Solitary cutaneous malignant schwannoma. Immunohistochemical and ultrastructural studies.

We describe two cases of malignant schwannoma in the skin that did not originate from a nerve trunk and was not associated with neurofibromatosis. Light microscopy showed that both tumors were composed predominantly of atypical spindle-shaped cells. Immunohistochemical analysis was performed with various monoclonal antibodies against mesenchymal determinants. In both patients, tumor cells were positive for S-100 alpha and S-100 beta, neuron-specific enolase, vimentin, neurofilament protein, and myelin basic protein, and negative for HMB-45, glial fibrillary acidic protein, epithelial membrane antigen, and desmin. Electron microscopy revealed that the tumor cells possessed indented nuclei with an electron-lucent cytoplasm. Immunoelectron microscopy showed diffuse fine granular staining for S-100 alpha, not detectable in normal Schwann cells, in the cytoplasm of the tumor cells.