Proposal of a a neurosecretory system in the pancreas. An electron microscope study in the dog.

The nerve fibers supplying the islets of the dog pancreas were examined by electron microscopy. Axons with swollen portions containing various types of synaptic vesicles, including a cholinergic type, two varieties of presumable aminergic type and a possible peptidergic type, rush to the pericapillary space to end there, whereas only a small part of them seem to terminate on the endocrine cells of the islet. Schwann cell cytoplasm invests the axons and often separates these from the endocrine cells. The terminal portions filled with vesicles usually become free of the Schwann sheath on the side facing the blood capillary. Here the neuronal secretions are believed to be released into the blood stream through the pored endothelium of the capillary. The neurosecretions, together with the islet hormones are distributed to the exocrine pancreas in high concentrations via the insulo-acinar portal system. It seems thus sufficient for the nerves to supply the islet in order to control the exocrine function of the pancreas.