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Abstract
Normal human tendon sheath and synovium were studied by scanning and transmission electron microscopy. The lining cells of the two tissues appear to be identical ultrastructurally. The most superficial cells (B-cells) possess long cytoplasmic extensions that clothe the membrane surface. Intermingled with deeper B-cells are the so-called A-cells, which have similar cytoplasmic features but lack long processes and instead have many filopodia. The frequent occurrence of intermediate forms indicates that the two cells form part of a morphologic spectrum. Comparison with cells of tumors that have been ascribed to synovium or tendon sheath (synovial sarcoma, epithelioid sarcoma, clear cell sarcoma) do not reveal any close similarities that might support a histogenetic relationship.