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Timeline of rat lung development. Representative images of female rat lungs at postnatal day 0 (P0), P6, P14, P21, and P65. At P0, the rat lung is at the saccular stage of lung development. Alveolar formation takes place from approximately P4 to P14. At P21 and P65, alveoli have characteristic histological features of long, straight alveolar walls and numerous, long secondary septa. All panels original magnification 80x (scale bar = 50 μm).
