MUCOUS CELL METAPLASIA IN RAT NASAL EPITHELIUM AFTER A 20-MONTH EXPOSURE TO OZONE - A MORPHOMETRIC STUDY OF EPITHELIAL DIFFERENTIATION

The present study was designed to examine the effects of long-term ozo ne exposure on nasal epithelia and intraepithelial mucosubstances (IM) throughout the nasal airways of F344/N rats. Animals were exposed to 0 (controls), 0.12, 0.5, or 1.0 ppm ozone, 6 h/day, 5 days/wk, for 20 mo. Rats were killed 1 wk after the end of the exposure, and nasal tis sues were processed for light and electron microscopy. Standard morpho metric techniques were used to determine epithelial cell densities and the amounts of IM in the surface epithelium lining the nasal airways. No mucous cells or IM were present in the epithelia lining the nasal lateral meatus and maxillary sinus of rats exposed to 0 or 0.12 ppm oz one. In contrast, rats exposed to 0.5 or 1.0 ppm ozone had marked muco us cell metaplasia (MCM) with numerous mucous cells and conspicuous am ounts of IM in the surface epithelium lining these upper airways. Ozon e-induced increases in total epithelial cells (i.e., epithelial hyperp lasia) were present only in rats exposed to 1.0 ppm. The results of th is study indicate that rats chronically exposed to 1.0 or 0.5 ppm; but not 0.12 ppm, ozone can develop marked MCM with significant increases in IM in both proximal and distal nasal airways. The epithelial chang es observed throughout the nasal passages of ozone-exposed rats may be adaptive responses in an attempt to protect the upper and lower respi ratory tract from further ozone-induced injury.