The Process of Cornification Evolved From the Initial Keratinization in the Epidermis and Epidermal Derivatives of Vertebrates: A New Synthesis and the Case of Sauropsids

During land adaptation of the integument in tetrapods, an efficient stratum corneum was originated through the evolution of numerous corneous proteins in addition to the framework of intermediate filament-keratins present in keratinocytes. The new genes for corneous proteins were originated in a chromosome region indicated as epidermal differentiation complex (EDC), a locus with no apparent relationship to keratin genes. The addition of EDC proteins to IF-keratins transformed the process of epidermal keratinization present in anamniotes into a new process of cornification in the epidermis and skin appendages of amniotes, including hairs and feathers. In sauropsids among other EDC proteins a peculiar type of small proteins evolved a central region of 34 amino acids conformed as beta-sheets that, differently from the other EDC proteins, allowed the formation of long polymers of filamentous proteins customarily termed beta-keratins but in the present review reclassified as EDC corneous beta proteins. To the initial beta-sheets present in the corneous beta proteins specific N- and C-regions were later added in the proteins of different sauropsids in relation to the evolution of the corneous layer and skin appendages. Cornification contributed to the evolutive success of amniotes in the terrestrial environment