Morphogenesis and distribution of the endocrine pancreas in adult lampreys

The endocrine pancreas of larval lampreys appears as islets of cells isolated in the submucosa and those both continuous with, and within, the gut epithelium at the intestinal-oesophageal-bile duct junction. The islets, and occasionally follicles, are composed of only insulin-secreting (B) cells. During metamorphosis, the bile duct either completely degenerates or its epithelium transforms into a caudal endocrine pancreas while a cranial pancreas appears as a specialization and expansion of the larval pancreas. Immunocytochemistry and histochemistry demonstrates that there is a wide variation in the distribution of the pancreatic tissue in adults of lamprey species, and this variation may result from interspecific differences in morphogenetic events at metamorphosis. Despite species variability in its distribution, the endocrine pancreatic tissue in all adult lampreys is composed of equal numbers of B cells and somatostatin-secreting (D) cells, but there are no glucagon-secreting (A) cells. Immunocytochemistry reveals that B and D cells of the caudal pancreas differentiate from cells of the larval bile duct during metamorphosis of the sea lamprey,Petromyzon marinus.