Cartilage is a connective tissue found in the skeleton. It is semi-transparent and flexible, yet is also strong and able to provide support. It is made of a gelatinous glyco-protein base called chondroitin, which contains collagen and elastic protein fibers. These components form a matrix which allows for cartilage to be very flexible and also very durable. The cartilage cells, called chondrocytes, lay scattered within this matrix. Cartilage also has an outer covering, a dense, fibrous membrane called the perichondrium. Cartilage is avascular, meaning it lacks blood vessels and therefore does not posses the regenerative abilities seen in bones and other connective tissue types.
In some vertebrates, such as sharks, the skeleton is made up entirely of cartilage. In mammal embryos, the skeleton initially consists of cartilage, although most of it is gradually replaced by bone, through a process known as ossification. There are three different types of cartilage: hyaline, elastic, and fibrocartilage. The type of cartilage is determined by the protein fibers embedded within the cartilage matrix.
Hyaline: In hyaline cartilage, the protein fibres are large and predominantly collagen. It is also commonly encased within a perichondrium. This type of cartilage is the most abundant, and can be found in the nose, trachea, larynx, and bronchi. In the respiratory system, the cartilage is found in C-shaped rings around these tubes, allowing them to remain open. Hyaline cartilage can also can be found on the articular surfaces of bones in synovial joints. It is then known as articular cartilage, the function of which is to reduce friction and absorb shock. Hyaline cartilage also forms the costal cartilage which attaches the ribs to the sternum, and is found in mammalian embryos prior to ossification.
Elastic: Elastic cartilage consists of predominantly elastic fibers embedded in ground substance. It is quite similar to hyaline cartilage, and can also be found within a perichondrium. Elastic cartilage can easily maintain its shape and provide support, while also being extremely flexible. It can be found in the external ear, the epiglottis, and in certain parts of the larynx.
Fibrocartilage: Fibrocartilage consists of fine collagen fibers set in a layered arrangement. These layers posses a more open arrangement than that of hyaline cartilage, which makes fibrocartilage a good shock absorber. It is a very tough tissue, and can be found in the intervertebral discs of the spine, and in the pubic symphysis. It can also be found in ball-and-socket joints, such as those of the shoulder and hip, where it serves to deepen the joint and descrease the likelihood of dislocation.