Keratin is a protein used by numerous groups of animals as a structural element, and is a classic example of a fibrous protein. It is the main component ofmammal hair, reptile scales, (though not fish scales), bird feathers, claws (including nails and hooves), horn (but not antlers), baleens (the sieve-like structures with which whales filter food out of water), the enamel of the teeth. Keratin molecules are helical and fibrous, twisting around each other to form strands called intermediate filaments. These proteins contain a high percentage of sulfur-containing amino acids, largely cysteine, which form disulfide bridges between the individual molecules resulting in a fairly rigid structure. Human hair is approximately 14% cysteine.
Keratin is the collective name for a family of tough proteins which are found in a number of structures. Keratin is found in hair, feathers, nails, claws, hooves and reptile scales. The keratinized part of the epidermis is known as the stratum corneum. Keratinization occurs when underlying epidermal cells produce and accumulate keratin within their cytoplasm. As they are being pushed towards the surface by the production of new epidermal cells from beneath (only the layer of epidermal cells immediately next to the dermis divide), their organelles and nucleus disappear and they become flattened. The keratin then becomes external from the broken down cells, forming the acellular layer of keratin seen on the surface of mammalian epithelium. As the outer keratinized cells are worn off, they are replaced by the cells coming up from below. The keratin layer serves to form an impervious barrier between the external body and the environment, as well as protecting the skin from damage. In the lower image, there are ducts visible travelling through the stratum corneum. These are the ducts of the sweat glands found lower down in the dermis (see the upper image). Sweat glands are coiled tubular glands which produce a watery secretion and serve an important function in thermoregulation for mammals.
Keratin is the protein which forms the hair, nails and outer layer of the skin. It is tough and fibrous and its function is to cover the body and protect it from damage and infection. Keratin can take many forms in nature, from the thin waterproof coating, which is what we see of our skin, to the hard, horny hooves of cattle, the feathers of birds and the fur of animals.
Keratin is produced in the cells deep in the basal layer of the skin. As these cells are pushed up to the surface they die and form the minute white skin-scales which cover our bodies. As these top scales wear away they are constantly replaced by more cells from below. Only when something goes wrong with the production of keratin does the condition known as keratosis arise.