Histological analysis of lung tissue from E15.5 to E18.5 wild-type and cPGES/p23−/− embryos by light microscopy. (A) Total RNA was extracted from E18.5 embryo lungs. Quantitation of RNA revealed a significant increase in the cPGES/p23−/− lungs, suggesting hypercellularity. (B) Lungs were removed from embryos at E15.5, E16.5, E17.5, and E18.5 and fixed overnight in 10% formalin. Lung sections were stained with hematoxylin-eosin. The cPGES/p23−/− lung development appears arrested at the canalicular stage (E16.0 to 17.5). Notable is the failure of the development of terminal sacs. Original magnification, ×10.