Melanotic schwannoma coexpression of vimentin and glial fibrillary acidic protein.

A 35-year-old man was operated for a thoracic wall tumor which bulged into the pleural cavity. Histologic examination showed a spindle cell tumor with numerous psammoma bodies and a heavy melanin pigmentation. Electron microscopy revealed melanosomes and a continuous basal lamina surrounding all tumor cells, compatible with the diagnosis of a melanotic schwannoma. The tumor cells were S-100 protein-positive, and there was an abundant pericellular laminin-positivity. Practically all tumor cells were vimentin-positive, while cytokeratin, desmin and neurofilaments were absent. Glial fibrillary acidic protein was found in single tumor cells, suggesting the presence of focal glial differentiation in melanotic schwannoma.