The ability of human joints to function normally depends on the presence of a layer of specialized supporting connective tissue called cartilage. All supporting connective tissues have two major constituents: 1) cells, and 2) extracellular material. Chondrocytes are the cells responsible for cartilage production (Fig.1.4). The integrity of the cartilage and its matrix is absolutely dependent on the viability of chondrocytes.
Chondrocyte maturation is most advanced in the centre of the growing cartilage mass. Upon completion of chondrocyte maturation, cartilage comprises chondrocytes embedded in a large mass of extracellular matrix. As cartilage is largely avascular (i.e. devoid of blood vessels), since chondrocytes produce a chemical that inhibits vascularization, the exchange of nutrients and metabolic waste-products between chondrocytes and surrounding tissues is diffusion-dependent through the aqueous medium contained in the ground substance (i.e. amorphous gel) [7]. Consequently, cartilage layer thickness is limited to that which will maintain the viability of the innermost cells. Furthermore, the avascular nature of cartilage prevents most cartilage from repair and self-regeneration after severe injury. Cartilage is characterized by small clusters of chondrocytes embedded in an amorphous matrix of ground substance that is reinforced by collagen fibres. The clusters are separated by large amounts of cartilage matrix whereas the cells of each cluster are separated by a thin layer of matrix.
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Chondrocytes fully occupy spaces in the matrix, called lacunae, such that each lacuna contains a single chondrocyte [7]. Mature chondrocytes are characterized by small nuclei and glycogen-rich cytoplasm, whereas lipid droplets are a distinguishing feature of larger chondrocytes (Fig. 1.4). These distinctions demonstrate the chondrocytes’ active role in the synthesis and maintenance of the cartilage matrix (i.e. the ground substance and fibrous elements). The extracellular matrix determines the material properties of the tissue [8]. Collagen fibres within the extracellular matrix provide tensile strength, while the combined characteristics of the extracellular fibres and the ground substance give it flexibility and resilience. The ground substance is an amorphous, transparent material that has the properties of a semi-fluid gel. It is composed of long, unbranched chains of proteinated sugars called glycosaminoglycans (GAGs) in the form of hyaluronic acid and proteolglycans [7]. Due to the nature and predominance of the ground substance in the extracellular matrix, cartilage is a semi-rigid form of supporting connective tissue.