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Cartilage // is a flexible connective tissue in animals, including the joints between bones, the rib cage, the ear, the nose, the bronchial tubes and the intervertebral discs. It is not as hard and rigid as bone, but it is stiffer and less flexible than muscle.
Because of its rigidity, cartilage often serves the purpose of holding tubes open in an animal's body. Examples include the rings of the trachea, such as the cricoid cartilage and carina, the torus tubarius at the opening of the pharyngotympanic/auditory tube, the ala of the nostrils, and the auricle/pinna of the ear.
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 in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in relative amounts of cartilage. Chondroblasts that get caught in the matrix are called chondrocytes.
Cartilage does not contain blood vessels. The chondrocytes are supplied by diffusion. The compression of the articular cartilage or flexion of the elastic cartilage generates the pumping action, which assists the diffusion of the chondrocytes. Thus, compared to other connective tissues, cartilage grows and repairs more slowly.
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.
Following the initial chondrification that occurs during embryogenesis, cartilage growth consists mostly of the maturing of immature cartilage to a more mature state. The division of cells within cartilage occurs very slowly, and thus growth in cartilage is usually not based on an increase in size or mass of the cartilage itself. The articular cartilage function is dependent on the molecular composition of the extracellular matrix (ECM). The ECM consists mainly of proteoglycans and collagens. The remodeling of cartilage is predominantly affected by changes and rearrangements of the collagen matrix. The collagen matrix responds to tensile and compressive forces that are experienced by the cartilage. Cartilage growth thus refers to the matrix deposition, but can also refer to both the growth and remodeling of the extracellular matrix.
Cartilage does not absorb x-rays under normal In vivo conditions, but a dye can be injected into the synovial membrane that will cause the x-rays to 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 are enough to contrast the presence of cartilage due to the refraction of the x-ray.
Several diseases can affect cartilage. Chondrodystrophies are a group of diseases, characterized by the disturbance of growth and subsequent ossification of cartilage. Some common diseases that affect the cartilage are listed below.
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.
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 cartilage damage 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.
The mechanical properties of articular cartilage in load bearing joints such as knee and hip have been studied extensively at macro, micro and nano-scales. These mechanical properties include the response of cartilage in frictional, compressive, shear and tensile loading. Cartilage displays viscoelastic properties.