Cartilage is an avascular, flexible connective tissue located throughout the body that provides support and cushioning for adjacent tissues.
It consists of the following three elements: cells, fibers, and a ground substance or extracellular matrix. Cartilage is a flexible connective tissue that differs from bone in several ways. For one, the primary cell types are chondrocytes as opposed to osteocytes. Chondrocytes are first chondroblast cells that produce the collagen extracellular matrix ECM and then get caught in the matrix.
They lie in spaces called lacunae with up to eight chondrocytes located in each. Chondrocytes rely on diffusion to obtain nutrients as, unlike bone, cartilage is avascular, meaning there are no vessels to carry blood to cartilage tissue. This lack of blood supply causes cartilage to heal very slowly compared with bone. The base substance of cartilage is chondroitin sulfate, and the microarchitecture is substantially less organized than in bone.
The cartilage fibrous sheath is called the perichondrium. The division of cells within cartilage occurs very slowly, and thus growth in cartilage is usually not based on an increase in size or mass of the cartilage itself. Articular cartilage function is dependent on the molecular composition of its ECM, which consists mainly of proteoglycans and collagens.
The remodeling of cartilage is predominantly affected by changes and rearrangements of the collagen matrix, which responds to tensile and compressive forces experienced by the cartilage. Cartilage types: Images of microscopic views of the different types of cartilage: elastic, hyaline, and fibrous. There are three major types of cartilage: hyaline cartilage, fibrocartilage, and elastic cartilage. Hyaline cartilage is the most widespread cartilage type and, in adults, it forms the articular surfaces of long bones, the rib tips, the rings of the trachea, and parts of the skull.
This type of cartilage is predominately collagen yet with few collagen fibers , and its name refers to its glassy appearance. In the embryo, bones form first as hyaline cartilage before ossifying as development progresses.
Hyaline cartilage is covered externally by a fibrous membrane, called the perichondrium, except at the articular ends of bones; it also occurs under the skin for instance, ears and nose. Hyaline cartilage is found on many joint surfaces. It contains no nerves or blood vessels, and its structure is relatively simple.
If a thin slice of cartilage is examined under the microscope, it will be found to consist of cells of a rounded or bluntly angular form, lying in groups of two or more in a granular or almost homogeneous matrix. These cells have generally straight outlines where they are in contact with each other, with the rest of their circumference rounded. They consist of translucent protoplasm in which fine interlacing filaments and minute granules are sometimes present.
Embedded in this are one or two round nuclei with the usual intranuclear network. Fibrous cartilage has lots of collagen fibers Type I and Type II , and it tends to grade into dense tendon and ligament tissue. White fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions.
It owes its flexibility and toughness to the fibrous tissue, and its elasticity to the cartilaginous tissue. MRI is superior to conventional radiography and computed tomography because of its superior soft tissue contrast, multiplanar capabilities, and lack of ionizing radiation.
Standard MRI pulse sequences T1 and T2 use intrinsic relaxation times and are a reflection of the local tissue properties. T1-weighted spin echo sequences provide excellent anatomic detail and high contrast between cartilage and subchondral bone. T2 relaxation time is a reproducible and quantifiable parameter that reflects the internuclear dephasing that occurs as a result of transverse relaxation of the exited hydrogen dipoles.
The collagen organization of the ECM may be measured using this technique. To assess the glycosaminoglycan content of articular cartilage, delayed gadolinium-enhanced MRI of cartilage may be used. Tracking areas of depleted glycosaminoglycans are indirectly measured, and the use of delayed gadolinium-enhanced MRI of cartilage shows promise in its ability to detect injured or diseased articular cartilage.
Another imaging technique used to assess the proteoglycan content of articular cartilage is sodium MRI. When the excitational radio frequency is peaked on a specific sodium species 23 Na , the relative fixed charge density of cartilage may be quantified, which is a function of the spatial resolution of charged proteoglycans. The imaging of articular cartilage remains challenging owing to the zonal changes in structure and biochemical composition over a few millimeters.
The potential for artifacts associated with MRI adds another dimension of complexity to this imaging dilemma. Articular cartilage is a highly specialized connective tissue of diarthrodial joints. Its principal function is to provide a smooth, lubricated surface for articulation and to facilitate the transmission of loads with a low frictional coefficient. The mechanical behavior of this tissue depends on the interaction of its fluid and solid components.
The unique and complex structure of articular cartilage continues to make its treatment and repair a significant challenge. No potential conflict of interest declared.
National Center for Biotechnology Information , U. Journal List Sports Health v. Sports Health. Alice J. Sophia Fox. Scott A. Author information Article notes Copyright and License information Disclaimer.
Keywords: articular cartilage, basic science, extracellular matrix, collagen, proteoglycan, chondrocyte. This article has been cited by other articles in PMC. Open in a separate window. Figure 1. Gross photograph of healthy articular cartilage in an adult human knee.
Structure and Composition of Articular Cartilage Articular cartilage is hyaline cartilage and is 2 to 4 mm thick. Figure 2. Regions In addition to zonal variations in structure and composition, the matrix consists of several distinct regions based on proximity to the chondrocytes, composition, and collagen fibril diameter and organization.
Chondrocytes The chondrocyte is the resident cell type in articular cartilage. Proteoglycans Proteoglycans are heavily glycosolated protein monomers. Figure 3. Noncollagenous Proteins and Glycoproteins Although a number of noncollagenous proteins and glycoproteins are found within articular cartilage, their specific function has not been fully characterized. Metabolism In adults, the articular cartilage matrix is separated from the subchondral vascular spaces by the subchondral plate. Biomechanical Function Articular cartilage is a thin layer of specialized connective tissue with unique viscoelastic properties.
Figure 4. Figure 5. Age and Development Age determines the composition of the ECM as well as the organization of chondrocytes and their response to external factors such as cytokines. Mri In Articular Cartilage Noninvasive imaging techniques are an important tool for the evaluation, diagnosis, and monitoring of articular cartilage. Summary Articular cartilage is a highly specialized connective tissue of diarthrodial joints. Footnotes No potential conflict of interest declared.
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Arthritis Rheum. Mankin HJ. The response of articular cartilage to mechanical injury. The picture above is a section of elastic cartilage, stained so that you can see the elastic fibres. For example, the Van Giesen stain stains elastic fibres black. In elastic cartilage, the chondrocytes are found in a threadlike network of elastic fibres within the matrix. Elastic cartilage provides strength, and elasticity, and maintains the shape of certain structure such as the external ear.
It has a perichondrium. Cartilage: The three types of cartilage There are three types of cartilage: Hyaline - most common, found in the ribs, nose, larynx, trachea. Is a precursor of bone. Fibro - is found in invertebral discs, joint capsules, ligaments.
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