Connective tissue forms the structural support of living things, especially vertebrates. Tissues meeting this definition serve a variety of functions throughout the body, and the building blocks of many of these connective tissues are collagen fibers. Collagen is a protein – in fact, it is the most plentiful protein found in nature. It should therefore not be surprising that about 40 subtypes had been identified as of 2018.
Not all types of collagen are formed into fibers, made up of fibrils (which are themselves made of groups of triplets of individual collagen molecules), but three of the five major types of collagen – labeled I, II, III, IV and V – are often seen in this arrangement. Collagen possesses the advantageous trait of resisting stretching or tensile forces. Owing to the sheer prevalence of collagen in the body, disorders affecting its synthesis, or biological manufacture, are numerous and can be severe.
Types of Connective Tissue
Connective tissue proper, which translates roughly to "anything not bone that most people might recognize as connective tissue," includes loose connective tissue, dense connective tissue and adipose tissue. Other types of connective tissue include blood and blood-forming tissue, lymphoid tissue, cartilage and bone.
Collagen is a form of loose connective tissue. This type of tissue includes fibers, ground substance, basement membranes and a variety of free-existing (e.g., circulating in blood) connective tissue cells. In addition to collagen fibers, the fiber type of loose connective tissue includes reticular fibers and elastic fibers. Collagen is not found in ground substance, but it is a component of certain basement membranes, which are the interface between connective tissue itself and to whatever tissue it is supporting.
As noted, collagen is a type of protein, and proteins consist of amino acids. Short lengths of amino acids are called peptides, whereas polypeptides are longer but are short of being full-fledged functional proteins.
Like all proteins, collagen is made on the surfaces of the ribosomes inside cells. These use instructions from ribonucleic acid (RNA) to make long polypeptides called procollagen. This substance is modified in the endoplasmic reticulum of cells in various ways. Sugar molecules, hydroxyl groups and sulfide-sulfide bonds are added to certain amino acids. Each collagen molecule destined for a collagen fiber is wound into a triple helix along with two other molecules, giving it structural stability. Before the collagen can become completely mature, its ends are trimmed off to form a protein called tropocollagen, which is simply another name for collagen.
Although over three dozen distinct kinds of collagen have been identified, only a small fraction of these are physiologically significant. The first five types, using Roman numerals I, II, III, IV and V, are overwhelmingly the most common in the body. In fact, 90 percent of all collagen consists of Type I.
Type I collagen (sometimes called collagen I; this scheme of course applies to all types) makes up collagen fibers, and is found in skin, tendons, internal organs and the organic (that it, non-mineral) portion of bone. Type II is the primary constituent of cartilage. Type III is the main component of reticular fibers, which is somewhat confusing since these are not considered "collagen fibers" like the fibers made from type I are; types I and III are often seen together in tissues. Type IV is found in basement membranes, while type V is seen in hair and on the surfaces of cells.
Type I Collagen
Because type I collagen is so widespread, it is easy to isolate from surrounding tissues and was the first type of collagen to be formally described. The type I protein molecule consists of three smaller molecular components, two of which are known as α1(I) chains and one of which is called the α2(I) chain. These are arranged in the form of a long triple helix. These triple helices in turn are stacked alongside each other to form fibrils, which are in turn bundled into full-fledged collagen fibers. The hierarchy from smallest to largest in collagen is therefore α-chain, collagen molecule, fibril and fiber.
These fibers are able to stretch considerably without breaking. This makes them extremely valuable in tendons, which connect muscles to bones and must therefore be able to tolerate a great deal of force without breaking while still offering a great deal of flexibility.
In a disease called osteogenesis imperfecta, either type I collagen is not made in sufficient quantities or the collagen that is synthesized is defective in its composition. This results in bone weakness and irregularities in connective tissue, leading to various degrees of physical debility (it can in some cases be fatal).
Type II Collagen
Type II collagen also forms fibers, but these are not as well organized as type I collagen fibers. These are found chiefly in cartilage. The fibrils in type II, rather than being neatly parallel, are often arranged in what is more or less a jumble. This is afforded by the fact that cartilage, while being the major home of type II collagen, is made mostly of a matrix consisting of proteoglycans. These are made up of molecules called glycosaminoglycans wrapped around a cylindrical protein core. The entire arrangement makes cartilage compressible and "springy," qualities well suited for cartilage's main job of cushioning the impact stress on joints such as the knees and elbows.
The cartilage formation disorders affecting the skeleton known as chondrodysplasias are thought to be caused by a mutation in the gene in DNA that codes with the type II collagen molecule.
Type III Collagen
The main role of type III collagen is the formation of reticular fibers. These fibers are very narrow, being only about 0.5 to 2 millionths of a meter in diameter. The collagen fibrils made from type III collagen are more branching than parallel in orientation.
Reticular fibers are found in abundance in myeloid (bone marrow) and lymphoid tissues, where they serve as scaffolding for the specialized cells involved in the generation of new blood cells. They are made by either fibroblasts or reticular cells, depending on their location. They can be distinguished from type I collagen on the basis of how they appear after being stained with certain chemical dyes.
One of the 10 or so subtypes of the disease called Ehlers-Danlos syndrome, which can lead to a fatal rupture of blood vessels, is caused by a mutation in the gene that codes for type III collagen.
Type IV Collagen
Type IV collagen is a major component of the basement membrane, as noted. It is organized into extensive branching networks. This type of collagen does not have what is called axial periodicity, meaning that along its length, it does not have a characteristic repeating pattern, and it does not form fibers at all. This type of collagen might therefore be seen as the most haphazard of the major collagen types. Type IV collagen makes of much of the innermost of the three layers of the basement membrane, called the lamina densa ("thick layer"). On either side of the lamina densa are the lamina lucida and the lamina fibroreticularis. The latter layer contains some type III collagen in the form of reticular fibers as well as type VI collagen, a less frequently encountered type.