Cartilage is a flexible connective tissue found in multiple areas of the body, including joints, the ear and nose, and intervertebral discs. Hyaline cartilage, the most abundant type of cartilage, plays a supportive role and assists in movement.
Cartilage is a robust and viscoelastic connective tissue that can be found in joints between bones, the rib cage, intervertebral discs, the ear, and the nose. While more rigid and less flexible than muscle, cartilage is not as stiff as bone. These properties allow cartilage to serve as a support structure for holding tubes open or for proper locomotion. Examples of tubes include the cricoid cartilage and carina of the trachea, the torus tubarius at the opening of the auditory tube, and the auricle/pinna of the ear.
Chondrogenesis is the process by which cartilage is formed from condensed mesenchymal cells expressing collagen I, III and V. This process also involves differentiation of chondroprogenitor cells secreting molecules (aggrecan and type II, IX and XI collagen) that form the extracellular matrix. The chondroblasts that are caught in the matrix are called chondrocytes, and are the main type of specialized cells found in cartilage. Chondrocytes are responsible for producing large quantities of collagenous extracellular matrix and ground substance that is rich in proteoglycans and elastin fibers. It is important to note for clinical purposes that the division of cells within cartilage is a very slow process, and cartilage growth consists of immature cartilage developing into a more mature state. Moreover, cartilage has a very slow turnover and is difficult to repair due to the fact that cartilage tissue is avascular (and also aneural). Its growth is not usually quantified by an increase in size or mass of the cartilage itself, but instead by its biomechanical properties.
Cartilage is classified as 3 types: elastic cartilage, hyaline cartilage, and fibrocartilage. Each type has varying amounts of elastin and collagen. Articular cartilage is specifically the smooth, white tissue covering the ends of bones where they come together to form joints. Healthy articular cartilage allows joints to move and glide over each other with very little friction, but is subject to damage and injury, as well as normal wear and tear. The function of articular cartilage is dependent on the molecular composition of the extracellular matrix (ECM), which consists mainly of proteoglycans and collagen. As mentioned previously, the main proteoglycan in cartilage is aggrecan, which forms large aggregates with hyaluronan and are negatively charged to hold water in the tissue. The collagen (mainly type II), acts to constrain the proteoglycans and helps it hold its structure. Consequently, the ECM functions to respond to the tensile, shear, and compressive forces that are experienced by cartilage during mechanical use such as normal gait or weight-bearing movements. Additionally, a glycoprotein known as lubricin that is abundant in the superficial layer of cartilage and synovial fluid plays a major role in bio-lubrication and wear protection of cartilage.
The layers of articular cartilage are defined by zones. Starting from the subchondral bone, there is a tidemark that is deep to the basal layer and separates true articular cartilage from the deeper cartilage, which is a remnant of cartilage anlage from longitudinal growth during childhood. The deep, basal layer is found next, which consists of type II collagen that is perpendicular to joint and crosses the tidemark. This basal layer also contains the highest concentration of proteoglycans, and round chondrocytes in this layer are arranged in columns. The intermediate zone is adjacent to the basal layer, with an oblique or random organization of type II collagen. This zone is the thickest layer of round chondrocytes, with abundant proteoglycan content. Finally, there is most superficial layer, also known as the tangential zone. This layer has type II collagen that is oriented in parallel to the joint. Instead of round chondrocytes, the superficial layer contains flattened chondrocytes, in addition to condensed collagen fibers and spare proteoglycans. This zone is also the only zone where articular cartilage progenitor cells have been identified.