Peripheral innervation of the mouse pancreas was studied by scanning and transmission electron microscopy, as well as by light microscopy (cholinesterase technique). Major nerve bundles usually ran with arteries in the connective tissue septa. They gave off delicate branches that formed plexuses around arteries and arterioles. When reaching the capillaries, nerve fibers left the arterioles and formed very loose networks in the interacinar spaces. The nerves accompanying the arteries also sent off branches toward the islets of Langerhans and formed a dense plexus around the islets. A few delicate nerve fibers were also present around the pancreatic ducts. Thus, the intrapancreatic nerves formed four plexuses: perivascular, periductal, periacinar and peri-insular. The plexuses were networks of unmyelinated nerve fibers consisting of axons with varicosities and Schwann cells. Intrapancreatic ganglia were found in the interlobular connective tissue; ganglia were often closely associated to islets of Langerhans.
Our findings indicate that the ''interstitial cells'' described by light microscopists correspond to Schwann cells. Axons in the nerve plexuses contain transmitter vesicles and therefore represent an autonomic terminal apparatus. The rich innervation of arterioles and islets suggests that neural regulation of secretory function is mediated by control of pancreatic blood flow.