Fig. 2. Analysis of airway branching in E18.5 Fgf9-/- lungs using epithelial casts. (A-C) Epithelial casts of whole control (A) and Fgf9-/- (C) lungs. The Fgf9-/- lung cast is smaller than the control but exhibits evidence of both proximal airway branching and distal airspace formation. Some proximal branches are observed in the upper part of the Fgf9-/- cast, and alveolar sacs covering the surface indicate generation of distal airways. (B) An underinjected epithelial cast of a control lung showing a network of proximal branches that is larger than the total size of the Fgf9-/- cast (C). (D,E) Accessory lobe casts dissected from whole lung epithelial casts of control (D) and Fgf9-/- (E) lungs. (E is shown at 1.2x the magnification of D.) The lung cast shown in (E) was overinjected to ensure filling of all airway branches. Note the absence of branches along much of the Fgf9-/- accessory lobe bronchus (arrow), while further iterations of branches obscure the main bronchus of the control accessory lobe. (F,G) High-power views of the dorsal side of the left lobe of control (F) and Fgf9-/- (G) lung casts showing a similar density of alveolar sacs on the lung surface. (H,I) Scanning electron microscope images of control (H) and Fgf9-/- (I) lung casts, showing similar structure of distal clusters of alveolar sacs. The normal structure of dense tufts of alveolar sacs in (I), and the normal density of surface alveolar sacs in (G), together suggest that significant distal airspace formation occurs in Fgf9-/- lung. (J) Schematic diagram of the progression of epithelial airways from proximal conducting airways to distal respiratory airways in embryonic lung. Respiratory airways are distinguished from conducting airways by the presence of alveolar sacs in the walls of respiratory airways. (Respiratory bronchioles (rb), which serve as both conducting and respiratory airways in humans and in many animals, are not found in rodents.) Alveoli are formed by septation of alveolar sacs, a process that occurs in mice only after birth. Scale bars: 50 µm in H,I. ad, alveolar duct; as, alveolar sac; br, bronchus; rb, respiratory bronchiole; tb, terminal bronchiole; tr, trachea.
