The synovial membrane (or synovium) is a thin connective tissue that lines the joint, tendon sheaths and bursae to enclose a cavity within the knee joint called the synovial cavity. The synovial membrane regulates substance passage into the cavity and secretes synovial fluid that lubricates the joint, according to About.com.
According to Orthopaedics One, the synovium consists of two types of cells: fibroblasts and macrophages. The fibroblast cells produce hyaluronic acid, which increases viscosity and elasticity of cartilages, and lubricin, which reduces friction between the joint and mediates cell growth. The macrophages are responsible for removing microbes and debris resulting from wear and tear of the joint. Pathologies such as rheumatoid arthritis result when the synovium is irritated or thickened.