Epithelioid sarcoma: ultrastructural similarity to nodular synovitis

The ultrastructure of nodular synovitis of the knee and epithelioid sarcoma of the hand are compared. Both lesions show a similar pattern of light and dark cells having filopodia and microvilli, an outer coat of finely granular matrix without well defined basal laminae, maculae adherentes and attachment sites, pinocytotic vesicles, cytoplasmic filaments and complex nuclear invaginations. These similarities suggest a common histogenesis and support the concept that the epithelioid sarcoma is derived from synovioblastic mesenchyme.