Mitochondria as a feature of smooth muscle differentiation: a study of 70 smooth muscle tumors.

We report that the presence of numerous mitochondria is an ultrastructural feature of smooth muscle neoplasms which is diagnostically useful as a marker for smooth muscle differentiation. The number of mitochondria, as well as the usual features of smooth muscle differentiation, were studied in 70 smooth muscle neoplasms from a variety of body sites. The tumors were sub-classified according to the number of mitochondria (in the majority of the tumor cells) as sparse, moderate, abundant or packed. Thirty-one percent of the leiomyomas had sparse mitochondria and 69% had a moderate number of mitochondria. Seven percent of the leiomyoblastomas contained a moderate number of mitochondria, 33% contained abundant mitochondria and 60% were packed with mitochondria. Twelve percent of the leiomyosarcomas showed sparse mitochondria, 67% showed a moderate number of mitochondria, and 21% had abundant mitochondria. For the above tumor types, the cells with abundant and packed mitochondria contained few or no visible filaments, and these cells were often round or polygonal. By contrast, adjacent spindle cells often contained numerous filaments including dense bodies. The recognition of mitochondria as a feature of smooth muscle differentiation is diagnostically useful in tumor samples showing numerous mitochondria and a paucity of filaments and dense bodies.