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The effect of maternal CO2 breathing on lung development of fetuses in the rabbit. Morphologic and morphometric studies.

We have examined male fetuses from rabbits exposed from Day 21 to Day 28 of gestation to 8% CO2 for 8 h each day. Fetuses of CO2-exposed mothers weighed less and had lungs that weighed less but when expressed per unit body weight were unchanged. Distended lung volumes were greatly increased in the CO2 group. Light microscope morphometry showed more mature lungs with increased volume proportion of air spaces, decreased air-space wall, and decreased nonparenchyma in CO2-exposed animals. Individual air-spaces were larger, gas exchanging surface area was increased, air-space walls were thinner, and higher ratios of mature to immature crests were found. Electron microscope morphometry showed decreased glycogen in alveolar Type II cells and increased volume proportion of lamellar bodies in the CO2 group. Type II cells were more cuboidal. We interpret these changes to represent increased tissue and cellular maturation in fetuses whose mothers breathed CO2. We speculate that the changes were brought about by increased fetal respiration, which may cause increased stretch and distention of the lung. Alternative mechanisms are discussed.