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Anatomical Structure and Function:
Cartilage is a firm but flexible tissue, suited for its primary roles to reduce friction at moveable joint surfaces, serve as a shock-absorber and provide flexibility to some skeletal elements (e.g., external ear and nose). Due to its unique properties, it also plays an important role in support of vertebrate tissue. Like other connective tissues, cartilage has few cells and large quantities of extracellular matrix, mainly collagens and proteoglycans. However, it differs from other connective tissues in the absence of nerves or blood vessels in its matrix, slowing its rate of growth and repair. As with any connective tissue, the nature and organization of cartilage’s component building blocks, account for its structural and functional properties. The cells and collagen-embedded matrix form a firm material that resists tension, compression, and shear.
Chondrogenesis, or the formation of cartilage, is a dynamic cellular process involving mesenchymal cell recruitment and migration, condensation of progenitor cells, and chondrocyte differentiation which leads to the establishment of various types of cartilage, including hyaline, fibrous, and elastic cartilage. The forming cartilage is contributed by three different embryonic cell populations. In the head region, the cranial neural crest cells (CNCs) account for most of the cartilaginous elements. The cartilage of the appendix region of the limbs and of the axial level of the body, is contributed by the lateral plate mesoderm and the paraxial mesoderm (via the sclerotome), respectively. Cartilage formation is tightly controlled by cellular interactions with the surrounding matrix, growth and differentiation factors, and other environmental factors that initiate or suppress cellular signaling pathways and transcription of specific genes in a temporal-spatial manner. The surrounding tissues, the epithelium in particular, influence the differentiation of mesenchymal progenitor cells to chondrocytes.
Cartilage serves as a precursor to endochondral bone, forming almost the entire embryonic skeleton. Although bone gradually replaces cartilage during further development, cartilage persists until adulthood in the growth plates within bone, that allow lengthen during the growing years. Cartilage also persists throughout life at the lubricated articular surfaces of most limb joints and in the head region and between aligned vertebras of the spine (i.e., intervertebral discs).