Histological changes of terminal bronchioles in rats during the exposure to nitrogen dioxide.

At long-term exposure to 5 ppm NO2 (w/w), courses of histological changes in the epithelium of the terminal bronchioles of rats were observed. In the course of the experiment the first alterative changes were expressed by the loss of cilia and cell differentiation. The changes were followed by epithelial hyperplasia, leading to conspicuous narrowing of the lumen of bronchioles. After the phase of hyperplasia a phase of reparation followed, leading to the development of nonciliated epithelium without determinable Clara cells, with areas of uniform multi-layered epithelium. At the same time, in the longest interval of 11 weeks new alterative changes analogous to those found at the beginning of the experiment were observed.