Morphogenesis of somatostatin- and insulin-secreting cells in the lamprey endocrine pancreas.

The development of the endocrine pancreatic tissue during the entire life cycle of lampreys was provided through a review of previous literature and through a description of new, preliminary immunohistochemical data on a number of species. There seems to be no firm conclusions on the method of development of the endocrine pancreas during embyrogenesis, but observations of 79-day old larvae suggest that somatostatin-positive cells are confined to the epithelium of the alimentary canal, which is also the site at which insulin-immunoreactive islets form. This type of cellular distribution and morphogenesis at the oesophago-intestinal junction continues throughout the larval period. In northern hemisphere species, a caudal pancreatic mass forms at metamorphosis from the transformed epithelium of the bile duct. Evidence is presented to suggest that the process of bile duct degeneration and transformation is highly synchronized and that interference with the morphogenetic event will alter the normal distribution of pancreatic islets in the adult. The position of the bile duct in larvae of southern hemisphere species results in no caudal pancreas in adults. The adult cranial pancreas of all lamprey species, forms from remnants of the larval islets and through proliferation of new islets from the epithelium of the developing intestinal diverticulum. In holarctic lampreys, the cranial and caudal masses are connected by a discontinuous intermediate cord of islets and all three regions contain equal numbers of somatostatin- and insulin-immunoreactive cells. Following metamorphosis, growth of the caudal pancreas occurs through cellular proliferation from within but the cranial pancreas continues to recruit islets from the diverticular epithelium through the upstream-migrant (prespawning) period.