Lung lymphatics cast from the airspace

Lymphatics are important in the resolution of pulmonary edema, but which lymphatics drain alveolar fluid and how they change during lung injury and edema is uncertain. To study this question 16 rats were exposed to 85% O₂ for 7 days. At 0, 3, 7, and 14 days after removal from the hyperoxic chamber, the lungs of the rats were cast by instilling methyl methacrylate into the trachea. The lungs of four similar room-air breathing rats served as controls. Tissue was taken for light microscopy and the casts were examined for lymphatic filling with a scanning electron microscope. Rats exposed to hyperoxia had diffuse damage and extensive edema. On removal from hyperoxia (day 0), 29% of the rat bronchioles had saccular lymphatic casts around them and 6% of bronchioles were surrounded by these lymphatics. Twenty-five percent of bronchioles had conduit lymphatic casts. Fourteen percent of arteries had lymphatic casts around them. All were different from the rats kept in room air (P < 0.0001). Rats exposed to hyperoxia had lymphatics on the pleural surface, near alveoli and alveolar ducts, and around veins. The peribronchial and periarterial saccular lymphatics formed separate groups with communicating conduit lymphatics. The perivenous lymphatics had their own separate conduit lymphatics. Fourteen days after returning to ambient air, the lymphatics were similar to those of control animals. In this model, airway casting allows three-dimensional analysis of the lung lymphatics. It shows that lymphatic compartments expand during hyperoxic lung injury and that peribronchial and perivascular saccular lymphatics connect to conduit lymphatics of the bronchovascular bundle.