Giant cell tumor of tendon sheath and pigmented villonodular synovitis: an ultrastructural study.

The ultrastructures of five examples of giant cell tumors of the tendon sheath and one example of pigmented villonodular synovitis were studied. The giant cell tumors were composed mostly of A and B types of synovial cells. Also present were fibroblastic cells with intracytoplasmic filaments with densities (myofibroblasts) as well as "monocytic" and "lymphocytic" cells. The giant cells seemingly were derived from the fusion of synovial cells of A type, whereas foam cells were derived from both A and B cells. Both the giant cell tumors and the pigmented synovitis are considered to be reactive and borderline proliferative lesions of the synovial cells.