Thickness of the blood-gas barrier in premature and 1-day-old newborn rabbit lungs

The pulmonary capillaries of neonatal lungs are potentially vulnerable to stress failure because of the complex changes in the pulmonary circulation that occur at birth. We studied the ultrastructure of the blood-gas barrier (BGB) in premature and 1-day-old rabbit lungs and compared it with the ultrastructure of adult lungs. Normal gestation of rabbits is 30 days. After extensive pilot measurements, three premature (27 days gestation) and three newborn (1 day old) rabbit lungs were perfusion-fixed at arterial, venous, and airway pressures of 25, 0, and 10 cmH2O, respectively, and the measurements were compared with those of three adult lungs. The thickness of the capillary endothelium, alveolar epithelium, and interstitium of the BGB was measured at right angles to the barrier at random points. A striking finding was the large number of measurements of the interstitial thickness in 1-day-old lungs that were very thin (0-0.1 microm). The percentages of occurrence of very thin interstitium in premature, 1-day-old, and adult lungs were 35.3 +/- 9.4, 71.7 +/- 5.2, and 43.0 +/- 2.6, respectively (P < 0.02 for 1 day old vs. premature and adult). Given the previously found relationship between stress failure and interstitial thickness, this large proportion of very thin interstitial layers in the capillaries of 1-day-old lungs is a reasonable explanation for their previously demonstrated vulnerability to stress failure.