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Respiratory Failure in Newborn Rats with the Semilethal Mutation Fused Pulmonary Lobes as a Cause of Neonatal Death

The present study aimed at elucidating the cause of pre‐weaning mortality of fpl/fpl rats having fusion of the right lung lobes with hypoplasia of the intermediate lobe and many other associated malformations. In the first experiment, fpl/fpl pups were examined for postnatal viability and growth. Cumulative mortality of fpl/fpl pups obtained from three types of mating pairs was 13.8–30.8% on day 0 of lactation and was raised to 41.9–76.7% on day 4. However, only few deaths were noted thereafter. Body weights of surviving fpl/fpl pups were significantly lower than those of +/‐ littermates through the lactation period. In the second experiment, moribund or dead fpl/fpl newborns at birth were closely examined for abnormalities in general condition and necropsied. Their lungs were weighed and examined for histological abnormalities. Several findings obtained from the fpl/fpl newborns demonstrated the occurrence of respiratory failure. They were decreased frequency of respiration, cyanosis, hepatization of the lung and inflation of the stomach by air. Lung weights of these newborns were significantly lower than those of +/‐ littermates. Histological examination of the lungs of dying fpl/fpl newborns revealed partial atelectasis occurred in the regions where fusion of lobes as well as hypoplasia of the lobe was grossly observed. In these lungs, however, epithelial maturation was apparently normal and hyaline membrane was not formed. These observations suggest that reduced gas‐exchanging capacity related to lung malformations leads fpl/fpl newborns to die of respiratory insufficiency in their early postnatal age.