This is one of a family of fibrous structural proteins.
This is the protein that protects epithelial cells from damage or stress.
This is extremely insoluble in water and organic solvents.
It is the key structural material making up the outer layer of human skin.
Its 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.
It derives from Greek κερατίνη from Greek keras (κέρας) (genitive keratos, κέρατος) 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, the 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.
the α-keratins are found in all vertebrates.
They form the hair (including wool), stratum corneum, horns, nails, claws and hooves of mammals and the hagfish slime threads.
the harder β-keratins are found only in the sauropsids, that is all living reptiles, including birds.
They are found in the nails,scales, and claws of reptiles, their shells (Testudines, such as tortoise, turtle, terrapin), and in the feathers, beaks, and claws of birds..
(These keratins are formed primarily in beta sheets. However, beta sheets are also found in α-keratins.)