Alveolar expansion imaged by optical sectioning microscopy

During lung expansion, the pattern of alveolar perimeter distension is likely to be an important determinant of lung functions as, for example, surfactant secretion. However, the segmental characteristics of alveolar perimeter distension remain unknown. Here, we applied real-time confocal microscopy in the isolated, perfused rat lung to determine the micromechanics of alveolar perimeter distension. To image the alveolar perimeter, we loaded alveolar epithelial cells with a fluorescent dye that we microinjected into the alveolus. Then we viewed single alveoli in a 2-mu m-thick optical section at a focal plane 20 mu m deep to the pleural surface at baseline. In each alveolus, we identified five to eight segments of the perimeter. For each segment, we determined length (L-seg) by means of image analysis. At baseline alveolar pressure (P-alv) of 5 cmH(2)O, Lseg averaged 46 mu m. We hyperinflated the lung to Palv of 20 cmH(2)O and identified the same optical section as referenced against morphological landmarks. Hyperinflation increased mean Lseg by 14%. However, segment distension was heterogeneous, even within the single alveolus. Furthermore, distension was greater in alveolar type 1 than type 2 epithelial cells. These findings indicate that alveoli expand nonuniformly, suggesting that segments that distend the most might be preferred alveolar locations for injury in conditions associated with lung overdistension.