The pleura: a combined light microscopic and scanning and transmission electron microscopic study in the sheep. II. Response to injury.

The effects of acid-induced injury to the full thickness of the ovine pleura were studied using light, scanning, and transmission electron microscopy. Thirty minutes after injury, the mesothelial layer had disappeared and necrosis extended down to the subjacent alveolar walls. This was followed by an exudative phase, with edema of the pleural interstitium and subjacent alveoli, together with a marked congestion of alveolar capillaries that peaked at 2 days. Subsequently, the edema and congestion subsided and proliferation of fibroblasts and histiocytes was seen in the pleural interstitium. The subjacent alveoli were considerably distorted by interstitial thickening and alveolar type II cell proliferation. Unlike other studies in the peritoneum, no evidence for a diffuse recolonization of the mesothelial surface was found. Our work indicates that the damaged surface is rapidly covered by a layer of fibrin. Cells from the wound edges became plump and their processes projected toward the denuded region, indicating a process of centripetal healing, with migration of cells in irregular wedges to recolonize the surface.