Cartilage is a connective tissue composed of cells (chondrocytes) and fibres (collagen or yellow elastic) embedded in a firm, gel-like matrix which is rich in a mucopolysaccharide. It is much more elastic than bone. It is found in many areas in the bodies of including the joints between bones, the rib cage, the ear, the nose, the bronchial tubes and the intervertebral discs.
It is not as hard and rigid as bone but is stiffer and less flexible than muscle.
A layer of fibrous tissue, called perichondrium surrounds it like periosteum surrounds the bone. The articular cartilage has no perichondrium, so that its regeneration after injury is inadequate.
Chondrocytes are primary cells that form cartilage.
Types of Cartilage
It is bluish white and translucent due to very fine collagen fibres. It distribution is most abundant, and has a tendency to calcify after 40 years of age. All cartilage bones are preformed in hyaline.
It is the most widespread type of cartilage is found in
- Articular surface of bones
- Forms the anterior portion of the ribs
- Support for the respiratory passages (ringlike structures of the trachea).
- Provides shape as in nose
Hyaline cartilage is flexible, elastic, bluish white, and opalescent. The cells are mainly spherical and occupy the entire lacuna, although in stained sections the cell membrane retracts and the contour appears angular or stellate.
Nearer the surface, the cells appear flattened and lie in a plane parallel to the surface. The cytoplasm contains long mitochondria, vacuoles, and, particularly in more mature larger cells, fat droplets, and glycogen. The vacuoles may be large and distend the cell. The nucleus contains one or several nucleoli.
Nearer the surface, the cells occur singly or in pairs, often within the same lacuna. Multiple cells are often aggregated into compact groups irregularly placed.
The interstitial substance appears homogeneous. This is because refractive indexes of both collagen and acid mucopolysaccharide are identical.
Glycosaminoglycans, chiefly chondroitin sulphate, are contained within the interstitial substance and are responsible for the basophilic staining.
Cartilage has no blood vessels except an occasional one passing through to other tissues. Its nourishment, depending upon the location could be from synovial fluid, perchondrium.
Fibrocartilage differs from hyaline cartilage by the presence of thick, compact bundles of collagenous fibers within its interstitial substance. These bundles are arranged parallel to each other, separated by clefts in which encapsulated cells are squeezed.
Fibrocartilage appears to be a transitional tissue between hyaline cartilage and collagenous tissue and, as such, occurs in special situations.
It is white and opaque due to abundance of dense collagen fibres. Wherever fibrous tissue is subjected to great pressure, it is replaced by fibrocartilage which is tough, strong and resilient. Examples are intervertebral disc, intra-articular discs, menisci and labra. It lines certain bony grooves in which the tendons play.
- Shock Absorbers: Absorbs the shocks between the vertebrae.
- Support: Provides sturdiness without impeding movement.
- Movement: The white fibrocartilage forms a firm joint between bones but still allows for a reasonable degree of movement.
- Deepens Sockets: In articular cavities such as the ball-and-socket joints in the hip and shoulder regions white fibrocartilage deepens the sockets to make dislocation less possible.
It is made up of numerous cells and a rich network of yellow elastic fibers pervading the matrix, so that is more pliable. Example: cartilages in the external ear, auditory tube, and small cartilages at the inlet of larynx. It performs following functions.
- Maintain Shape: In the ear, for example, it helps to maintain the shape and flexibility of the organ.
- Support: It also strengthens and supports these structures.
Formation of Cartilage Tissue
Mesenchyme the cells becomes rounded, and the collagenous fibrils in the intercellular substance becomes enclosed by a basophilic material. The cells accumulate vacuoles, frequent mitoses occur, and daughter cells in a group are separated only by a thin partition.
A thin, shining layer, the capsule, appears about the cell cavity and represents the recently formed intercellular substance. The mesenchyme surrounding the cartilage forms a connective tissue layer covering, the perichondrium. A constant transformation of these layers and their cells into cartilage occurs during embryonic life.
The collagenous fibers are acidophilic flat bundles. They are surrounded by basophilic intercellular substance (acid mucopolysaccharides).
Elongated cells within the perichondrium lose their spindle shape and are transformed to spherical cells, the chondrocytes, surrounded by capsules. This process is termed appositional growth of cartilage.
Other form of growth is appostional wherein the cells within the cartilage multiply within their capsules and add to the surrounding matrix.
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