Hyaline cartilage is the glass-like (hyaline) and translucent cartilage found on many joint surfaces. It is also most commonly found in the ribs, nose, larynx, and trachea. Hyaline cartilage is pearl-gray in color, with a firm consistency and has a considerable amount of collagen. It contains no nerves or blood vessels, and its structure is relatively simple.
Structure
Hyaline cartilage is the most common kind of cartilage in the human body. It is primarily composed of type II collagen and proteoglycans.
As cartilage does not have lymph glands or blood vessels, the movements of solutes, including nutrients, occur via diffusion within the fluid compartments contiguous with adjacent tissues. When arranged in groups of two or more, the chondrocytes have rounded, but generally straight outlines, where they are in contact with each other, and in the rest of their circumference, they are rounded.]]
Articular cartilage is hyaline cartilage on the articular surfaces of bones, and lies inside the joint cavity of synovial joints, bathed in synovial fluid produced by the synovial membrane, which lines the walls of the cavity.
Though it is often found in close contact with menisci and articular disks, articular cartilage is not considered a part of either of these structures, which are made entirely of fibrocartilage.
The articular cartilage extracellular matrix has a highly specialized architecture that is zonally organized: the superficial zone consists mostly of type II collagen fibers aligned parallel to the articular surface to resist shear forces, whereas the deep zone consists of the same fibers aligned perpendicularly to the bone interface to absorb compressive loads. Osteoarthritis affects over 30 million individuals in the United States alone, and is the leading cause of chronic disability amongst the elderly.
Articular cartilage development begins with interzone condensation of a type II collagen positive limb bud at the future joint site. This is followed by definition of specific cellular subtypes (meniscal progenitors, articular progenitors, synovial progenitors, and ligament progenitors) that will eventually form the joint capsule. Finally, the joint capsule matures and forms a cavity, with a central meniscus, and an encasement of synovium. This final structure will form several distinct layers of the articular cartilage found in all synovial joints including the deep zone (closest to the bone), middle zone, and superficial zone (closest to the synovial fluid).
Maintenance of articular cartilage is guided by a balance of anabolic (cartilage generating) and catabolic (cartilage degrading factors), in a manner similar to the maintenance of bone. Over the lifetime of the organism, anabolic factors and catabolic factors are generally in balance, however, as the organism ages, catabolism predominates and cartilage begins to degrade. Eventually, the loss of hyaline cartilage matrix and reduction in the chondrocyte content of the hyaline cartilage matrix results in the development of joint disease such as osteoarthritis. Overexpression of hyaline-cartilage specific anabolic factors, such as FGF18, appears to restore the balance between cartilage loss and generation.
Additional images
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File:Gray300.png|A synovial joint with bone, articular cartilage, and articular disc shown.
File:Elbow joint - deep dissection (anterior view, human cadaver).jpg|Elbow joint. Deep dissection. Anterior view.
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See also
- Articular cartilage damage
- Articular cartilage injuries
- Articular cartilage repair
- List of distinct cell types in the adult human body