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The fibrocytic derivation of the so‐called epithelioid sarcoma

Tumor cells of three examples of epithelioid sarcoma reveal ultrastructural features characteristically found in neoplastic and, to a lesser extent, nonneoplastic fibrocytes. Their appearance is unlike that observed in rhabdomyosarcoma, liposarcoma, and neurogenic and other neoplasms with which it might be confused. The conclusion that the epithelioid sarcoma represents a fibrosarcoma does not exclude a topographic origin from synovium. Yet, if the epithelioid sarcoma is synovial it would represent either an atavistic, fibrocytic form of synovioma or neoplasm of less‐differentiated fibrocytes of this structure since its cells lack pronounced cytoplasmic fenestratin, elongated filopodia, and surface deposits regarded by some as characteristic of synovioblasts. Further, pseudoacinar formation and histochemically demonstrable acid mycopolysaccharide—of the hyaluronic acid type—features also attributed to synovioblasts—were lacking in the epitheliod sarcoma. On the other hand, desmosomal attachment plates, structures not observed in synovioblasts, were noted in one epithelioid sarcoma studied.