Osmium dependent argentaffin reaction in the pancreatic islet.

By applying at the ultrastructural level the silver methenamine impregnation method to glutaraldehyde-osmium fixed pancreatic tissue of Snell-Bagg mice it was found that two islet cell types show deposition of reduced silver on their secretory granules: B cells and another cell type. Histochemical analyses indicate that very likely the osmium dependent argentaffinity is due to S-S groups. After alkaline treatment the reactivity of B cells is lost, while that of the other cells remains unaffected. It is suggested that the third cell type stores the pancreatic somatostatin, an S-S containing polypeptide hormone.