Light and electron microscopy of the islets of langerhans of the saimiri monkey pancreas

The distribution of alpha, beta, and other islet cells in the tail of the pancreas of the Central American monkey ( Saimiri sciurea ) revealed by the light and electron microscopes, is similar to that occurring in man. Differentiation of acinar, islet and duct cells is based on the presence or absence of specific granules and the special morphology of the cytoplasm and its membrane systems. Conspicuous lipid droplets appear in the islets, largely confined to beta and delta cells. An “intergrade” cell is present, possessing features of both alpha and beta cells, as well as a nongranular variety of delta cells. Nerve processes which display synaptic vesicles and clusters of mitochondria are present within the islets. Islet and acinar cells are sometime separated by connective tissue or a reticulum, as observed in light microscopy, but there may be simple apposition of their respective plasma membranes. The fine structure of this reticulum presents several levels of organization. Duct cells possess lipid masses similar to those in islet cells. These duct cells, unlike islet cells, are agranular and possess microvilli and cytoplasmic blebs, both of which show internal structures.