Mediastinal tumors: ultrastructural and immunohistochemical evaluation of intermediate filaments as diagnostic aids.

The histogenesis of six mediastinal tumors was investigated ultrastructurally and immunohistochemically using monospecific antibodies against intermediate filament proteins. Four of the tumors, showing different appearances by light microscopy, displayed desmosomes and cytoplasmic tonofilaments, by electron microscopy, compatible with an epithelial thymoma. These cases also showed keratin positivity by immunofluorescence microscopy. One spindle cell tumor showed zonula adherens-type junctions, prominent collections of intermediate filaments, and abundant cytoplasmic neurosecretory granules consistent with a neuroendocrine tumor. In this tumor, neurofilaments could be demonstrated by immunofluorescence microscopy, a feature also consistent with a neuroendocrine tumor. One malignant tumor, lacking tonofilaments and desmosomes but showing a few primitive junctions, did not contain keratin but showed vimentin positivity. This suggests a mesenchymal origin and a diagnosis of primitive sarcoma. These cases illustrate the diagnostic usefulness of electron microscopy and immunohistochemical evaluation of intermediate filaments.