THE DEVELOPMENT OF THE LUNGS OF LAMBS

Abstract

Lambs with estimated gestational ages from 55 to 154 days were delivered by cesarean section. Observations were made on animals in the fetal state and after the onset of respiration in lambs of more than 130 days gestation. Spontaneously born lambs were also studied. Measurements of arterial blood gas tensions, pH, lactic acid concentration, and blood pressure showed that the animals were always in good condition. Normal surface activity of lung extracts was first found in fetuses of about 126 days gestation and was present in the lambs of more than 130 days gestation which breathed. Electron microscopy showed that cells containing osmiophilic inclusion bodies first appeared at about 120 days gestation and preceded by only a few days the appearance of normal surface activity of lung extracts. The number of cells containing osmiophilic inclusion bodies as well as the total number of inclusion bodies increased toward term. There was no apparent difference in the number of these cells or the total number of inclusion bodies in the lungs of lambs which breathed as compared to fetuses of comparable gestational age. The water content of the lungs/unit of dry tissue decreased during gestation until about 95 days and then increased again until term. After the onset of respiration the water content rapidly decreased. The blood content of the lungs/unit of dry tissue increased significantly during gestation and there was a further probable increase after the onset of respiration.