The demonstration of B cells in pancreatic islets with orcein

Orcein stains granules in pancreatic islet cells selectively. The localization of Orcein-positive cells within islets differs from that of Grimelius-stained cells, but corresponds to the B cell type differentiated by Aldehyde Fuchsin. Usually there appear to be fewer Orcein-positive cells than Aldehyde Fuchsin-positive ones. This indicates either that Aldehyde Fuchsin is a more sensitive stain for B cells or that Orcein is a more selective stain for a B cell type subpopulation. The rationale of the Orcein reaction in B cells seems to depend on the oxidation of disulphide bonds present in insulin and its precursors rich in cystine.