An immunocytochemical and electron-microscopical study of endocrine cells in the gut and pancreas of a stomachless teleost fish, Barbus conchonius (Cyprinidae).

Four immunoreactive endocrine cell types can be distinguished in the pancreatic islets of B. conchonius: insulin-producing B cells, somatostatin-producing A1 (= D) cells, glucagon-producing A2 cells and pancreatic polypeptide-producing PP cells. The principal islet of this species contains only a few PP cells, while many PP cells are present in the smaller islets. Except for the B cell all pancreatic endocrine cell types are also present in the pancreatic duct. At least six enteroendocrine cell types are present in the gut of B. conchonius: 1. a cell type (I) with small secretory granules, present throughout the intestine, and possibly involved in the regulation of gut motility; 2. a C-terminal gastrin immunoreactive cell, probably producing a caerulein-like peptide; these cells are located at the upper parts of the folds, especially in the proximal part of the intestinal bulb; 3. a met-enkephalin-immunoreactive cell, present throughout the first segment; 4. a glucagon-immunoreactive cell, which is rare in the first segment; 5. a PP-immunoreactive cell, mainly present in the first half of the first segment; 6. an immunoreactive cell, which cannot at present be specified, located in the intestinal bulb. The latter four cell types are mostly located in the basal parts of the folds, although some PP-immunoreactive cells can also be found in the upper parts. Most if not all enteroendocrine cells are of the open type. The possible functions of all enteroendocrine cell types are discussed.