Fig. 1.

Expression dynamics of Shh and Bmp2 in developing embryonic feather buds of plumulaceous and pennaceous feathers. Shh (A) and Bmp2 (B) are expressed in overlapping regions during the development of the feather bud. (C) Shh expression is diffuse in early buds but becomes refined into longitudinal domains, or stripes, in the marginal plate epithelium at the edge of the forming barbs (white arrows) from the initial diffuse expression (black arrow). (D) In plumulaceous down feathers of the chick, the longitudinal Shh expression domains run parallel as the feather grows in length from the bottom. (E) The simple, tufted, plumulaceous, natal down feather of a chick lacks a rachis. (F and G) Pennaceous feather germs in embryonic ducks exhibit helical growth of barb ridges from the ventral (F) to dorsal (G) surfaces. Ventral bifurcation of Shh stripes (F, arrows) leads to the addition of new barb ridges, whereas dorsal extinction of Shh stripes (G, arrows) produces fusion of barb ridges to create the rachis ridge and branched feather form. (H) A pennaceous natal duck feather has a prominent rachis and planar form.