Figure 1

Lung histology shows levels of lung injury after different days of hyperoxia exposure (original magnification × 400). The alveolar-like structures in full-term newborn rat lungs exposed to normoxia were irregular, and there were a small number of septa on the first day (A). There was no obvious difference in pulmonary morphology in newborn rats exposed to hyperoxia compared with that of the normoxia group on day 1 (B). The septa became thinner and the numbers increased in normoxia group on the third day (C). The septa were thickened and there was neutrophils infiltration in the interstitial space in hyperoxia group on day 3 (D). On the fifth day, the alveoli grew in number, and the septa continued to be thinner in normoxia group (E). In the hyperoxia group, there was more severe interstitial edema and a few inflammatory cells infiltration in the alveolar space on day 5 (F). The alveoli were regular and uniformly distributed in newborn rats exposed to normoxia on the seventh day (G). In the hyperoxia group, there was more inflammatory cells and red blood cell infiltration in the alveolar space with proteinaceous debris filling the airspaces on day 7 (H). Lung injury scores following exposure of newborn rats to normoxia for 1d or hyperoxia for 1d, 3d, 5d and 7d. 20 random high-power fields (400 × total magnification) were independently scored in a blinded fashion for each condition (I). Data are the mean ± SEM (n = 6 in each group); ** P < 0.01 versus normoxia group; && P < 0.01 versus 1 day of hyperoxia exposure group; ## P < 0.01 versus 3 days of hyperoxia exposure group; $$ P < 0.01 versus 5 days of hyperoxia exposure group.