Cells of embryonic and regenerating germinal layers within barb ridges: implication for the development, evolution and diversification of feathers.

The formation of feathers occurs by the transformation of the embryonic epidermis of feather filaments into keratinized barbules and barbs. The present ultrastructural study directly documents this transformation in chick and zebrafinch downfeathers and juvenile feathers. The transformation of the epidermis in the feather filament (downfeathers) or in the follicle (juvenile feathers) is similar. The change in cell shape of subperiderm or subsheath cells and surrounding barb vane ridge cells derives from the re-organization of the linear embryonic epithelium into barb ridges. In the latter the stratification of the outer and inner periderm, of the subperiderm/subsheath, and of the germinal layer of the embryonic epidermis is altered. While the external layers produce the sheath and barb vane ridge cells, subperiderm/subsheath cells are displaced into barbule plates that converge medially in the ramus area of the barb ridge. Cells in the barbule plates elongate into barbule and barb cortical cells by the synthesis of longitudinally oriented feather keratin bundles. In the mid-central area of the barb ridge (the ramus area) cells become polygonal and pile up. The external cells accumulate numerous keratin filaments forming cortical cells and are in contact with barbule cells. The above process also occurs in barb ridges of juvenile feathers and of adult feathers before molting. However, barb ridges produced within follicles of juveniles and adult feathers are longer than in downfeathers, and possess long rami. The incorporation of tritiated histidine in barbule and barb cortical cells has been studied by ultrastructural autoradiography. Most of the labeling is cytoplasmic or is associated with bundles of keratin but is not concentrated over keratin. This indicates that together with keratin possible histidine-rich keratin-associated proteins are produced during the elongation from subperiderm/subsheath to barbule/barb cells. Barb cortical cells merge with medullary cells of the ramus area. The latter accumulate lipids and few keratin bundles before degenerating into empty cells. Separation between barbule and barb cortical cells derives from the degeneration of barb vane ridge cells while separation between barb ridges derives from degeneration of cylindrical cells of marginal plates. These supportive cells incorporate less tritiated histidine than barbule/barb cells and their periderm granules are unlabelled with tritiated histidine. This indicates both that supportive cells are metabolically less active than feather-producing cells, and that putative histidine-rich proteins are only present in cells synthesizing feather keratin. Based on the morphogenesis of barb ridges a hypothesis on the evolution of downfeathers and pennaceous feathers is presented. From conical scales, thin hairy-like filaments were produced in which barb ridges were formed. The evolution of barb ridge morphogenesis with no fusion among barb ridges initially produced naked or branched barb-feathers (plumulaceous). After the formation of a follicle, the modulation of barb ridges patterning and their fusion into the rachis produced all the phenotypes of pennaceous feathers, including those later selected for flight.