GDF2 and bronchopulmonary dysplasia: Treatment of hyperoxia-induced experimental BPD with BMP9 for 10 days improved aberrant alveolar development by increasing the number of alveolar crests [1.4-fold, p < 0.05; Figures 5D,I (per field) and 1.8-fold, p < 0.01; Figures 5D,J (per tissue ratio)] and reducing MLI (1.1-fold, p < 0.05; Figure 5K), and decreasing alveolar septal thickness (1.5-fold, p < 0.001; Figures 5D, 6M).