Origin of ciliated alveolar epithelial cells in bleomycin-induced lung injury

Bleomycin is known to induce diffuse pulmonary fibrosis and epithelial metaplasia. The reaction of the alveolar epithelium following a single intravenous or multiple intraperitoneal injections of bleomycin to mice is now examined in a combined morphologic and cytodynamic study. Necrosis of Type 1 cells was observed, followed by proliferation of Type 2 cells, a common reparative process. The proliferated cells transformed to a variety of epithelial forms, including ciliated cells and cells with morphologic features intermediate between alveolar and bronchiolar epithelium. No evidence of cell injury or increased cell division was found in the bronchial epithelium. It is concluded that the metaplastic ciliated epithelial cells are produced by an abnormal reparative process in the alveolar epithelium. The results suggest that, whereas the "resting" Type 2 cell is not vulnerable to bleomycin, in the postmitotic phase the drug may modify the synthetic mechanisms of cellular differentiation and thereby induce metaplasia.