A staining procedure for melanin in semithin and ultrathin epoxy sections

Staining for melanin at the ultrastructural level may act as an important diagnostic aid by helping to identify apparently amelanotic melanomas. A modified Warthin-Starry (WS) procedure for melanin has therefore been adapted for direct application to epoxy sections. Minute amounts of melanin can thus be demonstrated in individual melanosomes, indicating a high sensitivity. Using the usual types of fixation, specificity for melanin at the ultrastructural level is high. Primary osmium tetroxide (OsO4) fixation probably induces false positive staining of lipofuscin and should not be used prior to the WS procedure, but it does not influence the results of the procedure when employed as a post-fixative. It is not improbable, however, that the positive staining of lipofuscin and also of mast cell granules is due to the presence of melanin in these structures. The WS procedure yields additional diagnostic information, especially in suboptimally preserved material, as expressed by an increase in the number of convincingly identifiable melanosomes in one completely and four partly amelanotic melanomas.