Development of the duct system during exocrine pancreas differentiation in the grass snake Natrix natrix (Lepidosauria, Serpentes).

We analyzed the development of the pancreatic ducts in grass snake Natrix natrix L. embryos with special focus on the three-dimensional (3D)-structure of the duct network, ultrastructural differentiation of ducts with attention to cell types and lumen formation. Our results indicated that the system of ducts in the embryonic pancreas of the grass snake can be divided into extralobular, intralobular, and intercalated ducts, similarly as in other vertebrate species. However, the pattern of branching was different from that in other vertebrates, which was related to the specific topography of the snake's internal organs. The process of duct remodeling in Natrix embryos began when the duct walls started to change from multilayered to single-layered and ended together with tube formation. It began in the dorsal pancreatic bud and proceeded toward the caudal direction. The lumen of pancreatic ducts differentiated by cavitation because a population of centrally located cells was cleared through cell death resembling anoikis. During embryonic development in the pancreatic duct walls of the grass snake four types of cells were present, that is, principal, endocrine, goblet, and basal cells, which is different from other vertebrate species. The principal cells were electron-dense, contained indented nuclei with abundant heterochromatin, microvilli and cilia, and were connected by interdigitations of lateral membranes and junctional complexes. The endocrine cells were electron-translucent and some of them included endocrine granules. The goblet cells were filled with large granules with nonhomogeneous, moderately electron-dense material. The basal cells were small, electron-dense, and did not reach the duct lumen.