Cartilage is a type of dense connective tissue found in vertebrates, as well as such invertebrates as horseshoe crabs, marine snails, and cephalopods. Cartilage is composed mainly of two components: Water and a matrix made up of macromacules. The matrix component of cartilage contains collagenous fibers, and/or elastin fibers, and cells called "chondrocytes." The chondrocytes are embedded in the firm gel-like ground substance of the matrix.
Cartilage is distinctive in that it has only one cell type, is avascular (lacks blood vessels), aneural (no neurons and nerves), and alymphatic (no lymphatic system). Nutrients are diffused through the matrix.
In the human body, cartilage is found in many places, including joints, rib cage, ear, nose, bronchial tubes, and between intervertebral discs. It is a major part of the embryonic vertebrate skeleton, but is converted largely to bone as the organism matures. Sharks are vertebrates (fish) with a full cartilaginous skeleton as adults.
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 and is classified as either "hyaline," "elastic," or "fibrous" cartilage.
The presence of so many varieties of tissues, connective tissues, and cartilage, all providing different functions and working together harmoniously—and with some cartilage needed by embryos to even give rise to bones in adults—reflects the intricacy and complex coordination in living organisms.
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, though it becomes absent everywhere else.
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 for viewing in a microscope, the chondrocyte often shrinks and appears smaller than the lacuna, but in live tissues they occupy the entire area.
Cartilage is composed of collagen (type II) and elastic fibers. In hyaline cartilage, type II collagen makes up 40 percent of its dry weight and is arranged in cross-striated fibers, 15-45 nanometers in diameter that do not assemble into large bundles. Fibrous cartilage contains more collagen than hyaline cartilage, and elastic cartilage, as its name implies, contains elastic fibers, which lend it a greater deal of flexibility.
The majority of the wet weight of cartilage, ranging anywhere from 65 to 80 percent, consists of water. The matrix makes up the rest. The matrix is mainly composed of proteoglycans, which are large molecules with a protein backbone and glycosaminoglycan (GAG) side chains. Glycosaminoglycans are long unbranched polysaccharides consisting of a repeating disaccharide unit (saccharide is a synonym for sugars). The main proteoglycan in articular cartilage is aggrecan. This molecule fills all the spaces between the collagen fibers 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 during slide preparation.
There are three different types of cartilage, each with special characteristics adapted to their function.
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.
Articular cartilage is responsible for the almost friction-free movement of our bones against one another. It is crucial for this cartilage to remain healthy because it also acts as a shock absorber; however, it is extremely susceptible to injury and pathological degeneration. Some cartilage-engineering techniques being tested are attempting to introduce cartilage-precursor cells that will differentiate into chondrocytes in the affected area. Such treatments are still in the experimental stage and are not ready for clinical use.
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, larynx, and especially in the epiglottis (keeps food from entering the airways). Cartilage is present to keep these tubes permanently open. Elastic cartilage is similar to hyaline cartilage but contains elastic bundles (elastin) scattered throughout the matrix. This provides a tissue that is stiff yet elastic.
Fibrous cartilage, (also called fibrocartilage and white cartilage) is a specialized type of cartilage found in areas requiring tough support or great tensile strength, such as between intervertebral discs, between the hip and pelvis bones, 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 forms fiber bundles seen under the light microscope. Fibrocartilage gives the appearance of lacking a perichondrium, but indeed it has one which cannot 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.
In embryogenesis, the process by which the embryo is formed and develops, 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 majority of the skeleton is cartilaginous. Because this cartilage is replaced by bone later on, it is referred to as "temporary." In contrast, "permanent" cartilage found in the joints remains unossified during the whole of life.
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.
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."
There are several diseases that 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.
Cartilage cells can also give rise to benign (chondroma) tumors. Malignant chondrosarcomas are tumors of bone, not cartilage.
Aside from diseases, trauma and tumors can often cause head and neck cartilage defects. The nose, auricle, larynx, and trachea are common regions that can be adversely affected. Cartilage reconstruction techniques are often used, which transplant autologous rib cartilage to the affected area. These procedures are done over a several month span and do carry high donor site morbidity (scarring, thorax deformities).
Bioengineering techniques are being developed to generate new cartilage, using a cellular "scaffolding" material and cultured cells to grow artificial cartilage. However, most techniques are not being used for clinical purposes yet.
All links retrieved April 16, 2013.
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