The origin of phagocytic cells in the joint and bone.

Studies dealing with the concept of the mononuclear phagocyte system are described, and the origin of synovial type A cells and osteoclasts is discussed in some detail. All of the available evidence indicates that both derive from precursor cells in the bone marrow. It seems justifiable to conclude that monocytes transform into type A synovial cells and fuse to form osteoclasts.