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Cartilage

Cartilage
Light micrograph of undecalcified hyaline cartilage showing chondrocytes and organelles, lacunae and matrix.
Identifiers
MeSHD002356
TA98A02.0.00.005
TA2381
Anatomical terminology

Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage,[1] and is a structural component of many body parts including the rib cage, the neck and the bronchial tubes, and the intervertebral discs. In other taxa, such as chondrichthyans and cyclostomes, it constitutes a much greater proportion of the skeleton.[2] It is not as hard and rigid as bone, but it is much stiffer and much less flexible than muscle. The matrix of cartilage is made up of glycosaminoglycans, proteoglycans, collagen fibers and, sometimes, elastin. It usually grows quicker than bone.

Because of its rigidity, cartilage often serves the purpose of holding tubes open in the body. Examples include the rings of the trachea, such as the cricoid cartilage and carina.

Cartilage is composed of specialized cells called chondrocytes that produce a large amount of collagenous extracellular matrix, abundant ground substance that is rich in proteoglycan and elastin fibers. Cartilage is classified into three types — elastic cartilage, hyaline cartilage, and fibrocartilage — which differ in their relative amounts of collagen and proteoglycan.

As cartilage does not contain blood vessels or nerves, it is insensitive. However, some fibrocartilage such as the meniscus of the knee has partial blood supply. Nutrition is supplied to the chondrocytes by diffusion. The compression of the articular cartilage or flexion of the elastic cartilage generates fluid flow, which assists the diffusion of nutrients to the chondrocytes. Compared to other connective tissues, cartilage has a very slow turnover of its extracellular matrix and is documented to repair at only a very slow rate relative to other tissues.

There are three different types of cartilage: elastic (A), hyaline (B), and fibrous (C). In elastic cartilage, the cells are closer together creating less intercellular space. Elastic cartilage is found in the external ear flaps and in parts of the larynx. Hyaline cartilage has fewer cells than elastic cartilage; there is more intercellular space. Hyaline cartilage is found in the nose, ears, trachea, parts of the larynx, and smaller respiratory tubes. Fibrous cartilage has the fewest cells so it has the most intercellular space. Fibrous cartilage is found in the spine and the menisci.
  1. ^ Sophia Fox, AJ; Bedi, A; Rodeo, SA (November 2009). "The basic science of articular cartilage: structure, composition, and function". Sports Health. 1 (6): 461–8. doi:10.1177/1941738109350438. PMC 3445147. PMID 23015907.
  2. ^ de Buffrénil, Vivian; de Ricqlès, Armand J; Zylberberg, Louise; Padian, Kevin; Laurin, Michel; Quilhac, Alexandra (2021). Vertebrate skeletal histology and paleohistology (Firstiton ed.). Boca Raton, FL: CRC Press. pp. xii + 825. ISBN 978-1351189576.

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