Page "Keratin" Paragraph 14

These hard, integumentary structures are formed by intercellular cementing of fibers formed from the dead, cornified cells generated by specialized beds deep within the skin.

Hair grows continuously and feathers moult and regenerate.

The constituent proteins may be phylogenetically homologous but differ somewhat in chemical structure and supermolecular organization.

The evolutionary relationships are complex and only partially known.

Multiple genes have been identified for the β-keratins in feathers, and this is probably characteristic of all keratins.