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The Proteoglycans

Proteoglycans consist of small amounts of protein bound to negatively charged polysaccharide chains referred to as GAGs. In articular cartilage, proteoglycans form a large portion of the macromolecular framework (commonly about 30% to 35% of the tissue dry weight), but in ligaments they form only a small portion of this framework, usually less than 1% of the dry weight. Nonetheless, proteoglycans may have an important role in organizing the extracellular matrix and in interacting with the tissue fluid.

Like tendon, meniscus, and articular cartilage, ligaments contain two known classes of proteoglycans: large articular cartilage type proteoglycans containing long, negatively charged chains of chondroitin and keratin sulfate (syndecan) and smaller proteoglycans that contain dermatan sulfate.

Because of their long chains of negative charges, the articular cartilage type proteoglycans tend to expand to their maximum domain until restrained by the collagen fibril network. As a result, they maintain water within the tissue, alter fluid flow within the tissue during loading, and exert a swelling pressure, thereby contributing to the mechanical properties of the tissue and filling the regions between collagen fibrils.

Small dermatan sulfate proteoglycans (decorin and biglycan) usually lie on the surface of collagen fibrils and seem to affect the formation, organization, and stability of the extracellular matrix, including collagen fibril formation and diameter. These molecules may also affect the ability of mesenchymal cells to repair ligament injuries. They can inhibit fibroblast adhesion to other matrix macromolecules (especially the noncollagenous protein fibronectin) and thereby may limit the ability of the cells to bind to the matrix and form new tissue.

The concentration of GAGs present in rabbit knee ligaments differs significantly from that present in tendinous tissue. The ACL has the highest proportion of GAGs, two to four times the amount observed in tendons. Although the functional importance of these differences is unknown, it is clear that the higher the GAG content, the more water that is associated with the complex.

This naturally alters the viscoelastic properties of these tissues and may represent an additional “shock-absorbing” feature in ligaments (optimized in the cruciate ligaments) that is less important in tendon.

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