keratins

Keratins are a family of fibrous structural proteins that form the intermediate filaments of epithelial cells. They are the principal components of hair, nails, horns, claws, beaks, and the outer layer of the skin in vertebrates.

Keratins are categorized into hard keratins and soft keratins. Hard keratins, found in hair, nails, and horns, have higher cysteine content and form extensive disulfide cross-links that increase rigidity. Soft keratins are expressed in the epidermis and other epithelia and are more flexible, contributing to the protective barrier properties of these tissues.

Structurally, keratins are expressed as pairs of type I (acidic) and type II (basic to neutral) proteins that assemble into obligate heterodimers and then into 10–12 nanometer intermediate filaments. These filaments strengthen epithelial cells and contribute to tissue integrity and resilience.

Keratins are produced by keratinocytes in epithelial tissues as they differentiate. During keratinization, keratin filaments reorganize and contribute to the formation of the dead, protein-rich outer layers, such as the stratum corneum of the skin.

Genetics: dozens of keratin genes occur in clustered families; mutations can disrupt filament formation and cause inherited skin and nail disorders, including epidermolysis bullosa simplex and pachyonychia congenita, often with blistering, hyperkeratosis, or nail dystrophy.

Function and applications: keratins provide mechanical resilience and barrier function to epithelia. In cosmetics, keratin-containing products are marketed for hair and nail care, and keratin-derived biomaterials are explored for wound healing and tissue engineering.