Keratinization and Cornification are not equivalent processes but keratinization in fish and amphibians evolved into cornification in terrestrial vertebrates

Abstract The present account offers a generalized view of the evolution of process of terminal differentiation in keratinocytes of the epidermis in anamniotes, indicated as keratinization, into a further differentiating process of cornification in the skin and appendages of terrestrial vertebrates. Keratinization indicates the prevalent accumulation of intermediate filaments of keratins (IFKs) and is present in most fish and amphibian epidermis and inner epithelia of all vertebrates. During land adaptation, terrestrial vertebrates evolved a process of cornification and keratinocytes became dead corneocytes by the addition of numerous others proteins to the IFKs framework, represented by keratin‐associated proteins (KAPs) and corneous proteins (CPs). Most of genes coding for these types of proteins are localized in chromosomal loci different and un‐related from those of IFKs, and CPs originated from a gene cluster indicated as epidermal differentiation complex. During the evolution of reptiles and birds, the epidermis and corneous derivatives such as scales, claws, beaks and feathers mainly accumulate a type of CPs that overcome IFKs and containing a 34 amino acid beta‐sheet core indicated as corneous beta proteins, formerly known as beta‐keratins. Mammals did not evolve a beta‐sheet core in their CPs and KAPs but instead produced numerous cysteine‐rich IFKs in their epidermis and specialized KAPs in hairs, claws, nails, hooves and horns.