Sequential Changes in Lung Morphology during the Repair of Acute Oxygen-induced Lung Injury in Adult Rats

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Abstract
We studied changes in lung ultrastructure and collagen content during the repair of acute lung injury in adult rats exposed to 100% O2 for 60 h and recovering in ambient air. In the interstitium, during the first 3 days of repair, the number of neutrophils decreased 16-fold, and monocytes and lymphocytes increased to 7-fold and 4-fold the respective control values. Myofibroblasts increased about 5-fold and the volume of the interstitial matrix remained high. By 7 days, the differential count of inflammatory cells was normal although the number of total interstitial cells and myofibroblasts decreased more slowly. In the capillary endothelium, after 3 days of repair, the cells were hypertrophied and had organelle-rich cytoplasm, and total cell number had increased back to control values; endothelial cell number increased an additional 63% between 3 and 7 days of repair. In the epithelium, type 2 cells increased 150% during the first 3 days of repair before decreasing; type 1 cell number did not change. After 28 days of repair, the lungs appeared qualitatively almost normal; however, interstitial cell number and collagen content were still increased. We conclude that the repair of oxygen-induced lung injury involves a complex pattern of morphologic changes that has important similarities to those occurring during repair on other tissues such as the skin.