Fgfr3 and Fgfr4 activity is required in the mesenchyme but not the epithelium for alveologenesis. (A,B,E,F) Representative H&E-stained sections from the alveolar regions of P8 Cre control (A,E) and Nkx2-1cre;Fgfr3;4 (B) or Dermo1cre;Fgfr3;4 (F) conditional mutant lungs. Nkx2-1cre;Fgfr3;4 conditional lungs displayed normal alveoli (A,B), whereas Dermo1cre;Fgfr3;4 conditional lungs displayed simplified alveoli. Scale bars: 100 μm. (C,G) MLI analysis indicated no change in the Nkx2-1cre;Fgfr3;4 mutant compared with control (C; P=0.95, n=3) and a statistically significant increase in Dermo1cre;Fgfr3;4 conditional lungs compared with control (G; **P=0.001, n=4). (D,H) qRT-PCR analysis indicated a statistically significant decrease of Fgfr3 expression in conditional mutant lungs compared with control (D, *P=0.02, n=3 for Nkx2-1cre;Fgfr3;4; H, **P=0.002, n=3 for Dermo1cre;Fgfr3;4). Mean±s.e.m.