Introduction

Articular 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 extracellular 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 macromolecules: 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 immediately surrounding the chondrocytes, enabling them to attach to the macromolecular 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 (aggrecan), 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. 1982; Sommarin et al. 1989; Ramachandrula et al. 1992; Ruoslahti and Reed 1994; Enomoto-Iwamoto et al. 1997; Gonzalez et al. 1993). Further, embedded in the meshwork are small proteoglycans, including decorin, biglycan and fibromodulin. Decorin and fibromodulin both interact with the type II collagen fibrils in the matrix and play a role in fibrillogenesis and interfibril interactions. Biglycan is mainly found in the immediate surrounding of the chondrocytes, where it may interact with collagen type VI (Buckwalter and Mankin 1998; Roughley and Lee 1994). Modulation of the ECM proteins is regulated by an interaction of a diversity of growth factors with the chondrocytes (Jenniskens et al. 2006; Trippel et al. 1989; Isgaard 1992; Hunziker et al. 1994; Sah et al. 1994).