Together with bones, cartilage is one of the main supporting tissues in animals. This function mainly relies on the extracellular matrix. Cartilage is a semi-rigid structure that maintains the shape of several organs, covers the surface of bones in the joints, and is the main supporting tissue during embryonic development, when bones are not yet present. During development, by endochondral ossification, the fetal cartilage is substituted by bone.
Cartilage is mostly an avascular tissue, without blood or lymphatic vessels, and without nerve terminals. The mechanical and biochemical features of cartilage depend on the extracellular matrix, which is mainly composed of collagen (15-20 %), mainly Type I, proteoglycans, mainly aggrecan, and glycoproteins (10 %), and water (65-80%). Collagen is important for withstanding stretching, whereas aggrecan resists mechanical pressures and provides abundant hydration. Large molecules of hyaluronan are also present in the cartilage extracellular matrix.
Chondrocytes are that forms the cartilage. They are located through the cartilage in scattered small cavities known as lacunae. Chondrocyte are round to ellipsoid cells with many irregular microvillosities in the plasma membrane. Immature chondrocytes (chondroblasts) contain well-developed secretory organelles, such as rough endoplasmic reticulum and Golgi complex, for synthesizing and secreting collagen and elastic fibers,as well as proteoglycans. They also contain glycogen depots and lipid drops in the cytoplasm. Chondrocytes are surrounded by a thin layer of pericellular extracellular matrix, which shows a distinct molecular composition. Both chondrocyte and pericellular thin layer of extracellular matrix are together known as chondron or chondrome. Wrapping chondrons, there is a layer of extracellular matrix known as territorial extracellular matrix. The remaining cartilage extracellular matrix is known as interterritorial extracellular matrix, which accounts for most of the cartilage tissue.
Cartilage is surrounded by a layer of connective tissue known as perichondrium, excepting a type of cartilage known as fibrocartilage. Perichondrium has an external layer, known as fibrous perichondrium, composed of fibrous connective tissue containing collagen fibers and fibroblasts, and an internal layer called chondrogenic perichondrium, where chondrogenic cells and chondroblasts are found. Chondrogenic cells differentiate into chondroblasts, and chondroblasts become chondrocytes. Chondroblasts synthesize most of the new extracellular matrix. During their differentiation, chondroblasts get surrounded by their own extracellular matrix and become chondrocytes. This growth is known as appositional growth. In young cartilage, however, chondrocytes can proliferate and contribute to the synthesis of extracellular matrix. This type of growth is referred as interstitial growth.
There are three types of cartilage in animals: hyaline, elastic and fibrocartilage.
Hyaline cartilage is the most widely distributed type of cartilage through the animal body. It is usually associated with the bones. During development, hyaline cartilage forms the skeleton of vertebrate. In adults, it can be found in tracheal rings, bronchi, nose, laringe, articular surfaces, and in the joints between the sternum and ribs. As animals get old, hyaline cartilage loses water content and necrotic areas may appear in the central parts of the cartilage. Hyaline cartilage can grow and be repaired, provided that perichondrium is preserved.
Hyaline cartilage is made up of mature cartilage, which forms most of the cartilage, and perichondrium, covering the outer surface of the mature cartilage. The extracellular matrix of the hyaline cartilage shows uniform appearance. Type II collagen and proteoglycans are abundant, and other types of collagen are also present. Extracellular matrix is secreted by chondrocytes located in cavities known as lacunae. Chondrocytes are round to ovoid in shape, and are usually found in couples or tetrads, known as isogenic groups. Type IV collagen and proteoglycans are abundant in the extracellular matrix surrounding the isogenic groups, but type II collagen is scarce. Isogenic groups are separated between each other by the interterritorial matrix.
Perichondrium is a layer of dense connective tissue that covers the outer surface of the mature cartilage. The outer part of perichondrium is known as fibrous because it is composed of collagen fibers, some fibroblasts, and a net of blood vessels. The inner part of perichondrium is referred as chondrogenic because new chondrocytes arise from this layer and become part of the mature cartilage while they synthesized extracellular matrix. By this process, known as apposition growth, cartilage is able to grow. Interstitial growth of cartilage happens when chondrocytes are able to divide.
Articular cartilage is a type of hyaline cartilage found in the synovial joints (they support a lot of movement). Articular cartilage lacks perichondrium and its main function is to resist the mechanical loads and provide a smooth and lubricated surface for reducing the effect of rubbing during movements. Articular cartilage is made up of several layers. The outer one is in contact with the synovial liquid, the inner layer is in contact with the bone, the middle layer is in between. While the outer layer is to minimize the effect of rubbing, the middle layer and, particularly, the inner layer, withstands the mechanical loads. Extracellular matrix, besides water, contains abundant type II collagen and proteoglycans, mainly aggrecan.
Elastic cartilage contains a high amount of elastic fibers so that it can be stretched while keeping its structural integrity. It is found in outer ear, Eustachian ducts, epiglottis, and larynx. Elastic cartilage shows scarce extracellular matrix (around 20 % of the weight dry), where type II collagen is the most abundant. Elastic cartilage does not arise from chondrogenic centers, but directly from mesenchymal tissue. Isogenic groups, 2 to 4 chondrocytes, are not easily distinguished. In the outer part of the elastic cartilage, the perichondrium is located as a layer of very dense connective tissue. Elastic cartilage does not become bone and it is not capable of self-repairing.
Fibrocartilage is the third type of cartilage found in intervertebral discs, some joints, insertion of tendon into the epiphysis of bones, in the heart valves, and in the penis of some animals. It is usually surrounded by hyaline cartilage and shows mechanical properties similar to dense connective tissue and hyaline cartilage. It is the most resistant of the three cartilages. Fibrocartilage cells are found more scattered than in the hyaline cartilage, but they are also distributed in rows, and sometimes it is difficult to distinguish chondrocytes from fibroblasts. Type I collagen is abundant in the extracellular matrix, but other types are also found. Type I collagen fibers are oriented in the same direction as the stretching mechanical forces. Elastic fibers are not abundant and the most abundant component of the ground substance are proteoglycans, but less abundant than in the hyaline cartilage. The proportion of the ground substance is lower than in other cartilages, which allows to easily observe the collagen fibers. Fibrocartilage is less elastic than hyaline cartilage, but more than tendons.
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