Synovial sarcoma: a review and update, with emphasis on the ultrastructural characterization of the nonglandular component.

Classic biphasic synovial sarcoma is usually not a problem in identification, whereas the monophasic spindle cell form continues to be a challenge in the differential diagnosis of spindle cell neoplasms. Most synovial sarcomas do not arise from a joint or tendon sheath, and by electron microscopy and immunohistochemistry they differ in several ways from nonneoplastic synovium. The cell of origin of synovial sarcoma is unknown, but certain features are rather consistently observed in the biphasic tumors and are useful in identifying monophasic samples. These features are apparent by immunohistochemistry and electron microscopy, both of which indicate early epithelial differentiation in the nonglandular component of the neoplasm. With immunohistochemistry, some of these cells stain for keratin. By electron microscopy, a gradient of differentiation from unclassifiable spindle cells to fully differentiated epithelial lining cells is demonstrable. A review and illustration of the ultrastructural characteristics in this spectrum of intermediate cells constitute the main emphasis of the article. The cells tend to be oval and polygonal; to be arranged in clusters surrounded by basal lamina or flocculent matrix; to have junctions, including tight junctions, and to form villuslike filopodia, true microvilli, canaliculi, and microlumina. This range of ultrastructural features is usually diagnostic of the nonglandular phase of synovial sarcoma.