The origin of type A synovial lining cells.

Type A synovial lining cells resemble macrophages in their ultrastructure. Type B lining cells, in contrast, resemble tissue fixed synthetic cells such as fibroblasts. Although cells with intermediate features are well recognised, recent evidence suggests that types A and B cells have different origins. Experiments on mouse radiation chimeras suggest a bone marrow derivation for the type A cells, but not for the type B. Esterase activity, surface Ia, and immunoglobulin receptors have been demonstrated in relation to a proportion of lining cells. It seems reasonable at the present time to consider type A lining cells as belonging to the Mononuclear Phagocyte System.