Cartilage facts for kids

Quick facts for kids Cartilage
Hyaline cartilage showing chondrocytes and organelles, lacunae and matrix

Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs. It is not as hard and rigid as bone but is stiffer and less flexible than muscle.

Cartilage is composed of specialized cells called chondroblasts that produce a large amount of extracellular matrix composed of Type II collagen (except fibrocartilage which also contains type I collagen) fibers, abundant ground substance rich in proteoglycan, and elastin fibers. Chondroblasts that get caught in the matrix are called chondrocytes. They lie in spaces called lacunae with up to eight chondrocytes per lacuna. Cartilage is classified in three types, elastic cartilage, hyaline cartilage and fibrocartilage, which differ in the relative amounts of these three main components.

Unlike other connective tissues, cartilage does not contain blood vessels. Because of this, it heals very slowly. The chondrocytes are supplied by diffusion, helped by the pumping action generated by compression of the articular cartilage or flexion of the elastic cartilage. Thus, compared to other connective tissues, cartilage grows and repairs more slowly.

Growth and development

In embryogenesis, the skeletal system is derived from the mesoderm germ layer. Chondrification (also known as chondrogenesis) is the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondroblasts and begins secreting the molecules that form the extracellular matrix.

Imaging

Cartilage does not absorb x-rays under normal In vivo conditions, but a dye can be injected into the synovial membrane that will cause the x-rays to be absorbed by the dye. The resulting void on the radiographic film between the bone and meniscus represents the cartilage. For In vitro x-ray scans, the outer soft tissue is most likely removed so the cartilage and air boundary is enough to contrast the presence of cartilage due to refraction of the x-ray.

Histological image of hyaline cartilage stained with haematoxylin & eosin, under polarized light

Why has imaging of cartilage such a high clinical value and should be optimized? Accurately described by Link et al.: “Cartilage is one of the most important biomarkers in degenerative and traumatic joint disease. MR imaging has been established as the standard cartilage imaging modality, and techniques have been developed and optimized to visualize cartilage morphology, to quantify its volume and to analyze its biochemical composition. The substantial amount of research that is invested in the development of these morphologic and quantitative imaging techniques is motivated by new therapeutic modalities both on a surgical (cartilage repair) and a pharmacological level.” Recital of clinical indications for MRI in order to assess the cartilages actual condition: Osteoarthritis, chronic or acute osteo-chondral injury, osteochondritis dissecans, chondromalacia patellae, spontaneous osteo-necrosis of the femoral condyle (SONC or Ahlbaecks disease) and inflammatory arthropathies, evaluation of invasive surgery or monitoring of pharmacological therapies. The higher the field strength, the higher the diagnostic value of the image. With 3.0 Tesla systems the image gains on quality and spatial resolution. The signal-to-noise ratio (SNR) correlates linearly to the field strength and is thus double compared to a 1.5 Tesla system. Additionally the contrast increases while the expenditure of time and the appearance of artifacts decrease. Those enhanced performances are of high value and importance, especially for cartilage at smaller joints. The other side of the coin is an exacerbated difference of magnetic susceptibility in-between different tissues, a higher vulnerability to flow artifacts and safety concerns. In addition, doubling of the field strength comes along with doubling of the chemical shift. Imaging parameters must therefore be adjusted to the higher field strength, the increasing bandwidth and TR and to the decreasing flip angle and TE.
Naked-eye estimations of cartilage on MR arthrography seem to have the trend that thin regions get over- and thick regions get under-estimated. Hodler et al. noted that fact examining the humeral head, and Yeh et al. described it similarly, again concerning the humeral head. On the contrary, that trend was not found at the glenoid fossa, rather both thick and and thin regions were overestimated.

