Cartilage

Cartilage is type of dense connective tissue. Cartilage is composed of cells called chondrocytes which are dispersed in a firm gel-like ground substance, called the matrix. Cartilage contains no blood vessels and nutrients are diffused through the matrix. Cartilage is found in the joints, the rib cage, the ear, the nose, in the throat and between intervertebral disks. There are three main types of cartilage: hyaline, elastic & fibrocartilage.

Composition

Much like other connective tissue, cartilage is composed of cells, fibers and a matrix.

Cells

Chondrocytes are the only cells found in cartilage. They are responsible for the secretion and maintenance of the matrix. The matrix immediately surrounding the chondrocytes is referred to as the territorial matrix and stains darker than the interstitial matrix. Chondrocytes lie in a cavity called a lacuna. During slide preparations, chondrocytes often shrink and appear smaller than the lacunae but in live tissue, they occupy the entire area.

Fibers

Cartilage is composed of collagen and elastic fibers. In hyaline cartilage, Type II collagen make up 40% of its dry weight and are arranged in cross-striated fibers, 15-45nm in diameter that do not assemble into large bundles. Elastic cartilage also contains elastic fibers and fibrocartilage contains more collagen than hyaline cartilage.

Matrix

The matrix is mainly composed of proteoglycans, a special type of glycosaminoglycans. The most common type is chondroitin sulfate and keratan sulfate .

Types of Cartilage

There are three different types of cartilage, each with special characteristics adapted to local needs.

Hyaline Cartilage

This is the most abundant type of cartilage. The name hyaline is derived from the greek word hyalos, meaning glass. This refers to the translucent matrix or ground substance. Hyaline cartilage is found lining bones in joints (articular cartilage) . It is also present inside bones, serving as a center of ossification or bone growth.

Elastic Cartilage

Elastic cartilage (also called yellow cartilage) is found in the pinna of the ear and several tubes, such as the walls of the auditory and eustachian canals and larynx. Cartilage is present to keep the tubes permanently open. Elastic cartilage is similar to hyaline cartilage but contains elastic bundles (elastin) scattered throughout the matrix. This provides a tissue which is stiff yet elastic.

Fibrocartilage

Fibrocartilage (also called white cartilage) is a specialised type of cartilage found in areas requiring tough support or great tensile strength, such as between intervertebral disks, the pubic symphisys and at sites connecting tendons or ligaments to bones. There is rarely any clear line of demarcation between fibrocartilage and the neighboring hyaline cartilage or connective tissue. The fibrocartilage found in intervertebral disks contains more collagen compared to hyaline. Fibrocartilage lacks a perichondrium.

Perichondrium

The perichondrium is a tissue layer that lines most types of cartilage. There are two distinct layers, an outer fibrous layer and inner chondrogenic layer. The fibrous layer contains fibroblasts which can produce collagen fibers while the inner layer contains undifferentiated or osteogenic cells capable of forming chondrocytes. If cartilage is transformed to bone, the perichondrium becomes the periosteum.

Growth / Development

Chondrification

Most of the skeletal system is derived from mesoderm tissue. Chondrification is the process in which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondrocytes and begin secreting the materials that form the matrix.

Two types of growth can occur in cartilage: appositional and interstitial.

Appositional

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

Interstitial growth results in an increase of cartilage mass and occurs from within. Chondrocytes undergo mitosis within their lacuna but remain imprisoned in the matrix, which results in clusters of cells called isogenous groups.

Cartilage in fetal development

In the fetus, at an early period, the greater part of the skeleton is cartilaginous; as this cartilage is afterward replaced by bone, it is called temporary, in contradistinction to that which remains unossified during the whole of life, and is called permanent.

It has been said that the cartilage in ears and noses continues to grow in size throughout adult life; however, this seems to be an urban myth which is not substantiated by research.

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 limitation of movement 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 immunoglobins . 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.

See also

• Biomechanics
• connective tissue
• Endochondral ossification
• Chondroplasty

External links

• University of Kansas Medical Center Cartilage tutorial
• public domain text from Gray's anatomy dated 1918, so probably needs updating
• I've heard 'Ears and nose do not ever stop growing.' Is this true?