Effects of hyperoxia on neonatal mouse lung. Inflated lungs were paraffin embedded and five-micrometer tissue sections were stained with hematoxylin and eosin (H&E). All pictures were taken at 200 x magnification. Calibration bar is 50 μm. (A) Exposure to room air at postnatal day 14 (B) Exposure to hyperoxia at postnatal day 14 (C) Exposure to room air at postnatal day 29 (D) Exposure to hyperoxia at postnatal day 29 (E) Mean linear intercept (F) Alveolar septal thickness (n = 11 mice/group; mean ± SE, * p < 0.05 P14-O2 vs. P14-RA, ** p < 0.05 P29-O2 vs. P29-RA). (G) Relative mRNA expression of several biomarkers of BPD by RT-PCR comparing mouse lung exposed to room air with hyperoxia (n = 3, mean ± SE, # p < 0.05 vs. P14 room air controls, * p < 0.05 vs. P29 room air controls).