Mesotheliomas and metastatic adenocarcinomas involving the pleura are frequently difficult to distinguish by light-microscopic and histochemical methods. In a double-blind study, we have compared ultrastructural features of 10 mesotheliomas of epithelial type and 10 adenocarcinomas from the lung, breast, and upper GI tract, i.e., sites known to give rise to metastases which mimic mesothelioma. Mesotheliomas were observed to have a significantly greater microvillus length/diameter ratio (LDR) than adenocarcinomas (p 0.01) and more abundant intermediate filaments (p 0.001). Mesotheliomas had more complex microvilli than adenocarcinomas, whereas adenocarcinomas had rootlets (2/10 cases) and lamellar inclusion bodies (2/10 cases), both of which were absent in the mesotheliomas. This study provides quantitative and qualitative ultrastructural features of potential utility in the differential diagnosis of pleural mesotheliomas and adenocarcinomas.
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