Alkaline phosphatase (ALP) activity marks epithelial branching structures in embryonic mouse lung. (A) Mouse embryonic lungs from four different embryonic stages i.e. E11.5, E12.5, E13.5 and E14.5 were stained with ALP substrate NBT/BCIP. Endogenous ALP activity in blue-purple visualizes epithelial branching morphogenesis in the whole-mount lungs. (B) Histology of the stained E13.5 lung shows the specific ALP activity around the airway cavities (*), but not in the mesenchyme. (C) Higher magnification shows that the ALP activity is specific to the apical surface of the lung epithelial cells (arrows).