Cartilage is a connective tissue found in various parts of the adult skeleton including all joints between bones and structures which is deformable as well as strong e.g. the elbows, knees, and ankles, ends of the ribs, Between the vertebrae in the spine, ears, and nose, Bronchial tubes or airways.
It is weaker than bone, but it is flexible and can recover quickly. It is mostly found in the infant skeleton which replaced by bone during growth.
MICROSTRUCTURE OF CARTILAGE
Cartilage is a pliant, load-bearing connective tissue, covered by a fibrous perichondrium except at its junctions with bones and over the articular surfaces of synovial joints. It has a capacity for rapid interstitial and appositional growth in young and growing tissues.
Three types of cartilage(hyaline cartilage, white fibrocartilage, and yellow elastic cartilage) can be distinguished on the basis of the composition and structure of their extracellular matrices, but many features of the cells and matrix are common to all three types, and these features will be considered first.
– There are two types of which are the following:
– Chondroblast – It is a type of cell that develops into a chondrocyte or cartilage cell.
– Chondrocytes- These chondrocytes produce large amounts of extracellular matrix composed of collagen fibers, proteoglycan, and elastin fibers. There are no blood vessels in cartilage to supply the chondrocytes with nutrients.
The matrix is mostly comprised of collagen and, in some cases, elastic fibers, embedded in a highly hydrated proteoglycan gel. Large proteoglycan molecules have numerous side chains of glycosaminoglycans (GAGs), carbohydrates with remarkable water-binding properties.
A preponderance of fixed negative charges on the surface of GAGs strongly attracts polarized water molecules, causing wet cartilage to swell until restricted by tension in the collagen network, or by external loading.
In this way, cartilage develops a compressive turgor that enables it to distribute loading evenly on to the subchondral bone, rather like a water bed.
Effectively, water is held in place by proteoglycans, which are themselves held in place by the collagen network. Other constituents of cartilage include dissolved salts, non-collagenous proteins, and glycoproteins.
Most fibrous tissues contain collagen type I, which forms large fibers with a wavy ‘crimped’ structure; however, this type of collagen is only found in cartilage in the outer layers of the perichondrium and in white fibrocartilage.
More typical of cartilage is collagen type II, which forms very thin fibrils dispersed between the proteoglycan molecules so that they do not clump together to form larger fibers.
Collagen type II fibrils are often less than 50 nm in diameter and are too small to be seen by light microscopy. Transmission electron microscopy reveals that they have a characteristic cross-banding (65 nm periodicity) and are interwoven to create a three-dimensional meshwork.
The collagen network varies in different types of cartilage and with age.
The length of collagen fibrils and fibers in cartilage is unknown, but even relatively short fibrils can reinforce the matrix by interacting physically and chemically with each other and with other matrix constituents including proteoglycans (Hukins and Aspden 1985), reflecting the fact that the term collagen means ‘glue maker’.
Collagen type II is found in the notochord, the nucleus pulposus of an intervertebral disc, the vitreous body of the eye, and the primary corneal stroma.
Cartilage proteoglycans are similar to those found in general, i.e. non-specialized, connective tissue. The most common GAG side chains in cartilage are chondroitin sulfate and keratan sulfate.
The most common proteoglycan molecule, aggrecan, form huge molecular aggregates with other proteoglycans and with hyaluronan.
Formation Of Cartilage
Cartilage is usually formed in embryonic mesenchyme. Mesenchymal cells proliferate and become tightly packed; the shape of their condensation foreshadows that of the future cartilage.
They also become rounded, with prominent round or oval nuclei and a low cytoplasm: nucleus ratio. Each cell differentiates into a chondroblast as it secretes a basophilic halo of the matrix,
composed of a delicate network of fine type II collagen fibrils, type IX collagen, and proteoglycan core protein.
At some sites, continued secretion of matrix separates the cells, producing typical hyaline cartilage. Elsewhere, many cells become fibroblasts; collagen synthesis predominates and chondroblastic activity appears only in isolated groups or rows of cells that become surrounded by dense bundles of collagen fibers to form white fibrocartilage.
In yet other sites, the matrix of early cellular cartilage is permeated first by anastomosing oxytalan fibers, and later by elastin fibers. In all cases, developing cartilage is surrounded by condensed mesenchyme, which differentiates into a bilaminar perichondrium.
The cells of the outer layer become fibroblasts and secrete a dense collagenous matrix lined externally by vascular mesenchyme.
The cells of the inner layer contain differentiated, but mainly resting chondroblasts or prechondroblasts.
Cartilage grows by interstitial and appositional mechanisms. Interstitial growth is the result of continued mitosis of early chondroblasts throughout the tissue mass and is obvious only in young cartilage, where the plasticity of the matrix permits continued expansion.
When a chondroblast divides, its descendants temporarily occupy the same chondroitin. They are soon separated by a thin septum of the secreted matrix, which thickens and further separates the daughter cells.
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The continuing division produces isogenous groups. Appositional growth is the result of the continued proliferation of the cells that form the internal, chondrogenic layer of the perichondrium.
Newly formed chondroblasts secrete matrix around themselves, creating superficial lacunae beneath the perichondrium. This continuing process adds additional surface, while the entrapped cells participate in interstitial growth.
Apposition is thought to be most prevalent in mature cartilages, but interstitial growth must persist for long periods in growth-plate cartilage. Relatively little is known about the factors that determine the overall shape of cartilage structures.
The Major Types Of Cartilage
There are three major types of cartilage: hyaline, fibro, and elastic cartilage.
It 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. It 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. It 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).
They are 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.
It has lots of collagen fibers (Type I and Type II), and it tends to grade into the 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. It is the only type of cartilage that contains type I collagen in addition to the normal type II.
Fibrocartilage is found in the pubic symphysis, the annulus fibrosus of intervertebral discs, menisci, and the temporal-mandibular joint.
This type of cartilage contains elastic fiber networks and collagen fibers. The principal protein is elastin.
It is histologically similar to hyaline cartilage but contains many yellow elastic fibers lying in a solid matrix. These fibers form bundles that appear dark under a microscope. They give elastic cartilage great flexibility so it can withstand repeated bending.
Chondrocytes lie between the fibers. It is found in the epiglottis (part of the larynx) and the pinnae (the external ear flaps of many mammals, including humans).
Learn more: Britannica