The formation of periodic patterns is fundamental in biology. Theoretical models describing these phenomena have been proposed for feather patterning; however, no molecular candidates have been identified. Here we show that the feather tract is initiated by a continuous stripe of Shh, Fgf-4, and Ptc expression in the epithelium, which then segregates into discrete feather primordia that are more strongly Shh and Fgf-4 positive. The primordia also become Bmp-2 and Bmp-4 positive. Bead-mediated delivery of BMPs inhibits local feather formation in contrast with the activators, SHH and FGF-4, which induce feather formation. Both FGF-4 and SHH induce local expression of Bmp-4, while BMP-4 suppresses local expression of both. FGF-4 also induces Shh. Based on these findings, we propose a model that involves (1) homogeneously distributed global activators that define the field, (2) a position-dependent activator of competence that propagates across the field, and (3) local activators and inhibitors triggered in sites of individual primordia that act in a reaction-diffusion mechanism. A computer simulation model for feather pattern formation is also presented.

The most unique character of the feather is its highly ordered hierarchical branched structure 1, 2 . This evolutionary novelty confers flight function to birds [3][4][5] . Recent discoveries of fossils in China have prompted keen interest in the origin and evolution of feathers [6][7][8][9][10][11][12][13][14] . However, controversy arises whether the irregularly branched integumentary fibers on dinosaurs such as Sinornithosaurus are truly feathers 6,11 , and whether an integumentary appendage with a major central shaft and notched edges is a non-avian feather or a proto-feather [8][9][10] . Here we take a developmental approach to analyze molecular mechanisms in feather branching morphogenesis. We have used the replication competent avian sarcoma (RCAS) retrovirus 15 to efficiently deliver exogenous genes to regenerating chicken flight feather follicles. We show that the antagonistic balance between noggin and bone morphogenetic protein 4 (BMP4) plays a critical role in feather branching, with BMP4 promoting rachis formation and barb fusion, and noggin enhancing rachis and barb branching. Furthermore we show that sonic hedgehog (SHH) is essential for apoptosis of the marginal plate epithelia to become spaces between barbs. Our analyses show the molecular pathways underlying the topological transformation of feathers from cylindrical epithelia to the hierarchical branched structures, and provide first clues on the possible developmental mechanisms in the evolution of feather forms.With three branching levels, i.e. from rachis to barbs; from barbs to barbules and from barbules to cilia or hooklets 1 (Fig. 1a), feathers can develop into a variety of forms, including the downy, contour, flight feathers, etc. (Fig. 1b). As in hairs, the feather follicle is composed of a dermal papilla and epidermal collar (equivalent to the hair matrix, Fig. 1c-f). Through epithelial-mesenchymal interactions, the epithelial cells at the bottom of the follicle undergo active proliferation (proliferation zone, Fig. 1c). Immediately above, the epithelial cells start to form the rachidial ridge and the barb ridges (ramogenic zone, Fig. 1c, f) [16][17][18][19] . Further distal, the barb ridge epithelia actively proliferate and differentiate to form the marginal plates, barbule plates and axial plates (Fig. 1e, central part). The barb ridges grow to form barbs, composed of the ramus and barbules, while the marginal and axial plate cells die to become the intervening space. Individual barbule plate cells undergo further cell shape changes to form the cilia and hooklets 1 . The barb ridges fused proximally to form the Correspondence and requests for materials should be addressed to: Cheng-Ming Chuong, chuong@pathfinder.usc.edu. Competing interests statementThe authors declare that they have no competing financial interests. Fig. 1) illustrate this process. HHS Public AccessThe cellular and molecular mechanisms of epithelial organ morphogenesis are beginning to be understood 20,21 . While branching morphogenesis 21 has been studied...

Stem cells cycle through active and quiescent states. Large populations of stem cells in an organ may cycle randomly or in a coordinated manner. Although stem cell cycling within single hair follicles has been studied, less is known about regenerative behavior in a hair follicle population. By combining predictive mathematical modeling with in vivo studies in mice and rabbits, we show that a follicle progresses through cycling stages by continuous integration of inputs from intrinsic follicular and extrinsic environmental signals based on universal patterning principles. Signaling from the WNT/bone morphogenetic protein activator/inhibitor pair is coopted to mediate interactions among follicles in the population. This regenerative strategy is robust and versatile because relative activator/inhibitor strengths can be modulated easily, adapting the organism to different physiological and evolutionary needs.

Hair follicles facilitate the study of stem cell behavior because stem cells in progressive activation stages, ordered within the follicle architecture, are capable of cyclic regeneration. To study the gene network governing the homeostasis of hair bulge stem cells, we developed a Keratin 15-driven genetic model to directly perturb molecular signaling in the stem cells. We visualize the behavior of these modified stem cells, evaluating their hair-regenerating ability and profile their molecular expression. Bone morphogenetic protein (BMP)-inactivated stem cells exhibit molecular profiles resembling those of hair germs, yet still possess multipotentiality in vivo. These cells also exhibit up-regulation of Wnt7a, Wnt7b, and Wnt16 ligands and Frizzled (Fzd) 10 receptor. We demonstrate direct transcriptional modulation of the Wnt7a promoter. These results highlight a previously unknown intra-stem cell antagonistic competition, between BMP and Wnt signaling, to balance stem cell activity. Reduced BMP signaling and increased Wnt signaling tilts each stem cell toward a hair germ fate and, vice versa, based on a continuous scale dependent on the ratio of BMP/Wnt activity. This work reveals one more hierarchical layer regulating stem cell homeostasis beneath the stem cell-dermal papilla-based epithelial-mesenchymal interaction layer and the hair follicle-intradermal adipocyte-based tissue interaction layer. Although hierarchical layers are all based on BMP/Wnt signaling, the multilayered control ensures that all information is taken into consideration and allows hair stem cells to sum up the total activators/inhibitors involved in making the decision of activation.hair follicle stem cells | β-catenin | BMPR1A

Beak shape is a classic example of evolutionary diversification. Beak development in chicken and duck was used to examine morphological variations among avian species. There is only one proliferative zone in the frontonasal mass of chickens, but two in ducks. These growth zones are associated with bone morphogenetic protein 4 (BMP4) activity. By "tinkering" with BMP4 in beak prominences, the shapes of the chicken beak can be modulated.