Ultrastructure of human malignant diffuse mesothelioma

Eleven cases of malignant diffuse mesotheliomas, histologically classified into two groups, epithelial (5 pleural and 3 peritoneal) and biphasic or mixed (2 pleural and 1 peritoneal) forms, were stuied by electron microscopy to elucidate their ultrastructural characteristics. The neoplastic cells of the epithelial forms were varied in ultrastructure, from well differentiated (marked by polarity, micovilli, glycogen granules, junctional structures, tonofilaments, intracellular vacuoles, and a basement membrane) to poorly differentiated (which lacked some of these epithelial characteristics). In four of eight instances in epithelial type tumors, nonepithelial or mesenchymal neoplastic cells were recognized. The biphasic or mixed cases included three major cell types: epithelial, atypical epithelial, and mesenchymal. It appeared that there were transitional forms among the three cell types. The observations support the concept that the neoplastic cell of malignant mesothelioma can differentiate into a number of cell lines.