Review: cornification, morphogenesis and evolution of feathers

Lorenzo Alibardi

Feathers are corneous microramifications of variable complexity derived from the morphogenesis of barb ridges. Histological and ultrastructural analyses on developing and regenerating feathers clarify the three-dimensional organization of cells in barb ridges. Feather cells derive from folds of the embryonic epithelium of feather germs from which barb/barbule cells and supportive cells organize in a branching structure. The following degeneration of supportive cells allows the separation of barbule cells which are made of corneous beta-proteins and of lower amounts of intermediate filament (IF)(alpha) keratins, histidine-rich proteins, and corneous proteins of the epidermal differentiation complex. The specific protein association gives rise to a corneous material with specific biomechanic properties in barbules, rami, rachis, or calamus. During the evolution of different feather types, a large expansion of the genome coding for corneous feather beta-proteins occurred and formed 3-4-nm-thick filaments through a different mechanism from that of 8-10 nm IF keratins. In the chick, over 130 genes mainly localized in chromosomes 27 and 25 encode feather corneous beta-proteins of 10-12 kDa containing 97-105 amino acids. About 35 genes localized in chromosome 25 code for scale proteins (14-16 kDa made of 122-146 amino acids), claws and beak proteins (14-17 kDa proteins of 134-164 amino acids). Feather morphogenesis is periodically re-activated to produce replacement feathers, and multiple feather types can result from the interactions of epidermal and dermal tissues. The review shows schematic models explaining the translation of the morphogenesis of barb ridges present in the follicle into the three-dimensional shape of the main types of branched or un-branched feathers such as plumulaceous, pennaceous, filoplumes, and bristles. The temporal pattern of formation of barb ridges in different feather types and the molecular control from the dermal papilla through signaling molecules are poorly known. The evolution and diversification of the process of morphogenesis of barb ridges and patterns of their formation within feathers follicle allowed the origin and diversification of numerous types of feathers, including the asymmetric planar feathers for flight.