Cartilage

Cartilage is a type of dense connective tissue. It is composed of collagenous fibers and/or elastin fibers, and cells called chondrocytes, all of which are embedded in a firm gel-like ground substance called the matrix. Cartilage is avascular (contains no blood vessels) and nutrients are diffused through the matrix. Cartilage serves several functions, including providing a framework upon which bone deposition can begin and supplying 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 between intervertebral discs. There are three main types of cartilage: hyaline, elastic and fibrocartilage.

Composition

Much like other connective tissue, cartilage is composed of cells, fibers and a matrix. During embryonic development, cartilage is enclosed in a dense connective tissue called the perichondrium which also contains the cartilage cell precursors (chondroblasts). In adults, the cartilage of the ribs maintains its perichondrium, but not that of the joints.

Cells

Chondrocytes and their precursors, known as chondroblasts, are the only cells found in cartilage. Chondrocytes occur singly or in groups (called "cell nests") within spaces called lacunae. Chondrocytes are responsible for the secretion and maintenance of the matrix. During slide preparation, the chondrocyte often shrinks and appears smaller than the lacuna, but in live tissues they occupy the entire area.

Fibers

Cartilage is composed of collagen (type II) and elastic fibers. In hyaline cartilage, type II collagen makes up 40% of its dry weight and is 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, which are large molecules with a protein backbone and glycosaminoglycan (GAG) side chains. The main proteoglycan in articular cartilage is aggrecan. This molecule fills all the spaces between the collagen fibres and holds water, thus plumping out the extracellular matrix and giving articular cartilage its resistance to compression and its resilience (ability to spring back into shape after load). The most common types of GAGs in cartilage are chondroitin sulfate and keratan sulfate, both of which are found in aggrecan. The matrix immediately surrounding the chondrocytes is referred to as the territorial matrix, or capsule, and stains darker than the interstitial matrix.

Types of cartilage

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

Hyaline cartilage

Hyaline cartilage 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. It is avascular hyaline cartilage that is made predominantly of type II collagen. Hyaline cartilage is found lining bones in joints (articular cartilage or, commonly, gristle) and is also present inside bones, serving as a center of ossification or bone growth. In addition, hyaline cartilage forms most of the embryonic skeleton.

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 (Eustachian) tubes and larynx and especially in the epiglottis. 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 specialized type of cartilage found in areas requiring tough support or great tensile strength, such as between intervertebral discs, the pubic and other symphyses, 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. In addition to the type II collagen found in hyaline and elastic cartilage, fibrocartilage contains type I collagen that does form fiber bundles seen under the light microscope. Fibrocartilage seems to lack a perichondrium but indeed it has perichondrium but it can not be seen due to type I collagen. When the hyaline cartilage at the end of long bones such as the femur is damaged, it is often replaced with fibrocartilage, which does not withstand weight-bearing forces as well.

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.

Mineralisation

Adult hyaline articular cartilage is progressively mineralised at the junction between cartilage and bone. It is then termed articular calcified cartilage. A mineralisation 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 mineralising 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 lacuna, but remain imprisoned in the matrix, which results in clusters of cells called isogenous groups.

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.

Invertebrate cartilage

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

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 false?

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