Clinical Correlation: Osteoarthritis constitutes the bulk of the fibrils (see Fig. 7.2); type IX collagen facilitates fibril interaction with the matrix proteoglycan molecules; type XI collagen regulates the fibril size; and type X collagen organizes the collagen fibrils into a threedimensional hexagonal lattice that is  crucial to its successful mechanical function. In addition, type VI collagen is also found in the matrix, mainly at the periphery of the chondrocytes where it helps to attach these cells to the matrix framework. Because types II, VI, IX, X, and XI are found in significant amounts only in the cartilage matrix, they are referred to as cartilage-specific collagen molecules. (Review the types of collagen in Table 6.2.)  Proteoglycans. The ground substance of hyaline cartilage contains three kinds of glycosaminoglycans: hyaluronan, chondroitin sulfate, and keratan  sulfate. As in loose connective tissue matrix, the chondroitin and keratan sulfate of the cartilage matrix are joined to a core protein to form a proteoglycan monomer. The most important proteoglycan monomer in hyaline cartilage is aggrecan. It has a molecular weight of 250 kilodaltons. Each molecule contains about 100 chondroitin sulfate chains and as many as 60 keratan sulfate molecules. Because of the presence of the sulfate groups, aggrecan molecules have a large negative charge with an affinity for water molecules. Each linear hyaluronan molecule is associated with a large number of aggrecan molecules (more than 300), which are bound to the hyaluronan by link proteins at the N terminus of the molecule to form large proteoglycan aggregates. These highly charged proteoglycan aggregates are bound to the collagen matrix fibrils by electrostatic interactions and multiadhesive glycoproteins (Fig. 7.3). The entrapment of these aggregates within the intricate matrix of collagen fibrils is responsible for the unique biomechanical properties of hyaline cartilage.
Cartilage matrix also contains other proteoglycans (i.e., decorin, biglycan, and fibromodulin). These proteoglycans do not form aggregates but bind to other molecules and help stabilize the matrix.
 Multiadhesive glycoproteins, also referred to as noncollagenous and nonproteoglycan-linked glycoproteins, influence interactions between the chondrocytes and the matrix molecules. Multiadhesive glycoproteins have clinical value as markers of cartilage turnover and degeneration.
Examples of such proteins are anchorin CII (cartilage annexin V), a small 34-kilodalton molecule that functions as a collagen receptor on chondrocytes, tenascin, and fibronectin (see Table 6.5, page 180), which also help anchor chondrocytes to the matrix.
FIGURE 7.3 • Molecular structure of hyaline cartilage matrix. This schematic diagram shows the relationship of proteoglycan aggregates to type II collagen fibrils and chondrocytes in the matrix of hyaline cartilage. A hyaluronan molecule forming a linear aggregate with many proteoglycan monomers is interwoven with a network of collagen fibrils. The proteoglycan monomer (such as aggrecan) consists of approximately 180 glycosaminoglycans joined to a core protein. The end of the core protein contains a hyaluronan-binding region that is joined to the hyaluronan by a link protein.
Hyaline cartilage matrix is highly hydrated to provide resilience and diffusion of small metabolites.