Figure 8: Feed-forward signaling in which FGF-9 controls Wnt-2a expression and mesenchymal Wnt/β-catenin signaling, and in which mesenchymal Wnt/β-catenin signaling is requisite for mesenchymal FGFR expression and mesenchymal responsiveness to FGF-9. In absence of either FGF-9 or canonical Wnt ligands, the mesenchymal FGF-9-Wnt/β-catenin feed-forward network breaks up, occasioning loss of mesenchymal FGFR expression and FGF responsiveness. Wnt/β-catenin signaling has to be preserved in order to induce or maintain FGFR expression, FGF-9 responsiveness, and continuance of FGF-Wnt/β-catenin feed-forward signaling. Loss of mesenchymal FGF-Wnt/β-catenin signaling increases Noggin expression in both the subepithelial and submesothelial mesenchyme. Lack of either pathway decreases mesenchymal proliferation. The change in the density of the shading of the epithelium shows the proximal-distal extent of the lung bud. Details can be acquired from Yin et al. , Rajagopal et al. , Yi et al.  De Langhe et al. , and Ornitz and Yin . Modified after Ornitz and Yin .