Keratin, a structural protein that is found in hair, skin and nails, is made of thin protein filaments which are joined together by a type of chemical bond known as cross-links. Like all proteins, keratin is comprised of amino acids. It's especially high in the amino acids glycine and alanine.
Keratin is known for giving hair its toughness and for making skin and nails sturdy. Callused skin has higher levels of keratin than soft, delicate skin. Keratin's toughness is derived from its molecular structure. The thin filaments that combine to form keratin are very tightly wound together and held in place by intermolecular hydrogen bonds.
Many beauty products containing keratin have emerged in the recent years. These products typically contain a form of keratin known as hydrolyzed keratin, which is small enough to enter skin and hair when applied topically. These products are intended to make skin smoother and more elastic. Those designed for hair are intended to make it thicker and stronger.
Cornification is a process by which skin cells begin manufacturing more keratin in order to form a protective barrier known as a corn or callus. This often happens when the skin is exposed to continued stress, such as rubbing on a shoe. The outermost skin cells fill themselves entirely with keratin, eventually shutting down their metabolism and essentially becoming dead tissue.