What is the Synovial Membrane?

The functions of the synovial membrane (or synovium) include: - providing lubrication for the joints to limit friction, and acting as a cleaner of the joint cavity - it determines what can come into the joint cavity and what will stay on the outside.

Like other connective tissue it is composed of fatty and fibrous tissue, although it differs from them in that it has a smooth non-adherent surface that permits easy movement between tissues. The surface is permeable to proteins, water and small molecules.

A synovial membrane is a filmy-like material that generally has two layers - the intima (a thin cellular layer) and the subintima (a deeper vascular layer). The intima contains macrophages and fibroblasts. The latter differ from typical fibroblasts in that they create large volumes of hyaluronan - a major component of the extracellular matrix - which passes into the synovial fluid.

This is a clear, colourless, thick and stringy liquid. Many consider it to be similar to egg white, hence the name syn ovum - with egg.

This fluid has two components - the foundation or base consists of a high molecular weight hyaluronate proteoglycin (hyaluronic acid) which contributes to the almost friction-free state. The second component is a cell that maintains the cleanliness of the joint and ingests foreign and potentially harmful substances.

The synovial membranes are subject to disease and injury, and are the major target in rheumatoid arthritis. This is an autoimmune disease in which the body's immune system goes on the attack when there's no threat from pathogens - and in this case strikes at the joints. It affects about 1 in every 100 people. The synovial membrane swells and it produces extra fluid which causes stretching of the ligaments around the joint. The overall result is a stiff, swollen and often painful joint, and it can also lead to damage to ajoining bone, ligaments and tendons.

The synovium is usually only a few cells thick, but this can increase when it becomes thickened and inflamed. When this happens the condition is known as synovitis.

The excess fluid that is released is watery and of low-viscosity; it is therefore pretty useless as a lubricant. Consequently the joint swelling occurs. The knee joint is often affected and is often described as 'water on the knee'.

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