Diverse Roles of Integrin Receptors in Articular Cartilage

1 Introduction Ar ticular cartilage is a specialised connective tissue with unique biological and mechanical properties which depend on the structural design of the tissue and the interactions between its unique resident cells, the chondrocytes, and the extrac- lular matrix (ECM) that makes up the bulk of the tissue (Buckwalter and Mankin 1998). Chondrocytes (Fig. 1 ) are the architects of the ECM (Muir 1995), building the macromolecular framework of the ECM from three distinct classes of mac- molecules: collagens, proteoglycans, and noncollagenous proteins. Of the collagens present in articular cartilage, collagens type II, IX, and XI form a fibrillar meshwork that gives cartilage tensile stiffness and strength (Eyre 2004; Buckwalter and Mankin 1998; Kuettner et al. 1991), whereas collagen type VI forms part of the matrix im- diately surrounding the chondrocytes, enabling them to attach to the macro- lecular framework of the ECM and acting as a transducer of biomechanical and biochemical signals in the articular cartilage (Guilak et al. 2006; Roughley and Lee 1994). Embedded in the collagen mesh are large aggregating proteoglycans (agg- can), which give cartilage its stiffness to compression, its resilience and contribute to its long-term durability (Dudhia 2005; Kiani et al. 2002; Luo et al. 2000; Roughley and Lee 1994). The extracellular matrix proteins in cartilage are of great significance for the regulation of the cell behaviour, proliferation, differentiation and morphogenesis (Kosher et al. 1973; Kosher and Church 1975; von der Mark et al. 1977; Hewitt et al.