Lung expansion and the formation of the alveolar lining layer in the fullterm newborn rabbit.

Light and electron microscopic studies of lungs from fullterm newborn rabbits, 0-24 hours after birth, revealed a patchy alveolar air expansion during the first few hours of extra-uterine life. Fairly uniform aeration of alveoli was noted 6-24 hours after birth but minor "unexpanded" areas were still present after 24 hours. In the fetal pulmonary fluid as well as in the alveolar lining layer formed after the onset of breathing there are multiple large phospholipid complexes, and a discontinuous multilamellar surface film can be demonstrated in some alveoli from 2 hours after birth. Apparently, the neonatal adaptation of the rabbit lung is a protracted process, not even complete at the age of 24 hours.