New paper - evo-devo of feathers

[Tim Williams <twilliams_alpha@hotmail.com>]

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."