Nonmelanized macromelanosomes in a cellular blue nevus. Light and electron microscopic observations.

We noted nonmelanized and partially melanized macromelanosomes in a cellular blue nevus and studied their light microscopic and ultrastructural features. Numerous intracytoplasmic eosinophilic inclusions were found in the lesion; individual cells contained up to nine, although most cells demonstrated two or three. The "macromelanosomelike" inclusions ranged in size from 1 to 15 microns. Most of them were partially melanized, and some were nonmelanized. These globules were periodic acid-Schiff positive and diastase resistant, with a centrally melanized core, best seen with the Fontana-Masson technique. S100 protein stained positively the cytoplasm of some nevus cells but not the inclusions. Electron microscopy confirmed the centrifugal melanization of these structures and their targetlike morphologic characteristics. Nine other cellular blue nevi from our files failed to show similar intracytoplasmic inclusions.