The number of cell layers that form the synovium varies but usually ranges between two and four cell layers thick (Fig. 1).

Figure 1
Normal synovium consisting of two cell layers of synoviocytes overlying fibrofatty supportive stroma. Note the presence of blood-filled capillaries (arrows) immediately beneath the synoviocytes. (Reproduced with permission from Carpenter CA, Rosenberg AE, Freiberg AA: Synovial conditions of the knee, in JJ Callahan, AG Rosenberg, HE Rubash, PT Simonian, TAWickiewicz (eds): The Adult Knee. Philadelphia, PA, Lippincott-Williams & Wilkins, in press.)

Synoviocytes that cover structures subject to pressure (such as tendons and ligaments) may be widely separated from one another. Loose cellular connections make the synovial lining semipermeable. The topography of the synovial lining consists of multiple villi and microvilli, which results in a huge surface area. The surface area of the synovial tissue in the knee alone is 100 m2. Arich vascular network, adipose tissue, and cells such as fibroblasts, macrophages, and lymphocytes, support the synovium. This subsynovial layer becomes more fibrous with dense bands of collagen as it merges with the underlying joint capsule. Diffusion is essential to the synovium’s role in articular nutrition, lubrication, and the removal of debris. The large surface area and semipermeable nature of the synovium are essential for efficient delivery of nutrients and removal of waste products from the articular chondrocytes. The synovial fluid provides a conduit for this exchange of materials. The articular surface does not have a blood supply or lymphatic system and thus depends on the synovial fluid to deliver and remove metabolites and to lubricate the articular interface to allow frictionless motion.