Keratin

Keratin (/ˈkɛrətɪn/) is one of a family of fibrous structural proteins. Keratin is the protein that protects epithelial cells from damage or stress. It is the key structural material making up the outer layer of human skin. Keratin monomers assemble into bundles to form intermediate filaments, which are tough and form strong unmineralized epidermal appendages found in reptiles, birds, amphibians, and mammals. The only other biological matter known to approximate the toughness of keratinized tissue is chitin.

Keratin derives from Greek κερατίνη from Greek keras () (genitive keratos, ) meaning "horn" originating from the Proto-Indo-European *ḱer- of the same meaning. It is composed of "horn like", i.e., kerato, to which the chemical suffix -in is appended. The Greek keras (or keros) is used in many animal names, e.g. Rhinoceros, meaning "nose with a horn".

Keratin filaments are abundant in keratinocytes in the cornified layer of the epidermis; these are proteins which have undergone keratinization. In addition, keratin filaments are present in epithelial cells in general. For example, mouse thymic epithelial cells (TECs) are known to react with antibodies for keratin 5, keratin 8, and keratin 14. These antibodies are used as fluorescent markers to distinguish subsets of TECs in genetic studies of the thymus.

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