Changes of epidermal proteins immunolocalization in the corneous layer from embryonic to definitive avian beak

The process of keratinization and cornification in the developing beak has been studied through immunofluorescence and immunogold electron microscopy in chick and zebrafinch embryos. After the curved beak anlagen appears at the tip of the maxillar bone, 5-8 layers of embryonic epidermis are generated from the basal layer of the epidermis. These cells are weakly immunoabeled for IFKs (Intermediate Filament Keratins) and more intensely for scaffoldin, a protein of the EDC (Epidermal Differentiation Complex) involved in the soft keratinization of the embryonic epidermis. Immunolabeling for CBPs (Corneous Beta Proteins) is visible in the transitional embryonic layers that are temporarily generated between the embryonic and definitive beak epidermis. The electron microscope reveals that intermediate layers contain immunolabeled periderm granules for scaffoldin mixed with bundles of corneous material immunolabeled for CBPs. Intense CBPs labeling occurs in the compacting corneous bundles of beta-keratinocytes in the definitive beak while scaffolding labeling disappears. The embryonic epidermis is sloughed before hatching. Sox (Sulfhydryl Oxidase) immunolabeling reveals that the enzyme is almost absent in embryonic layers but is present in transitional and definitive beta-keratinocytes. This indicates the formation of cross-linked disulfide bonds in the definitive corneous layer of the beak. Some calcium precipitation, suggested from von Kossa staining, occurs in the corneous layers only on the 18th day of development in the chick, in preparation for hatching.