There are three types of cartilage. The major type is hyaline (articular) cartilage and its principal proteins are listed in Table 1. Type II collagen is the major protein component (Figure 1), and a number of other minor types of collagen are also present. In addition to these components, the second type, elastic cartilage, contains elastin, and the third, fibroelastic cartilage, contains type I collagen. Cartilage contains a number of proteoglycans, which play an important role in its compressibility. Aggrecan (about 2 × 103 kDa) is the major proteoglycan. As shown in Figure 2, it has a very complex structure, containing several GAGs (hyaluronic acid, chondroitin sulfate, and keratan sulfate) and both link and core proteins. The core protein contains three domains: A, B, and C. Hyaluronic acid binds noncovalently to domain A of the core protein as well as to the link protein, which stabilizes the hyaluronate–core protein interactions. The keratan sulfate chains are located in domain B, whereas the chondroitin sulfate chains are located in domain C; both of these types of GAGs are bound covalently to the core protein. The core protein also contains both O- and N-linked oligosaccharide chains.
Table1. The Principal Proteins Found in Cartilage
Fig1. Schematic representation of the molecular organization in the cartilage matrix. Link proteins noncovalently bind the core protein (red) of proteoglycans to the linear hyaluronic acid molecules (gray). The chondroitin sulfate side chains of the proteoglycan electrostatically bind to the collagen fibrils, forming a cross-linked matrix. The oval outlines the area enlarged in the lower part of the figure. (Reproduced with permission from Mescher AL: Junqueira’s Basic Histology Text and Atlas, 16th ed. New York, NY: McGraw Hill; 2021.)
Fig2. Schematic diagram of aggrecan. A strand of hyaluronic acid is shown on the left. The core protein (about 210 kDa) has three major domains. Domain A, at its amino-terminal end, interacts with approximately five repeating disaccharides in hyaluronate. The link protein interacts with both hyaluronate and domain A, stabilizing their interactions. Approximately 30 keratan sulfate chains are attached, via GalNAc-Ser linkages, to domain B. Domain C contains about 100 chondroitin sulfate chains attached via Gal-Gal-Xyl-Ser linkages and about 40 O-linked oligosaccharide chains. One or more N-linked glycan chains are also found near the carboxyl terminal of the core protein. (Reproduced with permission from Moran LA, Scrimgeour KG, Horton HR, et al: Biochemistry, 2nd ed. Upper Saddle River, NJ: Pearson Hall; 1994.)
The other proteoglycans found in cartilage have simpler structures than aggrecan. Chondronectin is involved in the attachment of type II collagen to chondrocytes (the cells in cartilage).
Cartilage is an avascular tissue and obtains most of its nutrients from synovial fluid. It exhibits slow but continuous turnover. Various proteases (eg, collagenases and stromelysin) synthesized by chondrocytes can degrade collagen and the other proteins found in cartilage. Interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α) appear to stimulate the pro duction of such proteases, whereas TGF-β and insulin-like growth factor 1 (IGF-I) generally exert an anabolic influence on the tissue.
