ABSTRACT. A subcutaneous malignant fibrous histiocytoma (MFH) was observed in the region between the right posterior trunk and right hind limb of a 2-year-old male Djungarian hamster weighing 45 g. Histologically, the tumor consisted of bizarre multinucleated giant cells, histiocytic cells, and fibroblastic cells with a storiform pattern, and was considered to be of the storiform-pleomorphic type of MFH. Severe nuclear atypia with prominent nucleoli and many mitotic figures was also observed. Electron microscopy demonstrated fibroblastic cells and histiocytic cells. The fibroblastic cells were spindle-shaped, and sometimes had an invaginated nucleus. The histiocytic cells were polygonal with an oval or kidney-shaped nucleus. The cytoplasm of both cells contained numerous free ribosomes, small amounts of rough endoplasmic reticulum, and round mitochondria. Tumor cells were immunohistochemically positive for vimentin, and were thought to be of undifferentiated mesencymal cell origin. This is the first report of spontaneous MFH in a hamster.-KEY WORDS: Djungarian hamster, malignant fibrous histiocytoma.Malignant fibrous histiocytoma (MFH) is a malignant soft tissue tumor that frequently occurs in humans [4,21]. Among animals, MFH is seen mainly in cats, but rarely in other animal species [2,12]. Although experimental induction of MFH was previously reported in hamsters [2], there have been no published reports describing a spontaneous MFH in Djungarian hamsters. The present study reports a case of a subcutaneous MFH detected in a Djungarian hamster.A tumor mass measuring 4 × 3 × 2.5 cm was found between the right posterior trunk and right posterior limb of a 2-year-old male Djungarian hamster weighing 45 g that had been maintained at a home in the Tokyo metropolitan area. Because the tumor had become enlarged, it was surgically resected a week later. On preoperative X-ray examination, the tumor was localized in the subcutis and was not associated with the skeleton. The excised tumor was fixed in 10% buffered formalin, and paraffin sections prepared conventionally were stained with hematoxylin and eosin (HE), Masson-trichrome, Watanabe's silver impregnation method, alcian blue-PAS, and Van Gieson. Immunohistochemical evaluation was performed by the streptavidin-biotin-peroxidase complex (SAB-PO) method using the following antibodies: anti-human desmin, antihuman actin, anti-human vimentin, anti-α1-antitrypsin, anti-α1-antichymotrypsin, anti-lysozyme, anti-S-100 protein (Nichirei, Tokyo). For electron microscopy, a part of the formalin-fixed tumor tissue was fixed again with 1% glutaraldehyde•4% formalin solution, and postfixed with a buffered fixative containing 1% osmium tetroxide and 0.1 M cacodylic acid. Ultrathin sections were prepared after embedding in Epon, and were stained with uranyl acetate and lead citrate.Macroscopically, the tumor mass was 4 × 3 × 2.5 cm in size, and the cut surface was grayish white, with the white spongy center and dark red margin.Histologically, the tumor consisted predominantly of f...
