Grimelius' silver stain for endocrine cell granules, as shown by electron microscopy

The silver impregnation method of Grimelius has been applied to 100-150 μ thick sections of tissues fixed 2 hr to 1 mo in mixtures containing formaldehyde, glutaraldehyde or picric acid. After silvering, the sections (partly postfixed in 1% OsO₄, for 0.5 hr) were processed for electron microscopy. Endocrine granules of pancreatic A cells, enter-ochromaffin and some nonenterochromaffin cells of the gastrointestinal mucosa, thyroid C cells and adrenal medullary cells were found to be selectively stained by silver grains 10-30 nm in diameter, either as a peripheral "halo" or covering the entire granule. At least in some cells, the reactive material should not be identified with the hormonal products known to be stored in the granules.