Figure 1.

The synovial structure in healthy individuals and in individuals with rheumatoid arthritis (RA). (a) In the healthy joint, the synovial intimal lining consists of a thin cell layer of fibroblast synovial cells (FLSs) and macrophage-like synovial cells (MLSs) that together form a semi-permeable protective barrier. The sublining layer contains interstitial macrophages and fibroblasts, as well as blood vessels. (b) In RA, the macrophage barrier is lost and there is pathological expansion and remodeling of the synovial lining layer and sublining layer leading to synovial hyperplasia. The sublining of the synovium is heavily infiltrated by immune cells and undergoes neovascularization.