Development of Cartilage

Developmentally, cartilage can be followed histologically through three general phases [57, 58]. Prior to cell differentiation, it is a cellular condensation form which is known as cartilage condensation [58]. These cells proliferate and secrete the cartilage-specific matrix. The ECM calcifies as the cells enter the hypertrophic phase of differentiation. This calcified cartilage matrix is then replaced by bone [58].

Classifications of Cartilage

Cartilage is classified in three types, hyaline cartilage, fibrocartilage, and elastic cartilage, which differ in relative amounts of collagen and proteoglycan. The most abundant type is hyaline cartilage which is the typical cartilage that is generally referred to when one thinks of cartilage. It is widely distributed in the human fetus. Most of the fetal hyaline cartilage is replaced by bone, but some persists in the articular surface of bones, in the larynx, in the trachea, in the bronchi and on the ventral ends of ribs of the adult. Hyaline cartilage matrix is mostly made up of type

II collagen and chondroitin sulfate [59]. Fibrocartilage is a tough and inflexible form of cartilage, which makes up the intervertebral discs, the pubic symphysis, and appears in other high-stress areas. Fibrocartilage contains type I and II collagen [60]. Elastic can be found in the pinna of the ear, larynx, and epiglottis. Elastic cartilage is similar to hyamine cartilage in many respects, but has less matrix than hyaline cartilage. It shows expression of elastin in addition to type II collagen, which provides greater flexibility and thus have the ability to withstand repeated bending [61].

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