Spindle cell carcinoma: ultrastructural evidence of squamous origin and collagen production by the tumor cells.

Two cases of spindle cell carcinoma (SCC) of the esophagus and skin, respectively, were studied by transmission electron microscopy. The tumor cells were closely associated with collagen fibrils and had abundant rough endoplasmic reticulum with dilated cisternae. Except for their irregular shape and atypical nuclei they resembled actively synthesizing fibroblasts. In addition to these features, some cells contained numerous tonofibrils and occasional well-developed desmosomes. A gradual transition to typical squamous cells was noted in the skin tumor. These findings suggest that the pseudosarcomatous component of SCC originates from mesenchymal metaplasia of squamous cells and that collagen is produced by these metaplastic cells.