Sarcomatous type of malignant mesothelioma.

Thirteen malignant mesotheliomas of a sarcomatous type were studied by light microscopy and ten were studied by electron microscopy. The histologic patterns varied from tumor to tumor, often resembling other soft tissue sarcomas. Electron microscopic observation showed most of the tumors to be composed of primitive cells. Despite their mesenchymal character, many tumors contained foci of rudimentary epithelial differentiation. It is concluded that both histologic types of malignant mesothelioma, the epithelial as well as the sarcomatous, originate from the same precursor cell at various points of its differentiation and reflect the range of maturation from the mesenchymal reserve cell to the epithelial mesothelial cell.