An immunohistochemical study of the fibrosing process in paraquat lung injury

Twenty-nine autopsy cases of paraquat-induced lung injury were studied by histological and immunohistochemical methods. Two stages of injury were identified. In the early stage, the alveolar epithelium degenerates but the epithelial basement membrane remains intact. In the late stage, the epithelial basement membrane is focally disrupted, the mesenchymal cells grow into the alveolar space, and intra-alveolar fibrosis appears. In spite of these pathological changes, the original framework of the alveolar wall is preserved in many areas. Intra-alveolar fibrosis may follow as a consequence of damage to the epithelium without severe damage to the underlying basement membrane, which occurs at the stage of organization. Morphological variants of intra-alveolar fibrosis seem to occur not only to the size of the defect of the basement membrane but also to the difference in the stages of evolution at the time the lesion is studied. The epithelium regenerates along the basement membrane in the early stage of re-epithelialization, but grows over the luminal aspect of intra-alveolar fibrous tissue which has been laid on the remaining basement membrane in the late stage. It is speculated that the regeneration of epithelial cells may develop without any association with the basement membrane when a fibrous tissue covers the original basement membrane.