Routine electron microscopy and immunocytochemistry were used to describe the cell types in the islets of the endocrine pancreas of the gar Lepisosteus osseus, an actinopterygian fish of the order Semionotiformes, which has an ancient lineage. The general fine-structural features of cells composing the islets reflect their synthesis and packaging of protein for liberation at their perivascular surface. Cells are directly apposed to numerous capillaries and they are richly innervated with nerve terminals containing dense-cored vesicles. The islet tissue comprises many B cells, which are easily distinguished by their ubiquitous granules with polymorphous matrix cores and a loose-fitting membrane. These granules are only immunoreactive with an insulin antiserum. Only one type of D cell is found throughout the islets and it contains many granules of varying electron density, the most abundant granule profile being dumbbell-shaped. All granules in this cell type have a tight-fitting limiting membrane and they immunostain with antisomatostatin-14 and -34. Cells at the periphery of the islet contained granules of similar morphology to those in the D cells, but the granules were less numerous. Many granules in the cells were immunoreactive with both antiglucagon and antineuropeptideY, while others immunostained with only one of these antibodies. Since no cells stained exclusively for either glucagon or neuropeptide Y, it was concluded that there are only three cell types in the endocrine pancreas of the gar: B and D cells and a third cell type (A/F) that co-localizes peptides of the glucagon and pancreatic polypeptide family. Although this co-localization is not uncommon in the vertebrate endocrine pancreas, it may have some phylogenetic and (or) ontogenetic significance in this organism.
FINE STRUCTURE OF THE A AND D CELLS OF THE RABBIT ENDOCRINE PANCREAS IN VIVO AND INCUBATED IN VITRO