Unusual ultrastructural features of leiomyosarcoma of the lung

The electron microscopic features of a leiomyosarcoma occurring in the lung, a rare site, were studied. Unusual was the finding of extremely electron-dense bands that interrupted the longitudinally-arranged microfilaments and dense bodies of many of the tumor cells. The bands superficially resembled anomalous Z-bands seen in nemalin myopathy and aging rat cardiac muscle, but have not been described to occur in smooth muscle tumors. They are thought to represent condensations of dense bodies in degenerating tumor cells.