Wnt-7a in feather morphogenesis: involvement of anterior-posterior asymmetry and proximal-distal elongation demonstrated with an in vitro reconstitution model

How do vertebrate epithelial appendages form from the flat epithelia? Follo wing the formation of feather placodes, the previously radially symmetrical primordia become anterior-posterior (A-P) asymmetrical and develop a proxi mo-distal (P-D) axis. Analysis of the molecular heterogeneity revealed a su rprising parallel of molecular profiles in the A-P feather buds and the ven tral-dorsal (VD) Drosophila appendage imaginal discs. The functional signif icance was tested with an in vitro feather reconstitution model. Wnt-7a exp ression initiated all over the feather tract epithelium, intensifying as it became restricted first to the primordia domain, then to an accentuated ri ng pattern within the primordia border, and finally to the posterior bud. I n contrast, sonic hedgehog expression was induced later as a dot within the primordia, RCAS was used to overexpress Wnt-7a in reconstituted feather ex plants derived from stage 29 dorsal skin to further test its function in fe ather formation. Control skin formed normal elongated, slender buds with A- P orientation, but Wnt-7a overexpression led to plateau-like skin appendage s lacking an A-P axis. Feathers in the Wnt-7a overexpressing skin also had inhibited elongation of the P-D axes. This was not due to a lack of cell pr oliferation, which actually was increased although randomly distributed. Wh ile morphogenesis was perturbed, differentiation proceeded as indicated by the formation of barb ridges, Wnt-7a buds have reduced expression of anteri or (Tenascin) bud markers. Middle (Notch-1) and posterior bud markers inclu ding Delta-1 and Serrate-1 were diffusely expressed. The results showed tha t ectopic Wnt-7a expression enhanced properties characteristic of the middl e and posterior feather buds and suggest that PD elongation of vertebrate s kin appendages requires balanced interactions between the anterior and post erior buds.