Avian scale development. III. Ultrastructure of the keratinizing cells of the outer and inner epidermal surfaces of the scale ridge

Abstract

Abstract Ultrastructural features of keratinization differ for the inner and outer epidermal surfaces of developing avian scales. Only the alpha stratum develops along the inner scale surface, and it is retained after hatching and is still present in adult birds. The cells of the alpha stratum are characterized by lipid droplets, multigranular bodies, and peripherally located keratohyalin granules. In contrast, the epidermis of the outer scale surface develops both alpha and beta strata. The alpha stratum is lost soon after hatching and only the beta stratum is found in the mature bird. Bundles of 30 Å filaments fill the cells of the beta stratum and keratohyalin granules are not present. The outermost peridermal and subperidermal cell layers develop along both epidermal surfaces of the scale. These two cell layers are characteristic of the embryonic period and are lost at the time of hatching.