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New paper - evo-devo of feathers
Alibardi, L. (2005). Cell structure of developing barbs and barbules in
downfeathers of the chick: central role of barb ridge morphogenesis for the
evolution of feathers. Journal of Submicroscopic Cytology and Pathology.
37: 19-41
Abstract: "The present ultrastructural study shows how cells organize to
form the complex structure of down-feathers in chick embryos. The embryonic
epidermis of the apical part of feather filaments folds inward forming barb
ridges which extend toward the base of the feather. The stratification of
epidermal cells in barb ridges is maintained but the basal layer loses most
of the germinal activity. New cells for the growth of feather filaments are
mainly produced in its basal part. In barb ridges only the original four
epidermal layers of the embryonic epidermis remain to form feathers: 1) the
external periderm, 2) three-five layers of the feather sheath and barb vane
ridge cells, 3) subperiderm cells, and 4) basal or cylindrical cells.
Periderm, sheath, barb vane ridge and cylindrical cells synthesize only
alpha-keratin. Instead, cells of the subperiderm layer synthesize a small
type of beta-keratin: feather beta-keratin. At hatching, the subperiderm
layer is lost in most areas of the skin of the chick (apteric and scaled),
and is replaced by cells containing alpha-keratin (interfollicular-apteric
epidermis), scale beta-keratin (scales), beak beta-keratin (beak), and claw
beta-keratin (claws). Only in feathers, cells of the original subperiderm
layer remain and give origin to barb and barbule cells. The formation of
separated chains of barb and barbule cells is allowed by the presence of
barb vane ridge cells that function as spacers between merging cells of barb
and barbule cells. Subperiderm cells elongate and merge into a syncitium to
form barbules and barbs. While barbule and barb cells accumulate
feather-keratin, barb vane and cylindrical cells accumulate lipids, vesicles
and little alpha-keratin. These cells eventually degenerate by necrosis
leaving empty spaces and lipids between barbules and barbs. No apoptosis is
necessary to explain the process of carving out of barb and barbules in
feathers after dissolution of the external sheath. In fact, the retraction
of blood vessels nourishing the apical part of the feather filament
determines anoxia and eventually necrosis of all cells of the feather. While
sheath, barb vane and cylindrical cells degenerate, the keratinized
syncitium forming barbs and barbules simply remain in place to form the
ramifications of feathers. The formation of barb ridges is considered as the
evolutionary innovation necessary for the origin of feathers. The evolution
of the morphogenetic process of barb ridge formation within epidermal
tubular outgrowths of the integument of ancient archosaurians was an
evolutionary novelty, a true avian and theropod characteristic. Barb ridges
morphogenesis determines the contemporary formation of barb and barbule
cells as a unique and inseparable process so that intermediate forms of
evolving feathers with only barbs but not barbules are unlikely. Barb ridges
can merge with a large ridge (rachis) or into branched ridges, a process
which was at the origin of the ramogenic process from which pennaceous
feathers evolved."