The genetic program of human epidermal barrier formation is the product of evolution during which the epidermal differentiation programs of our evolutionary ancestors were modified by mutations of existing genes and the origin of new genes. To determine which epidermal differentiation genes were already present in the last common ancestor of modern terrestrial vertebrates, we determined genes that are upregulated in organotypic cultures of both human and chicken keratinocytes. Transcriptomes of non-differentiated and fully differentiated chicken keratinocytes were determined by RNA sequencing and compared to transcriptomes of human keratinocytes. The common keratinocyte differentiation program of human and chicken included keratins, desmosomal proteins, and proteins encoded by genes of the epidermal differentiation complex. However, only few of these structural proteins were 1:1 orthologs, suggesting a high degree of evolutionary diversification of skin barrier proteins. Evolutionarily conserved functions at the skin barrier were indicated by upregulation of ABCA12, aquaporins, arylacetamide deacetylases and a limited set of other genes during differentiation of both chicken and human keratinocytes. Among them, interleukin-36 receptor antagonist (IL36RN) was transcriptionally upregulated in skin equivalents of both species, suggesting that the expression of this anti-inflammatory protein represents a conserved link between cornification and suppression of inflammation. In conclusion, the comparative analysis of keratinocytes from phylogenetically diverse species under defined in vitro differentiation conditions identifies evolutionary ancient genes that play central roles in epidermal cornification.

Comparative transcriptomics of organotypic skin models defines components of the core molecular machinery of epidermal cornification