keratinous

Keratinous is an adjective describing tissue, structures, or materials composed of keratin, a family of fibrous structural proteins produced by epithelial cells. Keratinous tissues are common in vertebrates and include hair, nails, horns, scales, beaks, and feathers. In humans, keratinized structures include hair, nails, and the outer layer of the skin.

Keratin comprises a diverse family of proteins that assemble into robust intermediate filaments within epithelial cells. There are two main types: alpha-keratins (predominant in mammals) and beta-keratins (found in reptiles and birds). Keratins are rich in cysteine, enabling disulfide cross-links that confer strength and resilience. The keratinization, or cornification, process converts living epidermal cells into dead, keratin-filled cells that form the protective outer layer of the skin.

Examples of keratinous structures include horns of cattle and rhinos (keratinized appendages rather than bone), pangolin scales, and avian feathers and beaks. Keratinized nails and claws provide mechanical protection and functional advantages for locomotion and manipulation. The properties of keratinous materials depend on keratin type and the extent of cross-linking.

Keratinous tissues are typically insoluble and resistant to proteolysis, making them durable but challenging to digest. Degradation often requires specialized enzymes called keratinases produced by certain fungi and bacteria. Because keratin is common in many animal parts, keratinous remains can be informative in fields such as archaeology, paleontology, and zoology.