Fine structure of juvenile feathers of the zebrafinch in relation to the evolution and diversification of pennaceous feathers.

The present ultrastructural study describes the formation of feather ramification in developing juvenile feathers of the zebrafinch, a small passeraceous bird. The study stresses the importance of the detailed knowledge on the cell structure of barb ridges for the understanding of feather development and evolution. Feather formation depends on the morphogenesis of long barb ridges, in which cells are displaced into lateral barbule plates and a medial barb cells region. These cells merge into long chains and form a syncitium organized in a ramified structure that preserves the original cell disposition within the barb ridge. Barb vane ridge cells surround barb and barbule cells. Barbules separate after the degeneration of barb vane ridge cells. In barbule cells the formation of hooklets resembles the process of formation of climbing setae of digital pads of some lizards. The cytoplasm of barb vane ridge cells is localized among tile-like overlapped barbule cells that form barbule chains, and maintains a serrated outline. When barb vane ridge cells degenerate among keratinized barbules, keratinized hooklets remain. Hooklets allow the ordered grasping of barbules to form a close and planar vane of feathers. The rachis of juvenile feathers seems to be formed from the fusion of two or more barb ridges localized in the dorsal part of the follicle, but the process of fusion is unclear. Juvenile and adult feathers contain the same type of feather keratin present in downfeathers: this indicates that stem cells for the regeneration of a new feather remain in the follicle after shedding of downfeathers. The presence of embryonic organelles (periderm granules) in barb vane ridge cells of juvenile feathers further indicates that also stem cells for the regeneration of the latter cells remain in the follicle. Molting feathers are therefore derived from stem cells. The permanence of stem cells in the follicle and the modulation of barb ridges dimension and fusion into different patterns allow the production of different feather morphotypes such as contour, filoplumes, semiplumes, and bristles.