Cytodifferentiation of the chick pancreas: I. Ultrastructure of the islet cells and the initiation of granule formation

Abstract

Electron microscopic observations have shown that the characteristic granules of the alpha and beta cells of the chick (White Leghorn) pancreatic endocrine tissue are present on the third day of incubation. Granule formation in these cells is initiated concomitantly with or shortly after evagination of the intestinal endothelium to form the pancreatic diverticula. Evidence is also presented for the secretion of both types of granules on the fifth day of incubation. The sequelae of ultrastructural changes leading to the differentiation of the alpha and beta cells was difficult to assess. The islet tissue originated from primordial duct epithelium and differentiated before the acinar components. The elaboration of biosynthetic mechanisms to facilitate the production of secretory products was evidenced morphologically in the beta cells by an enlarged Golgi, ergastoplasm of four to eight members, and the synthesis of numerous smooth membrane vesicles. A locus of fine fibrillar material and one cilium per cell were present. Stages in the formation of the beta granules with their characteristic sheaves of material were also noted. Similary, the differentiation of alpha cells included enlargement of the Golgi and synthesis and accumulation of numerous vesicles with varied amounts of enclosed particulate material. Fine fibrillar material was near the Golgi and the rough reticulum appeared augmented.