Fig. 1. Nmyc expression and BrdU incorporation in the wild-type lung. (A) RT-PCR shows that Nmyc transcripts decrease during lung development, whereas those for differentiated epithelial cell markers (Sftpc, Sftpa and Aqp5) increase. (B) Immunohistochemistry at E12.5, E14.5 and E16.5 shows higher Nmyc expression in the distal compared with the proximal epithelium. Blue arrows mark the boundary between the two regions. At E18.5, low levels are seen in presumed type II cells (insets). (C) Immunostaining for Nmyc, BrdU, cyclin D1 and phosphohistone H3 (PHH3) in adjacent sections of E12.5 lung. (D) Percentage of BrdU-positive nuclei in Nmyc-high distal endoderm and Nmyc-low proximal endoderm of E12.5 lungs, as determined by counting nuclei in cross sections of tubules, where the identification of individual cells is unambiguous. ^*, difference is significant (P<0.001). Scale bars: 50 µm.