[Bronchiolar cells of the human fetal lung from 15 to 26 weeks of gestation]

The authors report an ultrastructural study about the human fetal bronchiole from seven human fetus which gestational age extend from fifteen to twenty-six weeks. It follows a precedent report concerning the pulmonary acinus. It is thus possible to definite the happening date of the different cellular populations of the bronchiolar layer, their morphology, their distribution, and their differentiation modalities. All those cells have the same endodermal origin from the undifferentiated columnar cell. From the fifteenth week, the three main cellular patterns with a full differentiated aspect are present: ciliated cell, Clara cell, and neurosecretory cell and as a fourth type: the intermediary aspect cell; but the undifferentiated columnar cell is still predominant. From the nineteenth week, those four cellular patterns are predominant. At last, the connection between alveolar and bronchiolar layers is studied at the bronchiolo-alveolar junction. From those data and the literature, the authors consider the bronchiolo-alveolar renewal, in which they think that the Clara cell and/ or an immediate precursor could take a leading part.