Evolution of Trichocyte Keratins

The evolution of trichocyte keratins was closely linked to the development of epithelia and epithelial appendages like hair. The primordial keratin gene originated from an ancient gene family encoding intermediate filament proteins. Through gene duplication and changes in exon-intron structure, type I and type II keratins emerged, evolving further through nucleotide sequence modifications. This diversification allowed for new, epithelium-specific cytoskeletal properties. In a common ancestor of reptiles, birds, and mammals, an increase in cysteine residues enabled extensive disulfide bond-mediated cross-linking in claws, which was later adapted for hair in mammals. The evolution of keratins, along with other structural proteins and developmental mechanisms, was fundamental to the development of the mammalian integument.