The synovium or synovial membrane is a tissue that lines the inside of the joints, it is the deep surface of the joint capsule. It is composed of areolar connective tissue containing elastic fibres. It secretes synovial fluid which lubricates and nourishes the cartilage. It therefore has mechanical functions but also defends against infection and inflammation. In many synovial joints, the synovial membrane accumulates fatty tissue called adipose bodies. The infrapatellar fat body of the knee is an example. The cells of the synovial membrane, the synoviocytes, are not frequently visualised in the synovial fluid, but they play a primordial role in the function of the hemosynovial barrier. They are thought to be the first to phagocytose the crystals and trigger the inflammatory process. The synovial membrane can be inflamed in the case of arthritis. In general, the inflammation of the synovial membrane is called synovitis. This can be caused by an intra-articular infection (septic arthritis), lupus erythematosus, inflammatory arthritis (such as rheumatoid arthritis), gout.