Spindle cell carcinoma of the renal pelvis. Immunohistochemical and ultrastructural study of a case demonstrating coexpression of keratin and vimentin intermediate filaments.

A carcinoma of the renal pelvis characterized histologically by a spindle cell sarcomatoid morphological growth pattern was studied by electron microscopy and immunohistochemical techniques. Ultrastructural examination revealed abundant perinuclear cytoplasmic tonofilament bundles in association with prominent rough endoplasmic reticulum. Immunohistochemical study demonstrated coexpression of keratin and vimentin, two intermediate filaments thought to be specific for epithelial and nonepithelial cells, respectively. It is proposed that the spindle transformation of the epithelial cells in such cases may be explained on the basis of the development by the tumor cells of nonepithelial characteristics, such as the expression of vimentin intermediate filaments, that may be responsible for the adoption of the morphological growth pattern characteristic of neoplasms following mesenchyme-derived lines of differentiation.