ABSTRACT:In the present study, we tested the hypothesis that exposure of newborn mice to sublethal hyperoxia would alter lung development and expressions of fibroblast growth factor receptors (FGFRs)-3 and FGFR-4. Newborn FVB mice were exposed to 85% O 2 or maintained in room air for up to 14 d. No animal mortality was observed, and body weight gains were not affected by hyperoxia. At postnatal d 7 and 14 (P7, P14), lungs of mice exposed to 85% O 2 showed fewer alveolar secondary crests and larger alveoli or terminal air spaces than did mice in room air. In pups kept in room air, lung levels of FGFR-3 and FGFR-4 mRNA were greater at P3 than at P1, but similar increases were not observed in hyperoxic mice. Immunoreactivity of FGFR-3 and FGFR-4 was lower in lungs of hyperoxic mice than in controls at P14. In pups kept in room air, lung fibroblast growth factor (FGF)-7 mRNA levels were greater at P14 than at P1, but similar changes were not observed in hyperoxic mice. The temporally and spatially specific alterations in the expressions of FGFR-3, FGFR-4, and FGF-7 in the mice exposed to hyperoxia may contribute to aberrant lung development. B ronchopulmonary dysplasia (BPD) was described initially as lung injury-and repair-related effects that were associated with radiologic findings of streaky, fibrous densities, alternating with hyperlucent areas (1,2). The primary risk factors were premature birth, respiratory distress, mechanical ventilation, and administration of supplemental oxygen (FIO 2 ÏŸ0.21). Inflammation, problems in nutritional support, and comorbid conditions also contribute to the development of BPD. Modifications of therapeutic strategies have decreased the incidence of BPD, as the disease was described originally, but the overall incidence of BPD has not declined. The "new BPD" of recent years shows little acute injury and repair described by Northway et al. (1,2), but is characterized by fewer and larger alveoli and less organized alveolar vascularization, suggesting arrested or disordered lung development. The events occurring during secondary septation of terminal gas exchange units and maturation of alveolar microvasculature are therefore of particular interest in research on mechanisms responsible for the new BPD (3-8).Lung development in humans normally progresses through sequential structural changes that are observed similarly in rats and mice, which, at term, exhibit lungs that structurally resemble human lungs at 26 -30 wk of gestation (9). Infants born at this age frequently encounter respiratory problems, arising from immature pulmonary surfactant metabolism, respiratory drive and coordination, or other deficiencies. Prematurely delivered nonhuman primate models of BPD may mimic human development more closely than do rodent models, but practical constraints limit the investigations that can be explored with primate models (4,5,10).The lungs of newborn rodents are sufficient for extrauterine life and are adequate for maturation into normal adult lungs. Studies in rats and mice indicate th...