[Electron microscopy study of the development of the pulmonary alveolus in the human fetus]

An ultrastructural study of distal human fetal lung between the fifteenth and nineteenth weeks of gestation is performed. All the cells of the epithelial lining layer have an endodermal origin and are provided by one cellular pattern: the columnar undifferentiated cell. From nineteenth week of gestation a differentiation into alveolar et bronchiolar ways is observed. Inside cells which are in type II pneumonocyte differentiation, cytoplasmic modifications has been observed. These modifications can agree with lamellar inclusion bodies precursors. Otherwise, respective part of type II pneumonocytes and Clara cells, in surfactant synthesis is discussed, as well as the chronological link connecting between them the precursors of the lamellar inclusions bodies, and the mode of renewal of alveolar cells.