What is the main structural molecule in hair and nails?

The main structural molecule in hair and nails is keratin, a tough fibrous structural protein.

Hair and nails are made largely from dead, keratinized cells produced in specialized growth regions: hair follicles for hair and the nail matrix for nails. As these cells move outward, they become densely packed with keratin and lose their living contents. The visible hair shaft and nail plate therefore cannot feel pain, although the living tissues beneath them can.

Keratin molecules assemble into strong intermediate filaments. Sulfur-containing cysteine residues form disulfide bonds between keratin proteins, helping give hair and nails their strength and resistance to water. Hair generally has fewer cross-links than harder structures such as fingernails, claws and hooves.

Keratin is not unique to humans. Related keratinized structures include feathers, horns, scales and the outer layer of skin in many vertebrates. Collagen is another important structural protein, but it is not the main material of hair or nails.