Influence of fixation and staining techniques on the ultrastructure of the insulin secretory granule.

The ultrastructure of beta-cell granules was examined after fixation of isolated mouse islets in fixatives of various compositions. Islets fixed in glutaraldehyde with or without post-fixing in osmium tetroxide, or in osmium tetroxide alone, exhibited the classical picture of electron-dense core surrounded by a lucid halo. After fixation in potassium permanganate, with or without post-fixing in osmium, the secretory vesicle was often completely filled with a fairly homogeneous material; the vesicle membrane was distinct in outline. When glutaraldehyde-fixed beta-cells were stained according to Rambourg to visualize reducing groups, possibly carbohydrates, two distinct classes of secretory granules were observed, one darkly stained and one faintly, without any noticeable precipitation of silver; the perigranular halo remained unstained in both types of granules. The perigranular halo of aldehyde and osmium fixed beta-granules also remained unstained in cells exposed to the osmiophilic amine, 5,6-dihydroxytryptamine.