FINE STRUCTURE OF MAMMALIAN AND AVIAN PANCREATIC ISLETS WITH SPECIAL REFERENCE TO D CELLS AND NERVOUS ELEMENTS

The pancreatic islets of the dog, guinea pig and pigeon were studied under the electron microscope and the following results were obtained. 1. Besides A and B cells, D cells are constantly found in the islets of these three species. The D cell granules, when fixed in glutaraldehyde especially by perfusion, are represented by a solid substance of medium electron density and fine granular texture closely encircled by a membrane sac. Generally acknowledged appearance of large D granules with coarse texture and low electron density and of interrupted membrane sacs is ascribed to be an artefact caused by OsO4 fixation. 2. Figures indicating emiocytotic release of the D granules were obtained in the dog and pigeon. 3. In the dog many nerve fibers containing accumulations of synaptic vesicles and mitochondria are either in close contact with or a certain distance apart from the islet cells. These nerve terminals are divided into two types: those containing mainly small cored vesicles and those with small non-cored vesicles. Large cored vesicles occasionally occur in both types. There was no special relation between a certain type of islet cell and a certain type of nerve terminal. In the guinea pig the nerves, though also in direct contact with islet cells, are much fewer than in the dog. 4. In the dog and guinea pig Schwann cells investing the axons are frequently found in close contact with the islet cells. Some of these cells are incorporated in the islet as an agranular, epithelial cell-like element with attenuated processes inserted among the islet cells. 5. The neuro-insular complex of type II (Fujita, 1959), i.e. the conglomeration of a nerve fiber bundle and islet cells was found frequently in the dog. Both axons, with or without synaptic vesicles, and Schwann cells are closely juxtaposed with islet cells. Every transitional type was found between this complex and the islet receiving an ordinary amount of nerve elements. 6. In the pigeon, no nervous element was found in the islet tissue. © 1969 Springer-Verlag.