Ontogeny and Regional Distribution of Hormone-Producing Cells in the Embryonic Pancreas of Alligator mississippiensis

Abstract

The hormones of the endocrine pancreas are believed to play an important role in early development. The development of the pancreas and the appearance of hormone-producing cells during embryogenesis have been extensively studied in mammals and birds. Relatively little work has been done in other vertebrates, and there are no published studies regarding the order Crocodilia. Given the pivotal phylogenetic position of crocodilians, Alligator mississippiensis provides an interesting species in which to study the embryonic development of the endocrine pancreas. The aims of the present study were (1) to investigate the morphological development of the pancreas and (2) to determine the initial appearance and regional distribution of the pancreatic endocrine cells in the embryonic alligator. At each stage of development serial sections of pancreatic tissue were stained with hematoxylin and eosin to aid in morphological description. Using immunocytochemistry sections were stained to detect the presence of insulin, glucagon, somatostatin, and pancreatic polypeptide. The dorsal pancreatic bud was first observed at stage 8, coincident with the appearance of insulin-containing and glucagon-containing cells. Somatostatin-containing cells were first detected at stage 10. At stage 13 the ventral pancreatic bud was first observed. At stage 14 the dorsal and ventral pancreatic buds fused and insulin, glucagon, and somatostatin were found throughout the pancreas. Not until stage 17 was pancreatic polypeptide first detected. Unlike the other hormones, pancreatic polypeptide was rare or absent in the dorsal region of the pancreas. In later stages of development, somatostatin-containing cells were the most abundant and constituted 35-40% of all hormone-containing cells. The sequence of appearance of insulin and glucagon found in the alligator is the same as that found in mammals and birds.