What is Cartilage?

Cartilage has two main components: water and matrix, and until it decreases with age, roughly 85 percent is the former substance. The remaining 15 percent is the extracellular matrix, a complex network of proteins such as collagen and proteoglycans that regulate functions such as cell adhesion, cellular growth, and metabolism. Collagen provides cartilage with its tensile strength while the proteoglycans give cartilage its elastic properties. Chondrocytes are usually the only cells in healthy cartilage; they are responsible for producing and maintaining the matrix. Cartilage lacks blood supply, nerves, or a lymphatic system.

Each type of cartilage is in a different area of the human body and provides a unique but similar purpose. Elastic or yellow cartilage contains many yellow elastic fibers within the matrix. The chondrocytes exist between the threadlike network of the fibers. Because of these fibers, elastic cartilage has a high degree of flexibility. This allows the cartilage to resist bending while still providing support and shape. Elastic cartilage forms parts of the body such as the outer area of the ear and the epiglottis in the throat.

Hyaline cartilage is translucent and exists on the ribs, larynx, trachea, and bronchi. It also serves as articular cartilage on the ends of bones that form synovial joints. Type II collagen and chondroitin sulfate are the primary components of hyaline cartilage. These also make up a portion of elastic cartilage, giving hyaline and elastic cartilage some histological similarities. Hyaline cartilage is the most common and prevalent type in the human body.

The articular surface is an area of a bone that normally makes direct contact with another skeletal structure as part of a synovial joint. Articular cartilage lines these areas to ensure the joint has a smooth, low friction environment along which to move. Additionally, articular cartilage acts as a shock absorber within the joint. Areas that receive greater stress typically have thicker articular cartilage. For example, the patella, also known as the kneecap, can have the thickest area of articular cartilage.

Compared to elastic and hyaline cartilage, fibrocartilage has some unique features. It is the only type of cartilage that possesses both type I and type II collagen. Fibrocartilage consists of a combination of white fibrous tissue and typical cartilaginous tissue. Because of this, fibrocartilage is relatively inflexible and strong. Many areas of the body contain varying amounts of fibrocartilage. For example, fibrocartilage serves as part of the transition gradient between entheses -- regions where tendons, ligaments, or joint capsules connect to bones. Other important fibrocartilage structures are the menisci, temporomandibular joints, and the pubic symphysis.

Cartilage begins developing during the embryonic stage. As an embryo develops into a fetus, its skeletal system forms from the mesoderm layer of the embryo. This layer slowly creates a fluid of mesenchymal cells, serous fluids, and a variety of tissue proteins. The serous fluid contains elements such as sodium and chloride. The mesenchymal cells become condensed mesenchyme tissue, which in turn becomes chondrocytes. These chondrocytes then secrete the molecules that form the extracellular matrix. During fetal development, the majority of the skeleton consists of cartilage. Eventually, bones replace much of the cartilage in the skeleton.

Because cartilage typically appears in areas of constant movement and stress, it is prone to damage. Direct and heavy trauma such as a car accident or a fall from a great height can damage cartilage. Athletes, particularly those involved in impact sports, are significantly more likely to have cartilage damage than others. However, sedentary lifestyles can also result in cartilage damage because joints require constant movement to remain healthy. Finally, basic wear and tear from long periods of stress commonly cause cartilage damage. Factors such as obesity or age can cause additional stress.

Cartilage damage in the joints from wear and tear can eventually lead to the most common form of arthritis, osteoarthritis. This disorder can affect any joint, but the most commonly affected areas are the hands, knees, hips, and spine. Because cartilage has a limited capacity for self-repair, osteoarthritis is typically irreversible. Treatment instead focuses on limiting and managing the symptoms. Usually, doctors recommend a combination of nonsteroidal anti-inflammatory drugs and exercises to maintain an active, healthy lifestyle.

Osteoarthritis and cartilage damage typically have similar symptoms. Inflammation around the affected area is common. Pain and soreness typically accompany the inflammation. A person may also have significant difficulty moving a joint with cartilage damage, either due to general stiffness or a limitation of the joint’s range of movement. In severe instances, pieces of cartilage break and cause the joint to lock. This can lead to bleeding in the joint. Osteoarthritis may eventually result in bone spurs.

Though cartilage cannot repair itself, some modern procedures may improve damaged cartilage. In autologous chondrocyte implantation, technicians take chondrocyte samples and then use bioengineering techniques to increase their number. They then implant the samples into an area of damaged cartilage. The chondrocytes grow into hyaline-like cartilage that resembles the native joint. Other procedures include debridement to smooth the damaged area and marrow stimulation to force the development of new cartilage. Some surgeons may perform a mosaicplasty, transplanting healthy cartilage to the damaged area.