There are over 11 types of collagen characterized by having 19 unique chains. They are divided into three general classes:
- A molecule containing a length greater than 30 nm in uninterrupted helical chain.
- 301 nm molecules in which the helical chain is interrupted.
- Relatively short molecules in which the helical region may be continuous and uninterrupted.
Types of Collagen
Class 1 (300 nm triple helix)
Type I – Skin, bone, ligament
Type II – Cartilage, disc, eye
Type III – Skin, blood vessels, ligament
Type V – With type I
Type XI – With type I
Class 2 (basement membranes)
Type IV – Basal lamina
Type VII – Epithelial basement membrane
Type VIII – Endothelial basement membrane
Class 3 (short-chain<300 nm molecules)
Type VI – Widespread
Type IX – Cartilage (with type II)
Type X – Hypertrophic cartilage
Type XII – Tendon, other?
Type XIII – Endothelial cells
In group 1, which includes types 1,2,3, and 5, type 2 was most commonly found in the musculoskeletal system, in arteries, in cornea, in neurorentinal tissues, in uterus and in placental membranes. As well as different structural appearances, they have varying amounts of hydroxylysine, glycosylated hydroxylysine, hydroxyproline, etc.
Type 1 collagen fibrils serve as a substrate for the deposition of the mineral component. Any alteration in this will produce weakening of the skeleton. This is particularly seen in osteogenesis imperfecta.
In Ehlers-Danlos syndrome, there appears to be disorganization of the cross-linkages due to reduction of the enzyme lysyl hydroxylase. Marfan’s syndrome and homocystinuria have protein collagen defects which may also be seen in scurvy, lathyrism, etc.
Type I, II and III make up the bulk of the collagen in the body. Type II is restricted to hyaline cartilage, the intervertabral disc, and the vitreous humour of the eye.
Types V and XI are fibrillar collagens.
Type V is also distributed in small amounts (about 3 percent of type I) wherever type I collagen appears, and type XI is similarly distributed with type II collagen.
Hybrid molecules containing chains of both types V and XI collagens may also occur in bone and cartilage.
Type IV collagen is the major structural component of basement membranes. Type X collagen appears exclusively in the calcified cartilage zones of the epiphyseal plate, articular cartilage and bone fracture callus.Type VI collagen appears in small amounts as filamentous material around cells and between the banded collagen fibrils of most soft connective tissues.
Type VI collagen is enriched in certain tissue, including the intervertebral disc and the cornea. It appears to act as a structural element between the cells and the matrix of soft connective tissues which can deform. Increased amounts of type VI collagen have been noted in inflamed skin, in skin with certain forms of Ehlers-Danlos syndrome, and in the articular cartilage of patients with osteoarthrosis.
The mechanical strength of the common collagens arises from the formation of two or three covalent intermolecular bonds (cross-linked) per collagen molecule. Even though type I collagen predominates in all the musculoskeletal soft tissues (synovium, muscle, tendon, ligament except cartilage), its intermolecular cross-linking varies among tissues, Skin, cornea and rat-tail tendon have collagens that are largely cross-linked by lysine-derived aldehydes.
Cross-links based on other pathways can be seen in some connective tissues, such as tendon and muscle. Similarly, ligaments that bear high loads, such as the antrerior cruciate ligament of the knee, contain the highest levels of such cross-links in type I collagen.. Other tissues that dissipate or transmit high mechanical force, such as fibrocartilages (knee meniscus, intervertebral disc) are also rich in mature hydroxypyridium, a cross-linking amino acid.
Synovial membrane is rich is type II collagen. Nearly equal amounts of type I and II collagens have been found in normal and rheumatoid human synovial-lining tissue.
Although turnover rates of collagen in adult connective tissues are poorly documented, it is said that bone turns over rapidly, articular cartilage collagen more slowly.
Collagen in Ligaments
Ligaments are composed of a complex macro-molecular network with water making up about two thirds of the weight and the fibrillar protein collagen making up the majority of the remaining dry weight.
A normal ligament consists of about 90 percent fibrillar types I collagen with less than 10 percent being type III collagen. Other collagen types are present in smaller quantities.
Collagen in growth plate structure
Five types (II, VI, IX, X and XII) have been identified in epiphyseal cartilage, the most prevalent collagen being type II.
Although type X collagen is thought to be required in cartilage calcification, its relation to other matrix constituents remains unclear.
Collagen in articular cartilage
Articular cartilage contains at least five genetically distinct types of collagen, types II, VI, IX, X and XI, which cross-link in a polymeric network to form a fibrillar framework of tissue. Type II collagen is a major component of this framework and represents more than 95 percent of all cartilage collagens.
Type X collagen is a minor fibril-forming collagen. It is only present in the hypertrophic zone of growth plate and basal calcified zone of articular cartilage. Type IX collagen is a minor fibril-nor-forming collagen, covalenty linked to the surface of type II collagen fibrils.
Type VI collagen was found in the pericellular capsule and matrix around the chondrocytes. Electron microscopy also showed type VI collagen anchored to the chondrocyte membrane at the articular pole, suggesting a dual role of this collagen in the maintenance of chondrocyte integrity and as part of a cell-matrix signaling system.
In normal articular cartilage, matrix molecules are constantly synthesized and degraded by chondrocytes. The rates of synthesis and degradation for different types of molecules varies with ageing and exercise.
The skeleton, the vertebral column and the pelvis are formed by endochondral ossification. Endochondral bone development begins as a condensation of mesenchymal cells derived from mesoderm, which form extracellualr matrix. The mesenchymal cells surrounding the cartilage become the periosteum. These cartilage cells go thorough a maturation process that can be visualized in the area of the developing long bone called the growth plate.
Growth plates consist of zones of rapidly proliferating chonrocytes secreting collagens II, IX and XI, maturing and hypertrophic chondrocytes secreting predominantly collagen X. Collagen I is the major extracellualr matrix (ECM) molecule of bone. Recent immunolocalozatio in developing mouse embryos demonstrated bone, but this action in bone structure and development is yet unknown.
Other molecules also may be important in bone development. Tenascin, a glycoprotein, is seen in both osteogenic and chondrogenic areas of developing endochondral bone. These data imply that tenascin variants are important is osteogenesis. However, the recent demonstration that mice with a disrupted tenascin gene developed normally in all respects suggests that the role of tenascin in bone development is not yet understood.
Aggrecan core protein, previously considered to be specific to cartilage ECM, in which it interacts with hyaluronate and link protein, is also expressed in chicken calvaria and osteoblasts. Cartilage-specific proteoglycan, aggrecan, has been demonstrated to be present in membranous bone, having dramatic implications for the role of proteoglycans in bone and cartilage formation.
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