Diffuse epithelial mesothelioma: a review of the ultrastructural spectrum.

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.