A Spontaneous Myxoid Liposarcoma in a Crj:CD(SD)IGS Rat

A very uncommon myxoid liposarcoma was found in a 41-week-old Crj:CD(SD)IGS male rat. At necropsy, an encapsulated elastic mass was observed in the subcutis of the left cervical region. The cut-surface was whitish, translucent and myxomatous with focal hemorrhage. No metastasis to other tissues or organs was noted. Histopathologically, the tumor was composed of numerous capillaries, round to oval cells and spindle cells with abundant myxoid matrix. Most tumor cells contained vacuoles of various size in their cytoplasm. These vacuoles were positive for oil red-O staining. Immunohistochemically, tumor cells were positive for S-100 protein and vimentin, but negative for cytokeratin. Ultrastructurally, tumor cells contained a large number and various sizes of lipid droplets, rough endoplasmic reticulum, and small numbers of smooth endoplasmic reticulum, Golgi complexes, and mitochondria. Thus the tumor cells were considered as lipoblasts at various differential stages. We therefore diagnosed this tumor as a myxoid liposarcoma, the tumor being similar to myxoid liposarcoma in humans. This is the first case report of such a myxoid liposarcoma in rats including Crj:CD(SD)IGS rats.