cartilage(redirected from xiphoid cartilage)
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A firm, resilient connective tissue of vertebrates and some invertebrates. Isolated pieces act to provide support and anchor muscles, or with bone to contribute its resilience and interstitial growth to skeletal functions. Cartilage comprises a firm extracellular matrix synthesized by large, ovoid cells (chondrocytes) located in holes called lacunae. The matrix elements are water bound by the high negative charge of extended proteoglycan (protein-polysaccharide) molecules, and a network of fine collagen fibrils. The elements furnish mechanical stability, give, and tensile strength, but allow the diffusion of nutrients and waste to keep the cells alive. See Bone, Collagen
Cartilage is modified in several ways. In elastic cartilage, elastic fibers in the matrix increase resilience, as in cartilages supporting the Eustachian tube, mammalian external ear, and parts of the larynx. Where cartilage joins bones tightly at certain joints with limited mobility, for example, at the pubic symphysis and between vertebrae, the matrix of fibrocartilage contains prominent collagen fibers and has less proteoglycan than the typical hyaline variety. Hyaline cartilage, named for its glassy translucence, is the major support in the airway; and throughout the embryo, pieces of it develop as a precursor to the bony skeleton, except in the face and upper skull. See Ear (vertebrate), Larynx
The primitive cartilaginous skeleton undergoes another modification, by locally calcifying its matrix. At sites of calcification, invading cells destroy the cartilage and mostly replace it by bone, leaving permanent hyaline cartilage only at the joint or articular surfaces, in some ribs, and, until maturity, at growth plates set back from the joints and perpendicular to the long axis of limb bones. The precarious physiological balance between chondrocytes and matrix materials in the heavily loaded articular cartilage breaks down in old age or in inflamed joints. See Connective tissue, Joint (anatomy)
a connective tissue that performs a mechanical (support) function; it is found in all vertebrates, including man, and in some invertebrates, for example, cephalopod mollusks. In cartilaginous fish and cyclostomes the entire skeleton consists of cartilage; in other vertebrates the cartilaginous skeleton occurs only in embryos. In adult mammals, including man, cartilage is preserved in the joint surfaces of bones, in the thoracic ends of the ribs, in the tracheal and bronchial walls, and in the auricle of the external ear. It also is present in the nasal wall, larynx, epiglottis, and eyelids.
Cartilage is formed from the mesenchyma. It is constructed from cells known as chondrocytes and by the intercellular substance elaborated by the cells. The substance consists of collagenous fibers (chondrin) and ground substance. Three types of cartilage—hyaline, elastic, and fibrous—are distinguished according to the characteristics of the intercellular substance. Hyaline cartilage is the most common. Its large quantity of ground substance and the similar values of the refractive index of ground substance and fibrous component determine its external features: homogeneity and glassiness. Elastic cartilage differs from hyaline cartilage in that it has elastic fibers. Fibrous cartilage has bundles of collagenous fibers that can be easily observed under a light microscope.
Cartilage is covered with a membrane of connective tissue, perichondrium, which contains cells capable of changing into chondrocytes. Cartilage grows mainly by such transformation and by the division of cartilage cells (intercalary growth). Cartilage does not have blood vessels; nutrients penetrate it by diffusion.
N. G. KHRUSHCHOV