Organogenesis and differentiation of the pancreas in the toad Bufo bufo L

In Bufo bufo at stage III₆ the first endocrine islets appear in the part of the pancreas corresponding to the dorsal anlage. At stage IV₂, 5 days later, the pancreatic duct develops and new islets arise by budding off from the ductal epithelium. The ultrastructural study of the secretory granules morphology of endocrine cells has distinguished four different cell types: B-cells (stage III₉), A-cells (stage IV₃), D-cells (stage IV₃) and a fourth type not yet identified (stage IV₃). By immunocytology insulin and corticotropin-releasing factor (CRF) cells have been demonstrated at stage III₉, and glucagon and somatostatin cells at stage IV₁. Lastly, endocrine islets can be homogeneous (predominantly containing insulin cells, rarely glucagon cells) or heterogeneous (insulin cells at the centre and glucagon or somatostatin cells at the periphery). Hypotheses are put forward for the origin and the constitution of the different generations of endocrine islets and isolated cells.