Cartilage

Histological appearance of hyaline cartilage under partially polarized light.

Cartilage is a type of dense connective tissue. It is composed of specialized cells called chondrocytes that produce a large amount of extracellular matrix composed of collagen fibers, abundant ground substance rich in proteoglycan, and elastin fibers. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in the relative amounts of these three main components.

Cartilage is found in many areas in the body including the articular surface of the bones, the rib cage, the ear, the nose, the bronchial tubes and the intervertebral discs. Its mechanical properties are intermediate between bone and dense connective tissue like tendon.

Unlike other connective tissues, cartilage does not contain blood vessels. The chondrocytes are fed by diffusion, helped by the pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Thus, compared to other connective tissues, cartilage grows and repairs more slowly.

Types of cartilage

Hyaline cartilage

Hyaline cartilage is a rather hard, translucent material rich in collagen and proteoglycan. It covers the end of bone(s) to form the smooth articular surface of joints. It is also found in the nose, the larynx and between the ribs and the sternum. Bones grow via a hyaline cartilage intermediate, a process called Endochondral ossification.

Elastic cartilage

Elastic cartilage contains large amounts of elastic fibers (elastin) scattered throughout the matrix. It is stiff yet elastic, and is important to prevent tubular structures from collapsing. Elastic cartilage is found in the pinna of the ear, in tubular structures such as the auditory (Eustachian) tubes and in the epiglottis.

Fibrocartilage

Fibrocartilage is the most common form of cartilage by weight. It is characterized by a dense network of Type I collagen. It is a white, very tough material that provides high tensile strength and support. It contains more collagen and less proteoglycan than hyaline cartilage. Thus, its properties are closer to those of tendon than hyaline cartilage. It is present in areas most subject to frequent stress like intervertebral discs, the symphysis pubis and the attachments of certain tendons and ligaments.

Growth and development

Main article: Chondrogenesis

In embryogenesis, the skeletal system is derived from the mesoderm germ layer. Chondrification (also known as chondrogenesis) is the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondrocytes and begins secreting the molecules that form the extracellular matrix.

Diseases and treatment

Main article: Chondropathy

There are several diseases which can affect the cartilage. Chondrodystrophies are a group of diseases characterized by disturbance of growth and subsequent ossification of cartilage. Some common diseases affecting/involving the cartilage are listed below.

• Osteoarthritis: The cartilage covering bones (articular cartilage) is thinned, eventually completely worn out, resulting in a "bone against bone" joint, reduced motion and pain. Osteoarthritis is very common, affects the joints exposed to high stress and is therefore considered the result of "wear and tear" rather than a true disease. It is treated by Arthroplasty, the replacement of the joint by a synthetic joint made of titanium and teflon. Chondroitin sulfate, a monomer of the polysaccharide portion of proteoglycan, has been shown to reduce the symptoms of osteoarthritis, possibly by increasing the synthesis of the extracellular matrix.
• Traumatic rupture or detachment: The cartilage in the knee is frequently damaged, and can be partially repaired through knee cartilage replacement therapy
• Achondroplasia: Reduced proliferation of chondrocytes in the epiphyseal plate of long bones during infancy and childhood, resulting in dwarfism.
• Costochondritis: Inflammation of cartilage in the ribs, causing chest pain.
• Spinal disc herniation : Asymmetrical compression of an intervertebral disc ruptures the sac-like disc, causing a herniation of its soft content. The hernia compresses the adjacent nerves and causes back pain.
• Relapsing polychondritis: a destruction, probably autoimmune, of cartilage, especially of the nose and ears, causing disfiguration. Death occurs by suffocation as the larynx loses its rigidity and collapses.

Tumors made up of cartilage tissue, either benign or malignant, can occur. They usually appear in bone, rarely in pre-existing cartilage. The benign tumors are called chondroma, the malignant ones chondrosarcoma. Tumors arising from other tissues may also produce a cartilage-like matrix, the best known being pleomorphic adenoma of the salivary glands.

The matrix of cartilage acts as a barrier, preventing the entry of lymphocytes or diffusion of immunoglobulins. This property allows for the transplantation of cartilage from one individual to another without fear of tissue rejection.

Repair

Cartilage has limited repair capabilities, because chondrocytes are bound in lacunae, they cannot migrate to damaged areas. Therefore if damaged, it is difficult to heal. Also, because hyaline cartilage does not have a blood supply, the deposition of new matrix is slow. Damaged hyaline cartilage is usually replaced by fibrocartilage scar tissue. Over the last years, surgeons and scientist have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement..

Bioengineering techniques are being developed to generate new cartilage, using a cellular "scaffolding" material and cultured cells to grow artificial cartilage

Cartilage in animals

Cartilaginous fish

Cartilaginous fish (chondrichthyes) like sharks, rays and skates have a skeleton composed entirely of cartilage. Shark cartilage is a popular but unproven dietary supplement.

Invertebrate cartilage

Cartilage tissue can also be found among invertebrates such as horseshoe crabs, marine snails, and cephalopods.

See also

• Connective tissue
• Biomechanics
• Shark cartilage

References

General references

External links

• University of Kansas Medical Center Cartilage tutorial
• Public domain text from Gray's anatomy
• I've heard 'Ears and nose do not ever stop growing.' Is this false?
• Cartilage Health - Information on Articular Cartilage Injury Prevention, Repair and Rehabilitation