What Is the Synovium?

The synovium, also called the synovial membrane, is the soft tissue that lines the spaces of diarthrodial joints, tendon sheaths, and bursae.

The synovium is where synovial fluid is produced, the substance that lubricates and nourishes the cartilage and bones inside the joint capsule.

Synovium Anatomy

The synovium is a special type of connective tissue located in articulated joints like the knees and elbows. The synovium lines the entire inner surface of the joint, except where the joint is lined with cartilage. The synovium is made up of an outer layer (subintima) and an inner layer (intima).

The subintima consists of intra-articular (within the joint) vessels, such as blood vessels and lymphatic vessels, and nerves. The cells of the intima are called synoviocytes. There are two types of synoviocytes, type A (macrophage-derived) and type B (fibroblast-derived). Underneath the layer of synoviocytes, there is either adipose tissue or fibrous tissue.

Synovium Function

In the joints, the synovium's purpose is the production of synovial fluid and the maintainance of the volume of fluid inside the joint. It also acts as a seal around the joint cavity to keep the fluid inside.

The synovium and cartilage are both parts of the joint. The synovium, however, produces fluid that helps protect the cartilage, while the cartilage helps protect and cushion the bones.

Synovial Fluid

The synovial fluid is the thick liquid produced by the synovium. Synovial fluid provides cushion for your joints and helps keep them lubricated. This is what protects the cartilage in your joints from friction damage.

Loss of synovial fluid can have a number of causes, including rheumatoid arthritis, normal aging, and lack of exercise.

The Synovium in Rheumatoid Arthritis

Like many other rheumatic diseases, rheumatoid arthritis is an autoimmune disease. In an autoimmune disease or condition, a person's immune system, which normally helps protect the body from infection and disease, attacks their own joint tissues for unknown reasons. In rheumatoid arthritis, immune system cells travel to the synovium and initiate inflammation (synovitis). Synovitis is caused by the proliferation of synovial cells, increased vascularization, and the infiltration of tissue by inflammatory cells, including lymphocytes, plasma cells, and activated macrophages. This manifests itself as typical symptoms of rheumatoid arthritis—warmth, redness, swelling, and pain.

As rheumatoid arthritis progresses, the inflamed synovium invades and destroys the cartilage and bone of the joint. The surrounding muscles, ligaments, and tendons that support and stabilize the joint become weak and unable to work normally. These effects lead to the joint pain and joint damage typically seen in people with rheumatoid arthritis. Understanding what happens to synovium in rheumatoid arthritis helps you to understand symptoms and disease severity.

Treatments to Target Inflamed Synovium - Are They Coming?

Researchers have been interested in developing tissue-specific treatments for rheumatoid arthritis. Potential drugs could target the synovium with increased efficacy and decreased systemic toxicity. If research into this process succeeds, imaging agents could be delivered directly to the synovium, allowing for an assessment of active synovitis in multiple joints. Although advancements have been made in this area, a specific synovial receptor has yet to be discovered.