Fig. 4. Wnt7b^lacZ–/– embryos exhibit thin mesenchyme and reduced distal mesenchyme proliferation at E12.5. Wnt7b^lacZ–/– embryos at E12.5 and E14.5 exhibit thinner mesenchyme than do Wnt7b^lacZ+/– littermates, with the airway epithelium growing almost directly next to the mesothelium of the lung (A,B,E,F, arrows). Samples were stained for ß-galactosidase expression to better visualize the airway epithelium (A,B,E,F). Staining of lung tissue with a monoclonal antibody to phospho-histone H3, which detects mitotic cells, shows decreased staining in the distal mesenchyme of Wnt7b^lacZ–/– embryos at E12.5 (C,D, yellow arrowheads). By E14.5, no significant difference in cell proliferation is observed between Wnt7b^lacZ–/– and Wnt7b^lacZ+/– embryos in the lung (G,H). Quantification of cell proliferation shows that distal lung mesenchyme proliferation is reduced by approximately two-thirds in E12.5 Wnt7b^lacZ–/– embryos (I). Scale bars: 150 µm in A,B; 125 µm in C,D; 250 µm in E,F; 200 µm in G,H. dm, distal mesenchyme; de, distal epithelium.