Embryonic keratinization in vertebrates in relation to land colonization

The embryogenesis and cytology of the epidermis in different vertebrates is variable in relation to the formation of a stratum corneum of different complexity. The latter process was essential for land colonization during vertebrate evolution and produced an efficient barrier in amniotes. Keratinocytes are made of cross‐linked keratins associated with specific proteins and lipids that are produced at advanced stages of embryogenesis when the epidermis becomes stratified. In these stages the epidermis changes from an aquatic to a terrestrial type, preadapted in preparation for the impact with the dry terrestrial environment that occurs at hatching or parturition. The epidermal barrier against water‐loss, mechanical and chemical stress, and microbe penetration is completely formed shortly before birth. Beneath the outer periderm, variably stratified embryonic layers containing glycine‐rich alpha‐keratins are formed in preparation for adult life. The following layers of the epidermis produce proteins for the formation of the cornified cell membrane and of the cornified material present in keratinocytes of the adult epidermis in reptiles, birds and mammals. The general features of the process of soft cornification in the embryonic epidermis of vertebrates are presented. Cornification in developing scales in reptiles, avian feathers and mammalian hairs is mainly related to the evolution of keratin‐associated proteins. The latter proteins form the resistant matrix of hard skin derivatives such as claws, beaks, nails and horns.