Collagens, the major extracellular matrix components in most vertebrate tissues, comprise a superfamily of proteins . A total of 29 genetically distinct collagens have been described so far in the vertebrate tissues and designated by Roman numerals I-XXIX in order of their discovery [2, 3]. The collagen molecules consist of three subunit polypeptides, so-called α-chains, and while some collagens are homotrimers, others can be heterotrimers containing two or even three genetically distinct subunit polypeptides. Consequently, there are well over 40 genes in vertebrate tissues that encode the subunits polypeptides of different, genetically distinct collagen molecules [1, 2].
A characteristic structural feature of all collagens is the presence of a protein domain in triple-helical conformation which provides stability to these molecules to serve as structural building blocks providing integrity to connective tissues. The triple-helical conformation resists non-specific proteolysis, such as digestion with pepsin. The folding of the individual α-chains into the triple-helical conformation is predicated upon the characteristic primary sequence, consisting of repeating Gly-X-Y triplet sequences. In some collagens, such as in type I collagen, the most abundant collagen in the skin and bones, the central collagenous domain of individual α-chains contains an uninterrupted Gly-X-Y repeat segment spanning approximately 1000 amino acids. In some collagens, such as in type IV (the basement membrane collagen) and type VII (the anchoring fibril collagen), the Gly-X-Y repeat sequence contains imperfections which interrupt the triple-helical conformation . These interruptions then provide flexibility to the rod-like collagen molecules and also provide sites susceptible to non-specific proteolytic cleavage of the primary sequence.
On the basis of their fiber architecture in tissues, the genetically distinct collagens have been divided into different subgroups . Collagens types I, II, III, V, and X align into large extracellular fibrils and are designated as fibril-forming collagens. Type IV collagen is arranged in an interlacing network within the basement membranes, while type VI collagen forms distinct microfibrils, and type VII collagen forms anchoring fibrils. FACIT collagens (fibril-associated collagens with interrupted triple helices)  include types IX, XII, XIV, XIX, XX, and XXI. Several of the latter types of collagens associate with larger collagen fibers and serve as molecular bridges, stabilizing the organization of the extracellular matrix.
The major collagens in human skin are types I and III which account for approximately 80% and 10% of the total bulk of collagen, respectively (Table 1). These two collagens associate to form broad extracellular fibers characteristic for human dermis. Type V collagen is present in most connective tissues, including dermis, where it represents less than 5% of the total collagen. In dermis, type V collagen is located on the surface of the large collagen fibers, formed by type I and III collagens, and type V collagen regulates the lateral growth of these fibers. Another major collagen in the skin is type IV collagen, present within the dermal-epidermal junction as well as in the vascular basement membranes.
In addition to these major collagens, human skin contains several minor collagens which demonstrate spatially restricted location, yet they play a critical role in providing integral stability to the skin (Table 1). One of them is type VII collagen, the major, if not the exclusive, component of anchoring fibrils . Another one is type XVII collagen, a transmembrane collagen in type II topography . Type XVII collagen resides in hemidesmosomes complexed with α6β4 integrin, plectin, and laminin-332 (laminin -5) . Finally, type XXIX collagen has been recently reported to be a putative epidermal collagen with the highest level of expression in suprabasal layers . This collagen has been suggested to play a role in atopic dermatitis but its characterization is currently incomplete.