Cytochemical study of secretory process in transplantable insulinoma of syrian golden hamster.

Electron microscopy, including phosphatase cytochemistry, indicates that the secretory granules of an insulinoma producing proinsulin and insulin are packaged by the endoplasmic reticulum (ER) and especially by a specialized region of ER which we call GERL because of the spatial relationship of this region to the Golgi apparatus and its apparent role in producing lysosomes. The granules are not derived from the Golgi apparatus. Preliminary evidence suggests this may be true also of pancreatic beta-cells.