An Electron Microscope Study of the Fetal Development of Human Lung

Results of a study of lung ultrastructure in 13 human fetuses of about 1 to 6 months of gestation are reported. Light microscope controls were obtained in all cases. The columnar epithelial cells of young fetuses contained large deposits of glycogen which tended to become less abundant between the fourth and the sixth month. The fine structure of epithelial glycogen is described. A gradual transition between the columnar or cuboidal epithelium and the attenuated cytoplasmic lining of fully developed alveoli started during the fifth month, when the first contacts between capillaries and epithelium were established. Further differentiation of the epithelial cells in two morphologically different types, as observed in adult lung, took place during the sixth month. Both types appeared to have the same endodermal origin. Lamellar inclusions were first observed during the sixth month. Their significance is discussed in the light of recent work on alveolar surface activity.