An immunocytochemical and ultrastructural study of the endocrine pancreas of Pseudemys scripta elegans (Chelonia)

Immunocytochemical and ultrastructural methods have shown four cell types in the endocrine pancreas of the turtle Pseudemys scripta elegans: insulin-, glucagon-, somatostatin-, and pancreatic polypeptide-immunoreactive cells. Each endocrine cell type was distributed differently in the duodenal or splenic regions of the turtle pancreas. Round or fusiform insulin- and glucagon-containing cells could be seen as single scattered cells which were more numerous in the duodenal regions, and the cell groups becoming progressively smaller from splenic to duodenal region. Round or fusiform somatostatin cells with thick processes and spindly pancreatic polypeptide cells with long protrusions were less numerous the nearer they were to the splenic regions; they were isolated in the duodenal zone. Insulin cells were surrounded by somatostatin cells and an outer layer of glucagon cells around the cell groups could be seen. Insulin cells were characterized by their round secretory granules which contained a polygonal, irregular or rod-shaped dense core. They also contained numerous clustered mitochondria, large multivesicular bodies, and cilium. Glucagon cells, joined by desmosomes to adjacent ones, had numerous filamentous mitochondria with longitudinal cristae and round electron-dense secretory granules with closely applied membrane. Somatostatin cells contained two kinds of secretory granules, some of which showed an electron-dense core, while others had moderately electron-dense floccular material. PP cells were characterized by round secretory granules, smaller than those of other cell types, and a large euchromatinic nucleus. Lysosomes, microtubules, bundles of microfilaments, a well-developed Golgi apparatus, and scarce rough endoplasmic reticulum were present in the cytoplasm of all these endocrine cell types.