title: Development of the human fetal airway tree: analysis of the numerical density of airway endtips.
abstract: BACKGROUND
  It has been long agreed that the lung contained, at the end of the pseudoglandular stage, mostly the future conductive airway tree and that the intra-acinar structures developed during the subsequent canalicular stage. Recent immunocytochemical investigations have disclosed that the epithelial cells lining the distal airways in the pseudoglandular stage have the characteristics of alveolar epithelial cells. The purpose of this study was to quantitatively evaluate the progress of fetal airway branching.
  
  
  METHODS
  The numerical density of endtips of the airway tree was estimated in three human fetal right lungs after 3D reconstruction of airways from serial histologic sections. The CRLs of the three fetuses were 103 mm, 132 mm, and 145 mm. The former two corresponded to the late pseudoglandular stage and the last to the early canalicular stage. Three-dimensional reconstructions were performed on two portions of the upper and lower lobes in each lung. The numbers of endtips of the airway tree contained in the reconstructed portions were counted an the numerical densities calculated.
  
  
  RESULTS
  The numerical densities in the parenchyma of the three right lungs were 1.2 x 10(3)/mm3, 1.7 x 10(3)/mm3, and 2.4 x 10(3)/mm3, respectively. The estimated total numbers of endtips in the whole right lung were 0.85 x 10(6), 2.25 x 10(6), and 5.17 x 10(6), which were equal to 2(19.8), 2(21.2), and 2(22.3), respectively.
  
  
  CONCLUSIONS
  Since this suggests that 20 generations of branching were completed, we concluded that the airway branching has already reached the level of the future alveolar ducts in the late pseudoglandular stage.
authors.affiliation: Institute of Anatomy, University of Bern, Switzerland.
authors.forename: H
authors.initials: H
authors.lastname: Kitaoka
authors.forename: P H
authors.initials: PH
authors.lastname: Burri
authors.forename: E R
authors.initials: ER
authors.lastname: Weibel
mesh_terms: D005333:Fetus; D006801:Humans; D008168:Lung / Q000166:cytology* / Q000254:growth & development