FIGURE 3.

Schematic diagram introducing the concept of feather morphogenesis. (a) An evolutionary novel event set up new branch types with qualitative advancement. The core branching module converts cylindrical feather filaments into barb branches. The rachis forming module sets up the rachis, converting radial to bilateral symmetric feather forms. The position of the barb generating module helps set up medial – lateral feather vane asymmetry. Distal feathers are formed first, with stem cells continuously generated from the proximal portion. This can be appreciated by viewing both the cross-sectional view (upper left) and whole feather view. The calamus in the base of the rachis has a cylindrical filament. (b) The concepts of morpho-regulation. Morpho-regulation modulates each dimension of feather forms quantitatively, and can generate diverse feather forms based on a prototypic feather. Molecules involved in each module are listed in the figure. By modulating the relative strengh of the molecular signal, each component can be amplified or shrunken or bent, topologically and disproportionately (i.e., by anisotropic positioned signaling modules) to generate different feather forms. (1) contour feather to (2) an elongate symmetric feather, (3) a fan shaped feather, (4) an asymmetric feather, (5) a filoplume or (6) a curved feather. The figure is drawn in the style of “On the Growth and Form” (Thompson, 1917).

Following molting and in each feather regeneration stage, these modules can be regulated by the surrouding stem cell niche, whose properties can be modulated by environmental factors such as sex hormones, circadian rhythm changes, seasonal changes etc. to generate diverse feather forms for best adaptation.