Keratinization and cornification of avian skin appendages during development. Insights from immunolabeling and...

The basal cytoskeleton of avian keratinocytes consists in a number of Intermediate Filament Keratins (IFKs, also indicated as alpha-keratins), poor (soft) or rich (hard) in cysteine. In keratinocytes of developing skin appendages Corneous Beta Proteins (CBPs, formerly termed beta-keratins), build most of the corneous material of developing scutate scales, claws, beak and feathers. CBPs derive from a gene locus termed Epidermal Differentiation Complex (EDC), unrelated to genes for IFKs. CBPs and IFKs belong to two different gene families that evolved independently during the evolution of birds. The evolution of feathers derived from the initial morphogenesis of barb ridges containing specialized proteins. During feather development, the framework of IFKs that combine with CBPs in differentiating keratinocytes, barb and barbule cells, give rise to resistant but flexible corneocytes in feathers and hard corneocytes in scales, claws and beaks. Here, we mainly deal with avian IFKs that are accumulated during the development of skin derivatives of birds, especially downfeathers. The latter are corneous appendages and, when mature, are composed from a prevalent mass of feather-CBPs (fCBPs, formerly indicated as feather beta-keratins). During development fCBPs are deposited over a IFKs cytoskeleton formed in barb and barbule cells, and these small beta-proteins rapidly overcame in amount IFKs, generating the corneous barbs and barbules of downfeathers. This process likely occurs through electrostatic interactions between acidic IFKs and basic CBPs, and later by the formation of covalent bonds (-S-S- and epsilon-bonds). Proteome and molecular studies have sequenced most of IFKs and CBPs of feathers in some species of birds. Most of the proteins extracted from feathers are fCBPs, while a lower amount is constituted from IFKs and other minor proteins.