Comparative study on the endocrine cells in the pancreas of Mauremys caspica (chelonia) in summer and winter.

Four endocrine cell types were identified using peroxidase-antiperoxidase (PAP) technique and ultrastructurally characterized in the pancreas of Mauremys caspica in both winter and summer. In winter, insulin-immunoreactive cells were more abundant and the cell groups larger in the splenic than in the duodenal region, whereas in summer, medium or small cell groups were evenly distributed. Glucagon- and somatostatin-immunoreactive cells were found throughout the gland; they were more numerous in the splenic than in the duodenal region. Polypeptide pancreatic (PP)-immunoreactive cells were found only in the duodenal region. Somatostatin-immunoreactive cells were mainly isolated in winter and grouped in summer. Glucagon- and PP-immunoreactive cells had a similar arrangement in both seasons. Somatostatin- and PP-containing cells showed cytoplasmic processes and could be found next to the pancreatic ducts; the latter were also observed near insulin-immunoreactive cells. Some large secretory granules and numerous, isolated and long rough endoplasmic reticulum (RER) cisternae were seen in winter B cells; in summer B cells numerous lysosomes and few, dilated RER cisternae were found. Summer A cells showed well-developed, dilated RER cisternae and numerous vacuoles; secretory granules were more numerous in winter A cells. In winter B cells and summer A cells some nuclear filamentous inclusions were observed. Few RER cisternae were observed in winter D cells and many in summer D cells; secretory granules were found, the shape and electron density of which differed with the season. PP cells were characterized by their small secretory granules, which were less numerous in winter than in summer, being clustered at the cell pole or dispersed in the cytoplasm, respectively; in winter, the well-developed RER cisternae were dilated and irregularly distributed.