Hyperoxia reversibly suppresses development of bronchiolar epithelium

The bronchiolar epithelium of rats is anatomically immature at birth. We now ask whether postnatal hyperoxia impairs the normal development of bronchiolar epithelium; and, if development is impaired, is the impairment permanent? To answer these questions, we exposed newborn rats to hyperoxia (greater than 95% O2, 1 atm) or air for 7 days and killed the rats at age 7 or 30 days. We used ultrastructural and morphometric means to assess maturation of the bronchiolar epithelium. Hyperoxia substantially diminished the postnatal increase in nuclear numerical density of bronchiolar Clara cells and ciliated cells. Hyperoxia also markedly delayed the rise in volume density of Clara cell secretory granules and rough endoplasmic reticulum but accelerated the increase in volume density of Clara and ciliated cell mitochondria. When rats exposed to hyperoxia from age 1 to 7 days were thereafter allowed to breathe air, by age 30 days all the differences were eliminated that were detected between the air- and O2-breathing groups at age 7 days. We conclude hyperoxia causes a marked but nonpermanent suppression of maturation of the bronchiolar epithelium.