Cartilage is a tough elastic tissue that serves as the main type of connective tissue within the body. There are three different types. The first, hyaline cartilage, is the most common. It covers and protects the ends of bones within joints, providing a low-friction articulating surface. Because of this, it is also known as articular cartilage. It is white and semi-transparent in color and is only 2 mm to 4 mm in thickness. An easily observable example outside the human body is the cartilage on the end of a chicken bone.
The second type is elastic cartilage, which is similar to hyaline but has elastic fibers that make it more flexible. Elastic cartilage forms the rigid but bendable auricle, or outer ear.
The third type is fibrous cartilage, which offers increased toughness and strength. It is found in the menisci of the knee, special cartilage pads that help to disperse body weight and reduce friction.
Cartilage is also used to hold anatomical structures, such as the trachea (air tube), open. These structures may be made from more than one type of cartilage.
At a molecular level, cartilage consists of a highly-organized matrix of fibers that forms a solid structure. The molecules that make up cartilage include collagens, proteoglycans, and other proteins. Cartilage is 65% to 80% water, which is attracted to the negatively-charged proteins within the fiber matrix. A small number of cells called chondrocytes produce and maintain this fiber matrix. In adults, these cells making up about 1% to 2% of the tissue volume. Cartilage contains no blood vessels (is avascular), no nerves (is aneural) and no lymphatic system (is alymphatic). Therefore, nutrients and waste must diffuse through the matrix.
This lack of access makes cartilage an interesting material from an engineering perspective. Most other cells within the body have access to capillary beds that provide each cell a fresh flow of nutrients (including oxygen and sugars) and remove waste. Because cartilage is avascular, it does not have an active transport system; nutrients necessarily must travel a much greater distance through diffusion. Limited access to raw materials, fuel, and waste removal, limits the rate of tissue maintenance and repair. Thus, damage to cartilage is slow to heal and in many cases, leads to increased damage rather than restoration (i.e., a positive feedback loop). Flow rates may be higher than that predicted by Fickian diffusion because many of the cartilage structures are stress fluctuations which can have a pumping effect.
