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Cartilage

Cartilage is a type of dense connective tissue. It is composed of collagen fibers and/or elastin fibers, and can supply smooth surfaces for the movement of articulating bones. Cartilage is found in many places in the body including the joints, the rib cage, the ear, the nose, the bronchial tubes and the intervertebral discs. There are three main types of cartilage: elastic, hyaline, and fibrocartilage.

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Types of cartilage

There are three different types of cartilage, each with special characteristics adapted to their function.

Hyaline cartilage

Hyaline cartilage has fibers scattered throughout its matrix, which looks glassy. Chondrocytes are found in the lacunae. Hyaline cartilage is found in the nose, thyroid and it connects the ribs to the sternum. It is also required in the endochondral development of bone.

Elastic cartilage

Elastic cartilage is similar to hyaline cartilage but contains elastic bundles (elastin) scattered throughout the matrix, which are responsible for its yellow color. This provides a tissue which is stiff yet elastics. Elastic cartilage is found in the pinna of the ear and several tubes, such as the walls of the auditory (Eustachian) tubes and larynx and especially in the epiglottis. Cartilage is present to keep the tubes permanently open.

Fibrocartilage

Fibrocartilage is very fibrous and found in between the vertebrae of the spinal column.

Growth and development

Cartilage in fetal development

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

Early in fetal development, the greater part of the skeleton is cartilaginous. As this cartilage is afterward replaced by bone, it is called temporary. In contrast, the cartilage in the joints remains unossified during the whole of life, and is called permanent.

Mineralization

Adult hyaline articular cartilage is progressively mineralized at the junction between cartilage and bone. It is then termed articular calcified cartilage. A mineralization front advances through the base of the hyaline articular cartilage at a rate dependent on cartilage load and shear stress. Intermittent variations in the rate of advance and mineral deposition density of the mineralizing front, lead to multiple tidemarks in the articular calcified cartilage.

Adult articular calcified cartilage is penetrated by vascular buds, and new bone produced in the vascular space in a process similar to endochondral ossification at the physis. A cement line demarcates articular calcified cartilage from subchondral bone.

Growth

Two types of growth can occur in cartilage: appositional and interstitial. Appositional growth results in the increase of the diameter or thickness of the cartilage. The new cells derive from the perichondrium and occur on the surface of the cartilage model. Interstitial growth results in an increase of cartilage mass and occurs from within. Chondrocytes undergo mitosis within their lacunae, but remain imprisoned in the matrix, which results in clusters of cells called isogenous groups.

Repair

Cartilage generally has very limited repair capabilities. Because chondrocytes are bound in lacunae, they cannot easily migrate to damaged areas and lay down new matrix. Damaged cartilage is usually replaced by fibrocartilage scar tissue.

Diseases / Medicine

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.

Arthritis: The cartilage covering bones in joints (articular cartilage) is degraded, resulting in movement limitation and pain.
Achondroplasia: Reduced proliferation of chondrocytes in the epiphyseal plate of long bones results in a form of dwarfism.
Costochondritis: Inflammation of cartilage in the ribs which causes chest pain
Herniated disk: Asymmetrical compression of a disk ruptures the cartilage ring, causing tissue to herniate into the spinal canal.

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.

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

Cartilage cells can give rise to benign (chondroma) tumors. Malignant chondrosarcomas are tumors of bone, not cartilage.