Proteoglycans are complex macromolecules that consist of a protein core with covalently bound polysaccharide (glycosaminoglycan) chains (Fig. 3).

Figure 3 Schematic diagram of a proteoglycan. The protein core has several globular domains (G1, G2, and G3), with other regions containing the keratan sulfate and chondroitin sulfate glycosaminoglycan chains. The N-terminal G1 domain is able to bind specifically to hyaluronate.This binding is stabilized by link protein. (Reproduced from Mankin HJ, Mow VC, Buckwalter JA, Iannotti JP, Ratcliffe A: Articular cartilage structure, composition, and function, in Buckwalter JA, EinhornTA, Simon SR (eds): Orthopaedic Basic Science: Biology and Biomechanics of the Musculoskeletal System, ed 2. Rosemont, IL, American Academy of Orthopaedic Surgeons, 2000, pp 443-470.)

 

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Collagen is a triple helix protein that is the major structural macromolecule of the ECM.8,9 There are at least 15 distinct collagen types composed of at least 29 genetically distinct chains.

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Water is the most abundant component of normal articular cartilage, making up from 65% to 80% of thewetweight of the tissue.

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Because the chondrocytes of articular cartilage occupy only a small proportion of the total volume of the tissue, the material properties of cartilage are determined primarily by the ECM.

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The formation and maintenance of articular cartilage depends on chondrocytes.1,4,5 These are derived from mesenchymal cells, which differentiate during skeletal morphogenesis.

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Unlike most tissues, articular cartilage does not have blood vessels, nerves, or lymphatics. It consists of a highly organized extracellular matrix (ECM) with a sparse population of highly specialized cells (chondrocytes) distributed throughout the tissue.

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The articular cartilage of synovial joints is subject to high loads applied repetitively for many decades. Thus, the structural molecules, including collagens, proteoglycans, and other molecules of articular cartilage, must be organized into a strong, fatigueresistant, and tough, solid matrix.

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Articular cartilage, the resilient durable tissue that forms the opposing articulating surfaces of synovial joints, provides these surfaces with the lowfriction, lubrication, and wear characteristics that make possible smooth painless movement.

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