Changes in the alveolar lining with onset of breathing

Changes in the alveolar lining of newborns were investigated by sacrificing rabbits at birth and at intervals shortly after birth, and examining their lungs in the electron microscope. An orderly, reproducible sequence of changes in the Type II alveolar lining cells and alveolar space was demonstrated, and the observations correlated well with biochemical events. Type II cell inclusions, dense and numerous at birth, enlarged and became less dense ((1/2)-1 hour), appeared to empty their contents into the alveolar space and disappear (1(1/2) hours) and then reappeared with a mixture of sizes and densities in each cell (2 hours). Few lamellar figures were present in the alveolus at birth, the number reaching a maximum at 1(1/2) hours and then decreasing. The first evidence of a noncellular alveolar lining was at 1(1/2) hours, when lamellar figures and granular material adhered to small corners of the alveolar wall. At 2(1/2) hours, a thick dense layer covered much of the wall. At 6 hours, the layer was thinner and denser, and at 24 hours the thin osmiophilic surfactant lining was present.