Cartilage – Dense connective tissue that provides the foundation for BONE in the developing fetus and covers bone ends in the joints in adults. The fetal SKELETONforms first as a translucent type of cartilage called hyaline cartilage, with conversion to bone beginning at about the fourth week of pregnancy. Calcification continues after birth. In the adult skeleton, hyaline cartilage forms the disks between the vertebrae in the back, the rings that give the TRACHEA stability, and the extensions that connect the ribs to the sternum. A type of cartilage that contains fibers of elastin that allow greater FLEXIBILITY, called elastic cartilage or yellow cartilage, gives shape to the outer ears (auricles), auditory canal (EAR canal), and end of the NOSE.
Cartilage consists of a thick, somewhat elastic base of collagen (an insoluble protein) with small clusters of cartilage cells (chondrocytes) suspended within it. Though cartilage does not have its own BLOOD or NERVE supplies, it continuously renews itself through a process called remodeling in which chondrocytes facilitate a slow turnover of collagen molecules and other substances. This remodeling provides chondrocytes with the NUTRIENTS they need to function.
Researchers believe imbalances in the remodeling process, in which the breakdown of collagen molecules exceeds rebuilding, is the basis for OSTEOARTHRITIS. Various mechanisms, notably repeated trauma such as through use of the joints and activation of CYTOKINES through the inflammatory process, contribute to this imbalance. Damage to cartilage is the foundation of osteoarthritis, HERNIATED NUCLEUS PULPOSUS, and most KNEE INJURIES.
For further discussion of cartilage within the context of musculoskeletal structure and function, please see the overview section “The Musculoskeletal System.”