FIGURE 5. Models of feather branching and evolution of feather forms.

a, Roles of noggin/BMP4, Shh and BMP2 in the three levels of feather branching. E, epithelial cells (blue); M, mesenchymal cells (pink). fs, feather sheath; mp, marginal plate; pe, pulp epithelium. b, The ratio of noggin and BMP4 may determine the number and size of barb ridges. A localized high BMP:noggin ratio, together with a helical growth mode of barb ridges¹⁷, can lead to the formation of a rachidial ridge through fusion of barb ridges. c, Hypothetical models of the evolution of feather forms. Top row, barb to rachis model; bottom row, rachis to barb model. The experimental data are in favour of the barb to rachis model.