A subcutaneous tumor was found in the right abdomen of a 16-week-old male TCRβ and p53
double-knockout mouse. The tumor had indistinct borders with the surrounding tissue. The
cut surface after formalin fixation was pale yellowish white, partially dark red and
partly white. Histologically, the tumor was composed of three distinct regions. The first
region showed pleomorphic cells arranged in sheets. The second region showed spindle cells
arranged in interlacing fascicles. The final region contained a mixture of the above
mentioned two types of cells. Furthermore, a small amount of collagen fibers, round cells,
multinucleated giant cells, and cells with eosinophilic granules were observed between
these tumor cells. Immunohistochemical examination and electron microscopy identified that
the pleomorphic cells and spindle cells were histiocytes and fibroblasts, respectively,
and that the round cells were undifferentiated mesenchymal cells. Based on these findings,
the tumor was diagnosed as a malignant fibrous histiocytoma