Cartilage

Cartilage is a flexible connective tissue in animals, including the joints between bones, the rib cage, the ear, the nose, the bronchial tubes and the intervertebral discs. It is not as hard and rigid as bone, but it is stiffer and less flexible than muscle.

Because of its rigidity, cartilage often serves the purpose of holding tubes open in an animal's body. Examples include the rings of the trachea, such as the cricoid cartilage and carina, the torus tubarius at the opening of the pharyngotympanic/auditory tube, the ala of the nostrils, and the auricle/pinna of the ear.

Cartilage is composed of specialized cells called chondrocytes that produce a large amount of collagenous extracellular matrix, abundant ground substance that is rich in proteoglycan and elastin fibers. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in relative amounts of cartilage.^[1] Chondroblasts that get caught in the matrix are called chondrocytes.

Cartilage does not contain blood vessels (avascular) or nerves (aneural). The chondrocytes are supplied by diffusion. The compression of the articular cartilage or flexion of the elastic cartilage generates the pumping action, which assists the diffusion to the chondrocytes. Compared to other connective tissues, cartilage has a very slow turnover and is not easy to repair.

Structure

Development

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 chondroblasts and begins secreting the molecules (aggrecan and collagen type II) that form the extracellular matrix.

Following the initial chondrification that occurs during embryogenesis, cartilage growth consists mostly of the maturing of immature cartilage to a more mature state. The division of cells within cartilage occurs very slowly, and thus growth in cartilage is usually not based on an increase in size or mass of the cartilage itself.^[2]

Articular Cartilage

The articular cartilage function is dependent on the molecular composition of the extracellular matrix (ECM). The ECM consists mainly of proteoglycan and collagen. The main proteoglycan in cartilage is aggrecan, which, as its name suggests, forms large aggregates with hyaluronan. These aggregates are negatively charged and hold water in the tissue. The collagen, mostly collagen type II, constrains the proteoglycans. The ECM responds to tensile and compressive forces that are experienced by the cartilage.^[3] Cartilage growth thus refers to the matrix deposition, but can also refer to both the growth and remodeling of the extracellular matrix.

Function

Mechanical properties

The mechanical properties of articular cartilage in load bearing joints such as knee and hip have been studied extensively at macro, micro and nano-scales. These mechanical properties include the response of cartilage in frictional, compressive, shear and tensile loading. Cartilage is resilient and displays viscoelastic properties.^[4]

Frictional properties

Lubricin, a glycoprotein abundant in cartilage and synovial fluid, plays a major role in bio-lubrication and wear protection of cartilage.^[5]

Repair

Cartilage has limited repair capabilities: Because chondrocytes are bound in lacunae, they cannot migrate to damaged areas. Therefore, cartilage damage 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 scientists 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.^[6]

Clinical significance

Disease

Several diseases can affect cartilage. Chondrodystrophies are a group of diseases, characterized by the disturbance of growth and subsequent ossification of cartilage. Some common diseases that affect the cartilage are listed below.

• Osteoarthritis: Osteoarthritis is a disease of the whole joint, however one of the most affected tissues is the articular cartilage. The cartilage covering bones (articular cartilage—a subset of hyaline cartilage) is thinned, eventually completely wearing away, resulting in a "bone against bone" within the joint, leading to reduced motion, and pain. Osteoarthritis 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 often made of a stainless steel alloy (cobalt chromoly) and ultra high molecular weight polyethylene (UHMWPE). Chondroitin sulfate or glucosamine sulfate supplements, have been claimed to reduce the symptoms of osteoarthritis but there is little good evidence to support this claim.^[7]
• Traumatic rupture or detachment: The cartilage in the knee is frequently damaged, and can be partially repaired through knee cartilage replacement therapy. Often when athletes talk of damaged "cartilage" in their knee, they are referring to a damaged meniscus ( a fibrocartilage structure) and not the articular cartilage.
• 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 often 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.

Imaging

Cartilage does not absorb x-rays under normal In vivo conditions, but a dye can be injected into the synovial membrane that will cause the x-rays to be absorbed by the dye. The resulting void on the radiographic film between the bone and meniscus represents the cartilage. For In vitro x-ray scans, the outer soft tissue is most likely removed, so the cartilage and air boundary are enough to contrast the presence of cartilage due to the refraction of the x-ray.^[8]

Histological image of hyaline cartilage stained with haematoxylin & eosin, under polarized light

Other animals

Cartilaginous fish

Cartilaginous fish (chondrichthyes) like sharks, rays and skates have a skeleton composed entirely of cartilage.

Invertebrate cartilage

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

Plants

• Vascular plants, particularly seeds.^[9]

See also

• Biomechanics

References

General references

External links

• Cartilage.org, International Cartilage Repair Society
• KUMC.edu, Cartilage tutorial, University of Kansas Medical Center
• Bartleby.com, text from Gray's anatomy
• MadSci.org, I've heard 'Ears and nose do not ever stop growing.' Is this false?
• CartilageHealth.com, Information on Articular Cartilage Injury Prevention, Repair and Rehabilitation
• About.com, Osteoarthritis
• Cartilage types
• Different cartilages on TheFreeDictionary