Collagen is a group of naturally-occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. Collagen, in the form of elongated fibrils, is mostly found in fibrous tissues such as tendons, ligaments, and skin and is also abundant in cornea, cartilage, bone, blood vessels, the gut, and intervertebral discs. The fibroblast is the most common cell which creates collagen.
Collagen is a composed of a triple helix Figure 1, which generally consists of two identical chains (α1) and an additional chain that differs slightly in its chemical composition (α2). The amino acid composition of collagen is atypical for proteins, particularly with respect to its high hydroxyproline content. The tropocollagen or collagen molecule is a subunit of larger collagen aggregates such as fibrils. The tropocollagen subunits spontaneously self-assemble, with regularly-staggered ends, into even larger arrays in the extracellular spaces of tissues. Collagen fibrils/aggregates are arranged in different combinations and concentrations in various tissues to provide varying tissue properties. In bone, entire collagen triple helices lie in a parallel, staggered array.
Elastic fibers (or yellow fibers) Figure 2 are bundles of proteins (elastin) found in extracellular matrix of connective tissue and produced by fibroblasts and smooth muscle cells in arteries. These fibers can stretch up to 1.5 times their length and snap back to their original length when relaxed. Elastic fibers include elastin, elaunin, and oxytalan. Elastic tissue is classified as "connective tissue proper." The elastic fiber is formed from the elastic microfibril and amorphous elastin. The microfibril scaffolds and organizes the deposition of amorphous elastin. Elastic fibers are found in the skin, lungs, arteries, veins, connective tissue proper, elastic cartilage, periodontal ligament, fetal tissue, and other structures.
Reticular fibers or reticulin is a histological term used to describe a type of fiber in connective tissue composed of type III collagen. Reticular fibers Figure 3 crosslink to form a fine meshwork (reticulin). This network acts as a supporting mesh in soft tissues such as liver, bone marrow, and the tissues and organs of the lymphatic system. Reticular fiber is composed of one or more types of very thin and delicately woven strands of type III collagen. These strands build a highly ordered cellular network and provide a supporting network.