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Proteoglycans of Articular Cartilage

Sulfated proteoglycan macromolecules constitute 12% of articular cartilage dry weight. The proteoglycans of articular cartilage serve as the “pump” of the highly pressurized cartilage system. As mentioned earlier, the characteristics of the proteoglycan molecules that permit this crucial function include their very large size and resulting immobility within the collagen fibril meshwork; their densely concentrated, fixed, negative sulfate and carboxyl charges; and the large number of hydroxyl groups contained.

These characteristics collectively serve to attract water and small positively charged ions into the cartilage. Donnan osmotic pressure results from this process.The negative charges on the proteoglycan molecules naturally create repulsive forces between each other, which are termed chemical expansive stresses.

The sum of the Donnan osmotic pressure and the chemical expansive stress constitutes the cartilage swelling pressure . Ogston  noted the rough equivalence of the pressure within articular cartilage with “motor tire pressure”!

The extraordinary size of the proteoglycan aggregate molecules of articular cartilage is achieved by supra-assembly of three different types of linear-chain molecular species: sulfated glycosaminoglycans, a core protein, and hyaluronan (a nonsulfated glycosaminoglycan).

The purpose of this section is to describe briefly the chemical structure of the functionally vital proteoglycan and its proteoglycan aggregate and to illustrate the manner in which the functional role derives from the chemical structure.

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