Fig. 8. A model for the role of Wnt7b in lung development. (A) Wnt7b is expressed at the distal tips of the airway epithelium in a pattern similar to that observed with BMP4 and overlapping that of SHH. In addition, Wnt7b is expressed in an increasing gradient from the proximal-to-distal airway epithelium. FGFs are expressed in the mesenchyme and are known to regulate epithelial branching and proliferation. However, because BMP-4 and SHH expression is unchanged in Wnt7b^lacZ–/– embryos and Wnt7b expression is unchanged in Shh-null mice, Wnt7b regulates mesenchymal proliferation and differentiation through a unique pathway. (B) Lung vasculature is composed of both endothelium (red) and vascular smooth muscle (VSMC, blue), and develops in parallel with the airways (green). Loss of Wnt7b function results in defects in vascular smooth muscle differentiation and/or survival leading to a hypertrophic response (change from dark blue to light blue), degradation of the vessel wall and eventual rupture of the weakened vessels.