocrine cell types at cranial somite levels. In addition, analysis of the material has provided new information about the location of the presumptive territories of certain gut regions and of the pancreas. Narrow transverse strips of full-thickness blastoderm two or three somites in length were excised at the levels of somites 1 to 5 of 8.5- to 18-somite chick embryos and cultured as chorioallantoic grafts to an age equivalent to 20 days of incubation. The grafts were analysed by immunocytochemistry, and their morphology was evaluated. Individual grafts exhibited up to five different types of gut morphology, including those of oesophagus, proventriculus, gizzard, pyloric region, small intestine, and pancreas. The morphologic survey yielded new information about the location, extent, or both, of the territories of the pyloric region, the small intestine, and the pancreas. In general, the progenitors of gut endocrine cell types identified were those expected for the different morphologic regions: in only a few instances were ectopic endocrine cell types detected. The available evidence points to the progenitors of bombesin/gastrin-releasing peptide cells being located cranial to somite 5 at the stages studied. Based on the morphology and the proportion of insulin cells, the development of pancreas in grafts appeared compromised compared with grafts of the intact dorsal pancreatic bud: this may relate to the likely exclusion of dorsal pancreatic bud mesoderm from the graft area. The results show that presumptive small intestinal endoderm in grafts can differentiate in the absence of homologous (i.e., small intestinal) mesoderm: this accords with the view that the primary source of positional information in the gut is in the endoderm.