Ultrastructural confirmation of malignant melanoma

We have recently had the opportunity to examine two cases, where malignant melanoma was suspected on light microscopy and Fontana-Masson stain was focally and faintly positive. Electron microscopy of four to five different blocks failed, however, to demonstrate the presence of premelanosomes and thereby to confirm the diagnosis of malignant melanoma. The areas in the paraffin blocks that on Fontana-Masson stain had showed a positive reaction were deparaffinized, embedded in plastic, and examined by electron microscopy. Now the characteristic premelanosomes were found and the diagnosis of malignant melanoma was confirmed.