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Hyaline cartilage is a type of body tissue, also called gristle. It is shiny, slippery, firm, translucent, and bluish-gray in color. It is simple in structure, with no nerves or blood vessels. It has high elasticity and helps cushion and protect bones. The word hyaline comes from the Greek for glassy, and refers to the translucence of the tissue.
This cartilage is one of the three main types of cartilage in the body, the others being elastic cartilage and fibrocartilage. Cartilage in turn is a form of connective tissue, which also includes bone, blood, and fat. Cartilage is stronger and more rigid than muscle, but more flexible than bone.
Hyaline cartilage appears on the ends of bones where they form joints, between the ribs and the sternum or breastplate, in the trachea and bronchii of the lungs, and in the ear and the larynx or voice box. It is also the forerunner of skeletal bones in the fetus. Hyaline cartilage becomes bone in a process called endochondral ossification. In the ear, hyaline cartilage helps to absorb loud sounds.
Hyaline cartilage, like elastic cartilage, is usually lined with perichardium, a layer of irregular connective tissue that aids in the growth and repair of cartilage. All types of cartilage gain most of their physical properties from the extracellular matrix, the material surrounding the cells, than from the cells themselves. The matrix of this type of cartilage is rich in type II collagen and proteoglycans, which contribute to its elasticity.
Cartilage is characterized by lacunae, literally "windows," in the matrix, in which the tissue cells are located. Each lacuna may hold from one, two, four, or eight cells. The fibers of the extracellular matrix form concentric lines around the lacunae. The cells, called chrondrocytes, are responsible for producing the extracellular matrix.
Cartilage does not grow as fast or as easily as some other body tissues because it does not have a blood supply. In addition, the chrondrocytes are unable to move outside of the lacunae to reach damaged areas. Damaged hyaline cartilage is often replaced by scar tissue consisting of the tougher and less flexible fibrocartilage, which can impair joint performance.