Traditionally, diffuse epithelial mesotheliomas are mainly identified at the ultrastructural level by the numerous, long, wavy-appearing surface microvilli. By electron microscopy of a series of diffuse mesotheliomas of varying subtype (epithelial, biphasic, sarcomatous, and poorly differentiated), it can be demonstrated that the differentiation of this specialized surface organelle is quite variable even in well-differentiated lesions. The presence of only a few, scattered, short microvilli does not exclude a diagnosis of epithelial mesothelioma, particularly if historical, surgical, and radiologic findings support this diagnostic conclusion. Indeed, even the complete absence of surface microvilli is compatible with a diagnosis of diffuse epithelial mesothelioma. It is important to become aware of the spectrum of tumor cell differentiation in serosal tumors, as all of the fine structural diagnostic criteria in mesotheliomas are expressed to varying degrees in individual cases.

In differentiating diffuse epithelial mesothelioma from metastatic adenocarcinoma in pleural and peritoneal biopsies, the number and form of microvilli and the amount and distribution of tonofilaments are thought to be the most useful criteria. This report details 5 cases of diffuse epithelial mesothelioma in which the characteristic fine structural features of neoplastic mesothelial cells were markedly modified. The majority or all tumor cells were poorly differentiated in electron micrographs, particularly with reduced prominence or absence of intermediate filaments, desmosomes, intracytoplasmic lumina, and microvilli. Immunohistochemistry revealed the absence of carcinoembryonic antigen and the presence of cytokeratin in all cases. Comparison with a better differentiated case suggests cytologic details that are useful in distinguishing the poorly differentiated type of epithelial mesotheliomas from adenocarcinoma. These include a mosaic pattern of closely associated tumor cells with a few long, narrow cytoplasmic processes lying parallel to adjacent plasma membranes, abundant cytoplasm with limited organelles usually having a polar arrangement, and nuclei with markedly disaggregated chromatin and prominent nucleolonemal-type nuclei.

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