Abstract
Articular cartilage is a thin layer of hyaline cartilage that covers the surface of the bones of diarthrodial joints. It is an avascular, alymphatic and aneural tissue, with a smooth opalescent appearance. Cartilage is a highly organised and specialised tissue allowing free articulation, painless movement and transmission of force through the skeleton. Compared to other tissues, articular cartilage has a low rate of metabolic activity (Pearle et al. 2005 ). The tissue is maintained by a single specialised cell, the chondrocyte (Buckwalter et al. 1999 ), and is comprised of a highly organised matrix with a large extracellular matrix (ECM) to cell volume ratio. The basic structure is composed of a 3D collagen scaffold and aggregating proteoglycans (Jeffery et al. 1991 ). The arrangement, direction and location of these collagen fibrils vary, along with the cell density, matrix composition and overall thickness throughout the tissue, providing different mechanical properties across the joint. The composition of the ECM reflects its mechanical properties such as tensile strength (mainly collagens type II, IX, and XI) and compressive stiffness (such as proteoglycans and aggrecan). Small proteoglycans, including decorin, biglycan and fibromodulin, bind to other matrix macromolecules and thereby help to stabilise the matrix (Buckwalter and Mankin 1998a ). Additionally, collagen type VI and non- collagenous proteins, such as anchorin CII, tenascin and fi bronectin, are important mediators of cell–matrix interactions (Poole et al. 2001 ). The ECM acts as a signal transducer for the chondrocytes, creating mechanical, electrical and physicochemical signals that help to direct the synthetic and degradative activity of chondrocytes (Buckwalter and Mankin 1998a ). Articular cartilage is divided into four zones: the superficial (tangential), transitional, radial and calcifi ed (Eyre 2002 ). The physical and biochemical differences between the zones are important to allow the cartilage to resist both extrinsic and intrinsic forces due to mechanical stress and swelling in the proteoglycan- rich areas (Knudson and Knudson 2001 ). Cartilage tissue contains a large proportion of water (65–80 % by wet weight). Chondrocytes comprise approximately 5–10 % of the tissue total volume and collagens form 10–30 %, whilst proteoglycans and other molecules consist of 5–10 % of the tissue wet weight (Eyre 2002 ; Archer et al. 2003a ; Bhosale and Richardson 2008 ; Hunziker et al. 2007 ).