Immunolocalization of epidermal differentiation complex proteins reveals distinct molecular compositions of cells that control structure and mechanical properties of avian skin appendages

The epidermal differentiation complex (EDC) is a cluster of genes that encode structural proteins of skin derivatives with variable mechanical performances, from the scales of reptiles and birds to the hard claws and beaks, and to the flexible but resistant corneous material of feathers. Corneous proteins with or without extended beta‐regions are produced from avian genomes, and include the largely prevalent corneous beta proteins (CβPs, formerly indicated as beta‐keratins), and minor contribution from histidine‐rich proteins, trichohyalin‐like proteins (scaffoldin), loricrin, and other proteins rich in cysteine or other types of amino acids. The light‐microscopic and ultrastructural immunolocalization of major and minor EDC‐proteins in avian skin (feather CβPs, EDKM, EDWM, EDMTFH, EDDM, and scaffoldin) suggests that each specific appendage consists of a particular mix of these proteins in addition to the main proteins containing a peculiar beta‐region of 34 amino acids, indicated as feather/scale/claw/beak CβPs (fCβPs, sCβPs, cCβPs, bCβPs). This indicates that numerous proteins of the EDC are added to the variable meshwork of intermediate filament keratins to produce avian epidermis with different mechanical and functional properties. Although the specific roles for these proteins are not known they likely make an important contribution to the final material properties of the different skin appendages of birds. The highest number of sauropsid CβPs is found in birds, suggesting a relation to the evolution of feathers, and additional epidermal differentiation proteins have contributed to the evolutionary adaptations of avian skin. A, cross‐section of downfeather immunolabeled for EDMTFH; B, developing scutate scale immunolabeled for CBPs (preCB ab); C, cross‐section of downfeather immunolabeled for EDDM; D, schematic drawing of downfether (left) with enlarged (arrow) barb ridge containing various EDC proteins; E, double immunogold labeling in barbule cell cytoplasm for feather CBP (fβK, large gold particles) and EDMTFH protein (small gold particles). ​