Weighert’s elastin staining of control (normoxia) and hyperoxic (hyperoxia) lungs at D5, D28 and D56. Elastin is stained black. Cytoplasm and nuclei of all cell types are stained yellow. Secondary septa (ss) are marked with black arrows, blood vessels surrounded by a layer of elastin (v) are marked with red arrows, and elastin depositions in the alveolar walls (ad) are marked with blue arrows. Scale bar = 100 μm.