Effect of hyperoxia and transgenic (TG) extracellular superoxide dismutase (EC-SOD) on alveolar and bronchiolar epithelial proliferation at postnatal days 3, 5, and 7 (P3, P5, and P7). Bromodeoxyuridine (BrdU) labeling indexes in cells lining distal air sacs (A) or bronchioles (B); means are 6 pups/group + SE. *P < 0.05 for indicated comparisons. Ki67 labeling indexes for cells lining air sacs (C) or bronchioles (D). E: BrdU labeling indexes for type II [surfactant protein C (SP-C)-positive cells], 5 pups/group, means + SE. *P < 0.05 wild type (WT) vs. TG. F: SP-C-immunostained (brown) cells are shown by black arrows; BrdU-labeled (purple) cells are shown by white arrows.