Chordoid sarcoma ultrastructural evidence supporting a synovial origin

A case of chordoid sarcoma which presented as a painful mass on the dorsum of the right hand in a 72-year-old man was studied by light and electron microscopy. The tumor was composed of lobules separated by thin connective-tissue bands. At the periphery of the lobules, the cells were epithelioid in appearacne and there were numerous nests of clear cells. From the periphery, stellate and spindle shaped cells were radially oriented and formed a loose cobweb arrangement in an abundant loose ground substance. Distinctive ultrastructural features included pseudoacinar formation, microvilli, long cytoplasmic filopodia (often with complex interdigitations), numerous desmosomes, partial basal lamina, scanty endoplasmic reticula, and numerous intracytoplasmic filaments 80--100 A in width. We believe these ultrastructural features indicate a synovial origin of this uncommon neoplasm.