A comparative study of cell renewal after exposure to ozone or oxygen. Response of terminal bronchiolar epithelium in the rat.

Cell renewal patterns of the terminal bronchiolar epithelium of rats were examined during 7-day exposures to either 0.8 ppm of ozone or 80 per cent O2. Terminal bronchiolar epithelial cells were classified and counted in ciliated, nonciliated secretory (Clara), and nonciliated, nonsecretory categories. Thymidine labeled with hydrogen-3 was used in conjunction with light microscopic autoradiography to obtain labeling indices in exposed rats at 6, 24, 72, and 168 hours compared to control rats at 6 and 168 hours. Results indicated that both ozone and O2 initiated a proliferative response of terminal bronchiolar epithelium, but that the response to O2 insult was delayed in onset and of lesser magnitude. Most importantly, the differential cell counts revealed that nonciliated secretory cells were the primary proliferating cell type at all periods after oxidant insult. This proliferative response continued with persistence of the insult through 168 hours. Shifts in proportions of cell types and labeling indices during exposure supported the view that nonciliated secretory cells are the reparative bronchiolar cells and that new ciliated cells are mainly derived from them.