Beta-keratin localization in developing alligator scales and feathers in relation to the development and evolution of feathers.

Beta-keratins form large part of the corneous material of scales and feathers. The present immunocytochemical study describes the fine distribution of scale- and feather-keratins (beta-keratins) in embryonic scales of the alligator and in avian embryonic feathers. In embryonic scales of the alligator both scale-keratin and feather-keratin can be immunolocalized, especially in the subperiderm layer. No immunolabeling for feather keratin is instead present in the adult scale after the embryonic epidermis is lost. The embryonic epidermis of feather folds into barb ridges while subperiderm or subsheath cells are displaced into two barbule plates joined to the central ramus. Subperiderm cells react with an antibody against feather keratin and with lower intensity with an antibody against scale keratin. The axial plate is colonized by barb ridge vane cells, which surround subperiderm cells that become barb/barbule cells. The latter cells merge into a branched syncitium and form the micro ramification of feathers. The lengthening of barbule cells derives from the polymerization of feather keratin into long bundles coursing along the main axis of cells. Keratin bundles in feather cells are however ordered in parallel rows while those of scales in both alligator and birds are irregularly packed. This observation indicates a different modality of aggregation and molecular structure between the feather keratin of subperiderm cells versus that of barbule/barbs. Barb vane ridge cells among barbule cells degenerate at late stage of feather development leaving spaces that separate barbules. Barb vane ridge cells contain alpha-keratin and lipids, but not beta-keratin. Cells of marginal plates do not contain beta-keratin, and later degenerate allowing the separation of barbs. The latter become isolated only after sloughing of the sheath, which cells contain bundle of keratin not reactive for both scale- and feather-keratin antibodies. The study confirms morphological observations and shows that subperiderm or subsheath cells differentiate into barb and barbule cells. The morphogenesis of barb ridges has to be considered as an evolutionary novelty that permitted the evolution of feathers from a generalized archosaurian embryonic epidermis.