Fibronexus in low-grade myofibrosarcoma: a case report.

Fibronexus (fibronexus junction) has been thought to be a characteristic ultrastructural feature of myofibroblasts, but it is controversial as to whether fibronexus is a characteristic of various myofibroblastic tumors. We report here a case of low-grade myofibrosarcoma with fibronexus arising in the right arm of an 80-year-old man. Histologically, the tumor was composed of relatively uniform and slender spindle cells arranged in fascicles. The nuclei with fusiform and tapered shapes were mildly hyperchromatic, but never exhibited pleomorphism. Mitotic figures were common, but no atypical mitosis was identified. At the tumor periphery, tumor cells had invaded into the surrounding skeletal muscle tissue. Tumor cells were positive diffusely for alpha-smooth muscle actin and less intensely for desmin, but were negative for h-caldesmon and S-100 protein. Ultrastructurally, tumor cells had well developed cytoplasmic organelles and varying amounts of peripheral or subplasmalemmal bundles of thin myofilaments with focal density. In addition, well formed, long fibronectin fibrils adjacent to the cell surface and fibronexus contacting intracellular myofilaments were easily identified. We believe that fibronexus is a useful ultrastructural feature for differentiating myofibrosarcoma from other myogenic sarcomas.