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|File:Hypertrophic Zone of Epiphyseal Plate.jpg|
|Hyaline cartilage showing chondrocytes and organelles, lacunae and matrix|
Cartilage [ˈkaːrtələdʒ] is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs. It is not as hard and rigid as bone but is stiffer and less flexible than muscle.
Cartilage is composed of specialized cells called chondroblasts that produce a large amount of extracellular matrix composed of Type II collagen (except fibrocartilage which also contains type I collagen) fibers, abundant ground substance rich in proteoglycan, and elastin fibers. Chondroblasts that get caught in the matrix are called chondrocytes. They lie in spaces, called lacunae, up to eight chondrocytes per lacuna. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in the relative amounts of these three main components.
Unlike other connective tissues, cartilage does not contain blood vessels. Because of this, it heals very slowly. The chondrocytes are supplied by diffusion, helped by the pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Thus, compared to other connective tissues, cartilage grows and repairs more slowly.
Growth and development
In embryogenesis, the skeletal system is derived from the mesoderm germ layer. Chondrification (also known as chondrogenesis) is the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondroblasts and begins secreting the molecules that form the extracellular matrix.
Cartilage does not absorb x-rays under normal In vivo conditions, but a dye can be injected into the synovial membrane so the x-rays will be absorbed by the dye. The resulting void on the radiographic film between the bone and meniscus represents the cartilage. For In vitro x-ray scans the outer soft tissue is most likely removed so the cartilage and air boundary is enough to contrast the presence of cartilage due to refraction of the x-ray. 
Diseases and treatment
There are several diseases which can affect the cartilage. Chondrodystrophies are a group of diseases characterized by disturbance of growth and subsequent ossification of cartilage. Some common diseases affecting/involving the cartilage are listed below.
- Osteoarthritis: The cartilage covering bones (articular cartilage - a subset of hyaline cartilage) is thinned, eventually completely worn out, resulting in a "bone against bone" joint, reduced motion and pain. Osteoarthritis affects the joints exposed to high stress and is therefore considered the result of "wear and tear" rather than a true disease. It is treated by Arthroplasty, the replacement of the joint by a synthetic joint often made of a Stainless Steel alloy (cobalt chromoly) and High Molecular Weight Polyethylene (HMWPE). Chondroitin sulfate, a monomer of the polysaccharide portion of proteoglycan, has been claimed to reduce the symptoms of osteoarthritis, possibly by increasing the synthesis of the extracellular matrix, but recent research has not produced evidence to support this claim 
- Traumatic rupture or detachment: The cartilage in the knee is frequently damaged, and can be partially repaired through knee cartilage replacement therapy
- Achondroplasia: Reduced proliferation of chondrocytes in the epiphyseal plate of long bones during infancy and childhood, resulting in dwarfism.
- Costochondritis: Inflammation of cartilage in the ribs, causing chest pain.
- Spinal disc herniation : Asymmetrical compression of an intervertebral disc ruptures the sac-like disc, causing a herniation of its soft content. The hernia often compresses the adjacent nerves and causes back pain.
- Relapsing polychondritis: a destruction, probably autoimmune, of cartilage, especially of the nose and ears, causing disfiguration. Death occurs by suffocation as the larynx loses its rigidity and collapses.
Tumors made up of cartilage tissue, either benign or malignant, can occur. They usually appear in bone, rarely in pre-existing cartilage. The benign tumors are called chondroma, the malignant ones chondrosarcoma. Tumors arising from other tissues may also produce a cartilage-like matrix, the best known being pleomorphic adenoma of the salivary glands. Conversely, chondrostatin, an ingredient of cartilage, is being investigated by Washington University researchers for its potential ability to shrink breast and musculoskeletal tumors.
The matrix of cartilage acts as a barrier, preventing the entry of lymphocytes or diffusion of immunoglobulins. This property allows for the transplantation of cartilage from one individual to another without fear of tissue rejection.
Cartilage has limited repair capabilities: Because chondrocytes are bound in lacunae, they cannot migrate to damaged areas. Therefore if damaged, it is difficult to heal. Also, because hyaline cartilage does not have a blood supply, the deposition of new matrix is slow. Damaged hyaline cartilage is usually replaced by fibrocartilage scar tissue. Over the last years, surgeons and scientists have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement.
Cartilage in animals
- "Supplements for osteoarthritis 'do not work'". BBC News. 16 September 2010. http://www.bbc.co.uk/news/health-11330747.
- International Cartilage Repair Society ICRS
- General references
- Keller-Peck, C. (2008). Vertebrate Histology, ZOOL 400. Boise State University.
- Cartilage.org, International Cartilage Repair Society
- KUMC.edu, Cartilage tutorial, University of Kansas Medical Center
- Bartleby.com, text from Gray's anatomy
- MadSci.org, I've heard 'Ears and nose do not ever stop growing.' Is this false?
- CartilageHealth.com, Information on Articular Cartilage Injury Prevention, Repair and Rehabilitation
- About.com, Osteoarthritis
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