Naturally occurring atriocaval mesotheliomas in rats.

Atriocaval mesotheliomas were observed in two control Sprague-Dawley rats out of a total of 2889 rats examined. Both neoplasms were located at the base of the interventricular septum. Microscopically, the neoplasms consisted of varying proportions of glandular structures and a fibrous tissue stroma. Glandular structures were lined by a single layer of cuboidal to low columnar epithelium with cells containing a moderate amount of eosinophilic fibrillar to granular cytoplasm and a pleomorphic vesicular round nucleus with coarsely clumped chromatin and often a single eccentric prominent nucleolus. Epithelial cells were stained with antibodies raised against both vimentin and cytokeratin. Electron microscopical features included the presence of microvilli, terminal bars composed of modified tight junctions and desmosomes at the lateral and apical borders, and numerous tonofilaments associated with desmosomes.