Scanning electron micrograph (SEM) of rheumatoid arthritis in the synovial membrane lining a joint, showing the type of cells that accumulate on the membrane surface. These cells have many projections (visible here) known as microvilli. Magnification: X 960 at 35mm size. Rheumatoid arthritis occurs when the function of the synovial membrane, which forms the lining enclosing freely moveable joints such as the elbows & ankles, fails. The cause is not known, although the effects are clearly seen. The membrane thickens, becomes inelastic and begins to encroach into the free spaces of the joint, impairing articulation. The normally smooth & folded membrane develops fingerlike microvilli which, in turn, become.
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