THE FORMATION OF ALVEOLI IN RAT LUNG DURING THE 3RD AND 4TH POSTNATAL WEEKS - EFFECT OF HYPEROXIA, DEXAMETHASONE, AND DEFEROXAMINE

Terminal gas-exchange units in the lung of many species are, at birth, relatively large structures termed saccules. Saccules septate postnatally forming smaller units that constitute the final alveoli. In the rat, septation occurs intensively during the first 2 postnatal wk after which it has been considered to stop. Treatment with dexamethasone or exposure to hyperoxia during the first 2 postnatal wk markedly inhibits septation as evidenced by the formation of fewer and bigger alveoli than in normally developed rats. Deferoxamine, an iron chelator, has been reported to protect the lung from the effects of exposure to hyperoxia in early postnatal life. In this study, we investigated the effects of these treatments during the 3rd and 4th postnatal wk, that is, after the early period of rapid alveolarization. Our results show that treatment with dexamethasone no longer had any inhibitory effect on alveoli formation; that exposure to hyperoxia continued to inhibit the formation of new alveoli, resulting in bigger and less numerous alveoli; that treatment of animals exposed to hyperoxia with deferoxamine still protected their lungs against hyperoxic inhibition; and that elastin fiber length density in the lung was significantly reduced in hyperoxic-exposed animals. These results suggest that septation continues beyond the 2nd postnatal wk and does not stop abruptly at age 14 d in air-breathing rats and that hyperoxic inhibition of alveolarization during the 3rd and 4th postnatal wk is due to the inhibition of septation of existing or newly generated gas-exchange units during that period of lung development.