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Cartilage or cartilaginous tissue is a resilient and type of connective tissue of mesodermal origin that forms an integral part within the musculoskeletal system and as a structural component in other organs.
Cartilage can be generally classified into the following main types:
- hyaline cartilage
- elastic cartilage
Generally, cartilage features a relatively simple anatomical structure and does not contain any blood vessels, lymphatics or nerves 1, even though some cartilaginous structures, such as the menisci of the knee, receive a blood supply.
It consists of a low density of cells, the chondrocytes surrounded by an abundant extracellular matrix. The chondrocytes make only a small part of the whole mass of the cartilage but are responsible for the synthesis and maintenance of extracellular matrix components. The extracellular matrix comprises a collagen network with variable amounts of elastin fibers, an amorphous ground substance ample in proteoglycans, a small amount of non-collagenous proteins and interstitial fluid 1-3.
The three types of cartilage vary in their density of the chondrocytes and extracellular matrix composition, including water content, amount of ground substance and elastin fibers and predominant collagen type. Thus, they also differ in their mechanical and viscoelastic properties 1,2,4.
Hyaline cartilage is the most common type of cartilage in the human body. It is characterized by a firm consistency and a smooth surface. It can distribute loading forces at a very low frictional resistance, ensuring a smooth motion within joints. It features a low density of chondrocytes surrounded by an abundant extracellular matrix. The chondrocytes are characterized by rounded, elliptic or partly angular outlines depending on whether they lie alone or are arranged in groups. They form only a small part of the whole mass of the hyaline cartilage but are responsible for synthesizing and maintaining extracellular matrix components 2,3.
About 60-80% of the hyaline cartilage tissue weight is made up of water bound into the extracellular matrix 1,5. The hyaline extracellular matrix consists of a collagen network making up the highest share of the dry weight. It also contains a high share of amorphous ground substance and a small amount of non-collagenous proteins 1-3.
The collagen network is mainly composed of type 2 collagen fibrils forming the backbone and to a lesser extent of other collagen types providing crosslinks and other functions 1,6.
The amorphous ground substance consists of proteoglycans, predominantly aggrecan made of negatively charged glycosaminoglycans (GAGs) such as chondroitin sulphate and keratan sulphate that are cross-linked to hyaluronic acid filaments to form aggregates that bind the water 2,3.
The gross appearance of hyaline cartilage is translucent, glass-like with a pearl-grey to bluish color 1,3. Hyaline cartilage is usually covered by perichondrium except within joints where it is covered by synovial fluid and can be found in the following locations 1-4:
- articular cartilage
- physeal cartilage
- costal cartilage
- nasal cartilage
- thyroid cartilage, cricoid cartilage and base of arytenoid cartilage
- tracheal and bronchial cartilage
- fetal skeleton
Fibrous cartilage is characterized by a high compressive and tensile strength. It features the lowest density of chondrocytes and is made up of densely interweaved collagen fibers high in type 1 in addition to type 2 collagen and only a small amount of ground substance 2.
Fibrous cartilage can be found in the following locations 1,2:
- annulus fibrosus of the intervertebral discs
- pubic symphysis
- menisci and articular disks
- glenoid and acetabular labrum
- bone-tendon interfaces
- ligament-tendon interfaces
Elastic cartilage can withstand repetitive bending forces and deformation stress due to high elasticity and flexibility 2. Elastic cartilage is also characterized by a prevailing fibrillar component made up of a high amount of type 2 collagen and elastin fibers with a multidirectional arrangement. It features larger chondrocytes and also a higher cell density than the two other cartilage types ref. Due to its high elastic fiber content, it features a yellowish appearance 1. Elastic cartilage is covered by perichondrium and can be found in the following locations 1,2,4:
- external ears
- Eustachian tube
- larynx (part of the arytenoid, corniculate and cuneiform cartilages)
Relations and/or boundaries
Depending on the location and type, cartilage is covered by the perichondrium, a dense membrane-like layer of fibrous tissue, surrounded by synovial fluid produced or is immediately adjacent to bone.
On CT, cartilage features a slightly higher density than normal soft tissue.
Hyaline cartilage features an intermediate signal intensity, which is similar or mildly hyperintense to muscle. Fibrocartilage and elastic cartilage feature a low signal intensity due to their lower water content.
History and etymology
The first description of cartilage has been attributed to Aristotle in the fourth century before Christ. Later, Galen described it in the second century and by the Persian physician Avicenna after the turn of the first millennium 7. The first scientific description about articular cartilage was composed by the English anatomist and surgeon William Hunter in 1743 7.
There is a wide spectrum of pathologies related to hyaline cartilage; this includes 1:
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