Together, the appropriate reflection of secreted lubricants by the synovial membrane and the appropriate lubricant secretion by cells are necessary for development of a mechanically functional SF (Blewis et al., 2007). The synovial matrix provides the permeable pathway through which exchange of molecules occurs (Levick, 1994), but also offers sufficient outflow resistance (Coleman et al., 1998 Scott et al., 1998) to retain large solutes of SF within the joint cavity. The extracellular matrix in these gaps contains collagen types I, III, and V (Ashhurst et al., 1991 Rittig et al., 1992), hyaluronan (Worrall et al., 1991), chondroitin sulphate (Price et al., 1996 Worrall et al., 1994), biglycan and decorin proteoglycans (Coleman et al., 1998), and fibronectin (Poli et al., 2004). The cells in the synovium form a discontinuous layer separated by intercellular gaps of several microns in width (Knight & Levick, 1984 McDonald & Levick, 1988). It is backed by a thicker layer (∼100 µm) of loose connective tissue called the subsynovium (SUB) that includes an extensive system of lymphatics for clearance of transported molecules. The synovium is a thin lining (∼50 µm in humans) comprised of tissue macrophage A cells, fibroblast-like B cells (Athanasou & Quinn, 1991 Revell, 1989 Wilkinson et al., 1992), and fenestrated capillaries (Knight & Levick, 1984). Other cells also secrete PRG4, including chondrocytes in the superficial layer of articular cartilage (Schmid et al., 2001b Schumacher et al., 1994) and, to a much lesser extent, cells in the meniscus (Schumacher et al., 2005).Īs a biochemical depot, SF is an ultra filtrate of blood plasma that is concentrated by virtue of its filtration through the synovial membrane. Synoviocytes secrete PRG4 (Jay et al., 2000 Schumacher et al., 1999) and are the major source of SAPL (Dobbie et al., 1995 Hills & Crawford, 2003 Schwarz & Hills, 1996), as well as HA (Haubeck et al., 1995 Momberger et al., 2005) in SF.

Molecules postulated to play a key role in lubrication alone or in combination, are proteoglycan 4 (PRG4) (Swann et al., 1985) present in SF at a concentration of 0.05–0.35 mg/ml (Schmid et al., 2001), hyaluronan (HA) (Ogston & Stanier, 1953) at 1–4 mg/ml (Mazzucco et al., 2004), and surface-active phospholipids (SAPL) (Schwarz & Hills, 1998) at 0.1 mg/ml (Mazzucco et al., 2004).

SF contains molecules that provide low-friction and low-wear properties to articulating cartilage surfaces. The synovial fluid (SF) of natural joints normally functions as a biological lubricant as well as a biochemical pool through which nutrients and regulatory cytokines traverse. The interaction between highly negatively charged cartilage proteoglycans and type II collagen fibrils is responsible for the compressive and tensile strength of the tissue, which resists applied load in vivo. Three classes of proteins exist in articular cartilage: collagens (mostly type II collagen) proteoglycans (primarily aggrecan) and other noncollagenous proteins (including link protein, fibronectin, COMP – cartilage oligomeric matrix protein) and the smaller proteoglycans (biglycan, decorin, and fibromodulin). Figure 2 sketches a section of the cartilage – a chondrocyte cell that permanently restructures/rebuilds its extracellular matrix. This property is derived from its high water entrapping capacity as well as from the structure and intermolecular interactions among polymeric components that constitute the cartilage tissue (Servaty et al., 2000). At walking, running, or sprinting the strokes frequency attain approximately 0.5, 2.5 or up to 10 Hz.Ĭartilage functions also as a shock absorber. those covering the bone ends in the knee joint – belong to mechanically highly stressed tissues in the human body. In a healthy synovial joint, heads of the bones are encased in a smooth (hyaline) cartilage layer.

Functions of synovial fluid