A spontaneous multicystic peritoneal mesothelioma was detected in a 108-wk-old male Fischer-344/DuCrj rat. Grossly, a tumor containing numerous, semitransparent, variously sized, thin-walled cysts was found on the splenic serosal surface. Microscopically, each cyst was surrounded by variable amounts of loose connective tissue. The luminal and free surfaces of the cysts were covered by a single layer of flattened or cuboidal mesothelial cells. No metastases were detected in any of the other organs. Diagnosis was supported by immunohistochemistry and ultrastructural features.
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