Function and structure of articular cartilage
What is the function, structure, and composition of articular cartilage?
Articular cartilage is avascular and aneural. It serves as a load-bearing connective tissue that can absorb impact and withstand shearing forces. This ability relates to the unique composition and structure of its extracellular matrix.
Normal cartilage is composed of a sparse population of specialized cells called chondrocytes that are responsible for the synthesis and replenishment of extracellular matrix. This matrix consists mainly of collagen and proteoglycans. Most of the collagen is type II (>90%), which makes up 50% to 60% of the dry weight of cartilage. Collagen forms a fiber network that provides shape, form, and tensile strength to the cartilage tissue.
Proteoglycans comprise the second largest portion of articular cartilage. The proteoglycan monomers (aggrecan) are large (molecular weight = 2–3 million) and contain mostly keratan sulfate and chondroitin sulfate GAGs. The proteoglycans are arranged into supramolecular aggregates consisting of a central hyaluronic acid filament to which multiple proteoglycan monomers (aggrecan) are noncovalently attached and stabilized by a link protein.
The entire structure looks like a large “bottle brush” and has a molecular weight of 200 million. These proteoglycans are stuffed into the collagen framework. The negative charge of the proteoglycans causes them to spread out until the elastic forces are balanced by the tensile forces of the collagen. Note that other collagens (types V, VI, IX, X, XI), proteins (chondroadherin, COMP, cartilage intermediate layer protein or CILP, proline/arginine-rich end leucine-rich repeat protein [PRELP], others), and lipids are also present in cartilage.
Water is the most abundant component of articular cartilage and accounts for 80% of the tissue wet weight. Water is held in cartilage by its interaction with the large matrix proteoglycan aggregates. This retained water is essential for cartilage to resist compression and distribute load.