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We report a biphasic malignant mesothelioma in an aged female F344/DuCrlCrlj rat. Macroscopically, multiple pale brown nodules were observed in the abdominal cavity with retention of bloody ascites. Histopathologically, the tumor cells spread over the peritoneum and formed masses on the surface and underlying adipose tissues. The tumor cells dominantly proliferated in a solid, nodular or nest-like pattern with modest amount of fibrillar connective tissues, which contained hyaluronan. The tumor consisted of ovoid, polygonal or spindle-shaped cells that possessed eosinophilic cytoplasms including glycogen; some tumor cells showed a signet-ring-like structure. Multinucleated cells and mitosis were found frequently, and direct invasion to intra-abdominal organs and intravascular metastasis to the liver were observed. Immunohistochemically, keratin and mesothelin were strongly positive in most of tumor cells, while vimentin was mainly positive in spindle-shaped cells. Podoplanin was also positive, particularly in the cell membrane of tumor cells. Electron microscopically, tumor cells showed an intercellular desmosome-like structure, basement membrane and microvillus. We diagnosed the case as a malignant peritoneal mesothelioma with a sarcomatoid growth pattern and signet-ring-like structure.

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Extraskeletal osteosarcoma is extremely rare in mice. This case report demonstrates a spontaneous murine extraskeletal osteosarcoma that exhibited various histological growth patterns in an ICR mouse. At necropsy, the tumor mass was located in the abdominal wall and was 45 × 30 × 25 mm in size. Histopathologically, the tumor showed the following four growth patterns: a solid pattern of polygonal cells embedded in an osteoid eosinophilic matrix with calcification, an irregular sheet pattern of short spindle cells accompanying some eosinophilic multinucleated cells, a fascicular pattern of spindle cells and a cystic pattern lined by short spindle cells. Immunohistochemically, most of the tumor cells were positive for vimentin, proliferating cell nuclear antigen and osterix. The multinucleated cells mentioned above were desmin positive and were regarded as regenerative striated muscles but not tumor cells. Since no clear continuity with normal bone tissues was observed, the tumor was diagnosed as an “extraskeletal osteosarcoma.”