The proposed organization of proteins and glycoproteins within the extracellular matrix of articular cartilage is shown below.
There are three classic types of cartilage: articular, fibrous, and elastic. Articular cartilage, also known as hyaline cartilage, is found in freely movable synovial joints such as the knee. Its function is to provide lubrication, absorb and distribute compressive load, and withstand shear stress during joint movement. Articular cartilage consists of one cell type, articular chondrocytes, and the extracellular matrix provided by these cells. The matrix is comprised primarily of collagen, proteoglycans, and water molecules. Collagens are the most abundant protein family in articular cartilage. Collagen II forms fibrils that provide tensile strength throughout the extracellular matrix. Collagen VI is thought to form hexagonal networks in close proximity to cells where it can be linked to Collagen II fibrils via Matrilin-4 and Biglycan. Proteoglycans are the major non-collagenous proteins found in articular cartilage. The most abundant is Aggrecan, which is modified with chains of chondroitin sulfate and keratan sulfate, and interacts with Hyaluronan. The chondroitin sulfate and keratan sulfate chains bind water molecules and the resulting hydrated Aggrecan provides lubrication within the joint and contributes to the load-bearing capacity of articular cartilage. Small leucine-rich proteins/proteoglycans (SLRPs), including Fibromodulin, Decorin, Biglycan, PRELP, and Chondroadherin, are also important for proper articular cartilage function. All SLRP family members are able to bind fibril-forming collagens. Chondroadherin and PRELP are also able to bind members of the Syndecan family of cell surface proteoglycans. Degeneration of articular cartilage accompanies several arthropathies including osteoarthritis.