The synovium or synovial membrane is a thin, weak layer of tissue which lines the non-cartilaginous surfaces within the joint space, sealing it from the surrounding tissue. The membrane contains a fibrous outer layer, as well as an inner layer that is responsible for the production of specific components of synovial fluid, which nourishes and lubricates the joint. The membrane is also responsible for the removal of undesirable substances from the synovial fluid. Small villi can sometimes be found on its surface.
It can become irritated and thickened in conditions such as rheumatoid arthritis. When this happens, the synovium can become more of a problem for the joint, and sometimes must be removed.