Magnetization transfer in MRI

Wet and fatty molecules are relatively small and feature in vivo conditions of high mobility. Macromolecular protons, for example proteins, have a wider range of Larmor frequency than protons of free water. Therefore can they be stimulated by radio frequency pulses. This leads to a saturated magnetization of the macromolecular protons, consequently to a decreased signal. This signal decay, called magnetization transfer, depends on the concentration of macromolecules and on interaction with free water. The decrease of signal intensity caused by magnetization transfer is in solid tissues distinct compared to the signal loss in wet and fatty tissues. MTC magnetization transfer contrast: The indirect effects of exchange of the magnetization saturation can be measured between the free and the bound protons. This technique is applied in cartilage imaging to improve the contrast between synovial fluid and cartilage. Thanks to the fact that synovial fluid has little amount of bound protons and cartilage has a large amount, it results in a pronounced magnetization transfer.
Damage of the cartilage starts whether on the surface as superficial fissures or deeper at the collagen structures. Second leads to a disadvantageous hyperhydratation with thickening and softening.

There are three different types of cartilage; elastic (A), hyaline (B), and fibrous (C). In elastic cartilage the cells are closer together creating less intercellular space. Elastic cartilage is found in the external ear flaps and in parts of the larynx. Hyaline cartilage has less cells than elastic cartilage, there is more intercellular space. Hyaline cartilage is found in the nose, ears, trachea, parts of the larynx, and smaller respiratory tubes. Fibrous cartilage has the least amount of cells so it has the most amount of intercellular space. Fibrous cartilage is found in the spine and the menisci.

Diseases and treatment

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.

• Osteoarthritis: The cartilage covering bones (articular cartilage - a subset of hyaline cartilage) is thinned, eventually completely worn out, resulting in a "bone against bone" joint, reduced motion, and pain. Osteoarthritis affects the joints exposed to high stress and is therefore considered the result of "wear and tear" rather than a true disease. It is treated by Arthroplasty, the replacement of the joint by a synthetic joint often made of a Stainless Steel alloy (cobalt chromoly) and Ultra High Molecular Weight Polyethylene (UHMWPE). Chondroitin sulfate, a monomer of the polysaccharide portion of proteoglycan, has been claimed to reduce the symptoms of osteoarthritis, possibly by increasing the synthesis of the extracellular matrix, but recent research has not produced evidence to support this claim
• Traumatic rupture or detachment: The cartilage in the knee is frequently damaged, and can be partially repaired through knee cartilage replacement therapy
• Achondroplasia: Reduced proliferation of chondrocytes in the epiphyseal plate of long bones during infancy and childhood, resulting in dwarfism.
• Costochondritis: Inflammation of cartilage in the ribs, causing chest pain.
• Spinal disc herniation : Asymmetrical compression of an intervertebral disc ruptures the sac-like disc, causing a herniation of its soft content. The hernia often compresses the adjacent nerves and causes back pain.
• Relapsing polychondritis: a destruction, probably autoimmune, of cartilage, especially of the nose and ears, causing disfiguration. Death occurs by suffocation as the larynx loses its rigidity and collapses.

Tumors made up of cartilage tissue, either benign or malignant, can occur. They usually appear in bone, rarely in pre-existing cartilage. The benign tumors are called chondroma, the malignant ones chondrosarcoma. Tumors arising from other tissues may also produce a cartilage-like matrix, the best known being pleomorphic adenoma of the salivary glands. Conversely, chondrostatin, an ingredient of cartilage, is being investigated by Washington University researchers for its potential ability to shrink breast and musculoskeletal tumors.

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.

Repair

Cartilage has limited repair capabilities: Because chondrocytes are bound in lacunae, they cannot migrate to damaged areas. Therefore if damaged, it is difficult to heal. Also, because hyaline cartilage does not have a blood supply, the deposition of new matrix is slow. Damaged hyaline cartilage is usually replaced by fibrocartilage scar tissue. Over the last years, surgeons and scientists have elaborated a series of cartilage repair procedures that help to postpone the need for joint replacement.

Bioengineering techniques are being developed to generate new cartilage, using a cellular "scaffolding" material and cultured cells to grow artificial cartilage.

Cartilage in animals

Cartilaginous fish

Cartilaginous fish (chondrichthyes) like sharks, rays and skates have a skeleton composed entirely of cartilage. Shark cartilage is a popular but unproven dietary supplement.

Invertebrate cartilage

Cartilage tissue can also be found among invertebrates such as horseshoe crabs, marine snails, and cephalopods.

Images for kids

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  Section from mouse joint showing cartilage (purple)

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  Human skeleton with articular cartilage shown in blue