Synovial tissue, or the synovium, is a specialized connective tissue that lines our joints. This thin membrane works alongside bone, cartilage, and ligaments to facilitate smooth, pain-free movement. The health of the synovial tissue is directly linked to joint mobility and overall comfort.
Where Synovial Tissue is Found
Synovial tissue is located lining the inner surface of synovial joints, which are the most common and movable type in the body, including the knees, hips, and shoulders. The synovium forms a boundary for the joint cavity, ensuring the lubricating fluid it produces is contained. This soft tissue membrane covers the entire inner surface of the joint, except for the areas covered by articular cartilage.
Beyond the major joints, synovial tissue is also found in other structures that require reduced friction. It lines bursae, which are small, fluid-filled sacs that cushion bones and tendons near joints. It is also present in tendon sheaths, which are tube-like structures that surround tendons to protect them from abrasive damage as they move.
Composition and Synovial Fluid Production
The structure of synovial tissue consists of two layers: the intima and the subintima. The intima is the innermost layer, in direct contact with the synovial fluid. The subintima is the outer layer that supports the intima with blood vessels, lymphatic vessels, and nerves. The intima is populated by specialized cells called synoviocytes.
Type A synoviocytes are macrophage-like cells that are part of the immune system. They are responsible for cleaning the joint space by removing debris, such as fragments of cartilage or other waste products. In contrast, Type B synoviocytes are fibroblast-like and are the primary producers of the components that make up synovial fluid.
Synovial fluid begins as an ultrafiltrate of blood plasma, meaning fluid from blood vessels in the subintima passes into the joint space. The Type B synoviocytes then enrich this fluid with specific molecules, most notably hyaluronic acid and lubricin. Hyaluronic acid gives the fluid its viscous, egg-white-like consistency, while lubricin is a protein that aids lubrication. A healthy knee contains approximately 2mL of this clear fluid.
Vital Roles of Synovial Tissue
The primary role of synovial tissue is the lubrication of the articular cartilage surfaces within the joint. This is a direct result of the synovial fluid it produces, which dramatically reduces friction between the bones during movement. This allows for the smooth, gliding motions characteristic of healthy joints and prevents wear and tear.
The synovial membrane also serves as a lifeline for the articular cartilage, which is avascular and lacks its own blood supply. The synovial fluid facilitates the transport of nutrients and oxygen from the blood vessels in the subintima to the cartilage cells. This process is necessary for maintaining the health and integrity of the cartilage.
Concurrently, the synovial tissue is responsible for removing metabolic waste products from the cartilage. The fluid absorbs these waste products and transports them away, where they can be removed by the vascular system in the synovium. The fluid also contributes to the joint’s ability to absorb shock, providing a cushioning effect during physical activities.
Synovial Tissue in Joint Disorders
When synovial tissue becomes inflamed, the condition is known as synovitis. This can lead to symptoms such as joint pain, swelling, warmth, and stiffness. The inflamed synovium may thicken and produce an excess amount of synovial fluid, which contributes to the discomfort in the affected joint.
In autoimmune conditions like rheumatoid arthritis, the synovial membrane is a primary target of the immune system. The resulting chronic inflammation causes the synovial tissue to grow abnormally and form a mass called a pannus. This pannus can invade and destroy both the cartilage and bone within the joint, leading to severe joint damage.
While osteoarthritis is characterized by the breakdown of articular cartilage, the synovial tissue is also frequently involved. The degradation of cartilage can release debris into the joint, triggering a secondary inflammatory response in the synovium. This synovitis can contribute to the pain in osteoarthritis and may accelerate disease progression. In other conditions, such as gout or septic arthritis, the synovium can become inflamed by crystals or infected by bacteria.