Expression of β-galactosidase in TOPGAL embryos shows dynamic changes in Wnt signaling during lung development. (a) Intact embryo at embryonic day 9.5 (E9.5). (b) High magnification of the region boxed in (a); the arrow marks a primordial lung bud. (c) A section of E9.5 embryo showing expression in the primordial lung bud and undivided trachea/esophagus (arrows). (d) At E11.5 expression is seen in the anterior trachea, distal lung buds and anterior stomach (arrows). (e) E12.5 whole lung, with a white line showing the level of section of the trachea in (f); the inset shows expression in the dorsal tracheal endoderm (D) and ventral mesoderm (V). (f) Section of E12.5 lung, at the position shown by the white line in (e). Note the heterogeneity of staining intensity in the endoderm. The inset shows immunolocalization of β-catenin in the nuclei of distal epithelial cells. (g) E15.5 whole lung. (h) Section of E15.5 lung, showing decreased expression in distal tubules. (i) Section of E18.5 lung, showing expression confined to the bronchi and bronchioles. (j) Section of postnatal (2 weeks) lung; the inset shows a higher magnification of the positive cells near the bronchiolar/alveolar junction. At all stages described here, non-transgenic tissue was negative for endogenous β-galactosidase activity. Scale bar, 100 μm (j) also applies to (c,f,h,i).