Cytokeratin-containing globular filamentous bodies in renal oncocytoma

Eighty-four cortical neoplasms were studied for cytokeratin and vimentin expression by immunohistochemistry and for intermediate filament aggregates by electron microscopy. Twenty oncocytomas expressed cytokeratin, 16 in a distinctive punctate pattern. These same 16 tumors also contained small globular filamentous bodies (GFB) by electron microscopy. The GFB were characterized by a matrix of intermediate-sized filaments with incorporation of diverse cell organelles such as endoplasmic reticulum, lysosomes, mitochondria, and lipid. The GFB were not within a unit membrane. Although 11 of 64 carcinomas also contained intermediate filament aggregates, only 2 of these solely expressed cytokeratin, and this was restricted to a few cells in small foci. Small GFB were also present in 5 carcinomas by electron microscopy. Three mixed clear and granular cell carcinomas contained only rare cells, whereas 2 sarcomatoid carcinomas, both of rhabdoid cell phenotype, contained numerous GFB that coexpressed vimentin and cytokeratin. Cytokeratin-containing GFB are common in oncocytomas but are uncommon in carcinoma, and, when numerous, may provide a diagnostically useful immunohistochemical feature with which to distinguish oncocytoma from its carcinoma congeners.