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Cartilage is a type of dense connective tissue. It is composed of cells called chondrocytes which are dispersed 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 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 and fibrocartilage. The main purpose of cartilage is to provide a framework upon which bone deposition could begin. Another important purpose of cartilage is to provide smooth surfaces for the movement of articulating bones. Composition Much like other connective tissue, cartilage is composed of cells, fibers and a matrix. Cells Chondrocytes and the precursor forms of chondrocytes, known as chondroblasts, are the only cells found in cartilage. Chondrocytes make up "cell nests," groups of chondrocytes within lacunae. Chondroblasts are responsible for the secretion and maintenance of the matrix. Chondroblasts encased in the matrix develop into chondrocytes. 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 preparation, the chondrocyte often shrinks and appears smaller than the lacunae, but in live tissues they occupy the entire area. Fibers Cartilage is composed of collagen 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 most common types of GAGs are 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 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) . It is also present inside bones, serving as a center of ossification or bone growth. In addition, hyaline cartilage forms 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 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 specialized type of cartilage found in areas requiring tough support or great tensile strength, such as between intervertebral disks, 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 lacks a perichondrium. Chondrification Most of the skeletal system is derived from the mesoderm germ layer. Chondrification 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. 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. 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. The cartilage in the ears and nose DO begin to grow with age as the production of sex hormones lowers. 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. 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) or malignant (chondrosarcoma) tumors. Fibrocartilage Fibrocartilage, as its name implies, is a type of cartilage arranged in a fibrous matrix that is similar to fibrous connective tissues. It is found in areas that require tensile strength, such as intervertebral discs. Chondrocytes are separated by dense bundles of collagenous fibers (collagen). Chondrocytes are usually arranged in short rows of 3 or 4. When hyaline cartilage, the shiny white gristle at the end of long bones, is damaged, it is often replaced with fibrocartilage, though it remains a poorer substitue. Invertebrate cartilage Cartilage tissue can also be found among invertebrates, for instance Limulus (horse-shoe crab), marine snails and cephalopods. See also | |||||||
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