cartilage is a tough, resilient material, found in various sites but especially important in large weight-bearing joints as it lines the articulating bone ends. Due to its thickness and elasticity it is thought to act as a ‘shock-absorber’, cushioning the impact of movement. In the knee there is an extra layer of cartilage separating the bone ends (meniscus), presumably because of the amount of mechanical stress this joint is subjected to. These can be torn by rotational injuries, particularly in football and rugby players, a condition commonly referred to as ‘torn cartilage’. It is sometimes possible to repair the meniscus by ‘key-hole surgery’ (arthroscopy) which avoids having to open the joint; if the damage is too great then the meniscus is removed (menisectomy), usually by arthroscopy. Wear and thinning of articular cartilage in the knee and hip is associated with the development of osteoarthritis. Unlike most tissues in the body, cartilage has no blood vessels within it (it is ‘avascular’) and relies on getting its nutrients, essential for the continued well-being of the cells within the cartilage matrix (chondrocytes), from the thin film of fluid lining the joint cavity (synovial fluid). This fluid is derived from the blood supplying the joint capsule, and its rapid turnover is important for keeping the chondrocytes supplied with oxygen and other essential substances. Cartilage is also found near the ends of long bones (epiphysis) in children, where it plays an important role in longitudinal bone growth after birth. New chondrocytes are generated, thickening the epiphysis producing lengthwise growth, whilst the cartilage matrix ‘left behind’ acquires mineral deposits and forms bone. Cartilage is also found in other sites such as the nose, ears, and larynx (externally visible as the ‘Adam's apple’ in males), where it provides lightweight support or flexibility.

William R. Ferrell

Cartilage

Cartilage is a fibrous and rubbery connective tissue found throughout the vertebrate skeletal system. As with other connective tissues, the general function of cartilage is to support and connect different parts of the body. Connective tissues originate from cells in the embryonic mesoderm, the middle layer of embryonic tissue.

Cartilage is made up of specialized cartilage cells called chondrocytes, which are suspended in an acellular matrix made up largely of a protein called collagen. All connective tissues have a matrix , and in the case of cartilage, the matrix is solid. A protective membrane named the perichondrium covers the surface of the cartilage and gives the subtance a shiny, cloudy-white appearance.

Early in development, cartilage makes up most of the vertebrate skeleton. As an individual grows older, calcium deposits form around the skeleton, and bone eventually replaces most of the cartilage. This process is called ossification. Ossification begins in humans when the fetus is still in the womb and is not complete until early adulthood. The skeleton of a young child tends to be less brittle than that of an adult because a certain amount of cartilage is still present.

This cartilage-to-bone conversion occurs in all vertebrates except for sharks, rays, and skates. These related "cartilaginous fishes" maintain a completely cartilaginous skeleton through adulthood. Cartilage is also found in branchiostomates such as tunicates, sea squirts, and lancelets , the closest relatives of the vertebrates. These animals have a cartilaginous rod called a notochord, which runs along the length of their back.

Cartilage is softer, more compressible, and more elastic than bone. In vertebrates whose skeletons do undergo ossification, cartilage is maintained in certain areas of the body that require this flexibility. Adults have cartilage in joints, in the nose, ears, breastbone, trachea, and larynx, and at the ends of bones.

Cartilage also helps to reduce friction between the bony elements of a joint. A lubricating liquid called synovial fluid helps the cartilage-covered bones of the shoulder slide over each other more easily. Cartilage found in joints with a large range of motion is called smooth cartilage. In joints that experience more limited motion, cartilage plays a different role. In this kind of joint, the cartilage that holds the bones together is called elastic cartilage. Immovable joints are held together by fibrous cartilage.

see also Bone; Skeletons.

Judy P. Sheen

cartilage (gristle) A firm flexible connective tissue that forms the adult skeleton of cartilaginous fish (e.g. sharks). In other vertebrates cartilage forms the skeleton of the embryo, being largely replaced by bone in mature animals (although it persists in certain areas). Cartilage comprises a matrix consisting chiefly of a glycosaminoglycan (mucopolysaccharide) called chondroitin sulphate secreted by cells (chondroblasts) that become embedded in the matrix as chondrocytes. It also contains collagenous and elastic fibres. Hyaline cartilage consists largely of glycosaminoglycan, giving it a shiny glasslike appearance; this type of cartilage gives flexibility and support at the joints. Fibrocartilage, in which bundles of collagen fibres predominate, is stronger and less elastic than hyaline cartilage; it is found in such areas as the intervertebral discs. Elastic cartilage has a yellow appearance due to the presence of numerous elastic fibres (see elastin). This cartilage maintains the shape of certain organs, such as the pinna of the ear.

car·ti·lage / ˈkärtl-ij/ • n. firm, whitish, flexible connective tissue found in various forms in the larynx and respiratory tract, in structures such as the external ear, and in the articulating surfaces of joints. ∎  a particular structure made of this tissue.DERIVATIVES: car·ti·lag·i·noid / ˌkärtlˈajəˌnoid/ adj.

cartilage (kar-til-ij) n. a dense connective tissue, consisting chiefly of chondroitin sulphate, that is capable of withstanding considerable pressure. In the fetus and infant cartilage occurs in many parts of the body, but most of this cartilage disappears during development. Cartilage is the precursor of bone following a fracture (see callus). elastic c. cartilage occurring in the external ear. fibrocartilage cartilage occurring in the intervertebral discs and tendons. hyaline c. cartilage found in the costal cartilages, larynx, trachea, bronchi, nose, and covering the surface of the bones at joints.