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Alveolar expansion imaged by optical sectioning microscopy

Departments of Medicine and Physiology and Cellular Biophysics, College of Physicians and Surgeons and St. Luke's-Roosevelt Hospital Center, Columbia University, New York, New York

Submitted 7 February 2007 ; accepted in final form 20 June 2007

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-µm-thick optical section at a focal plane 20 µm 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 cmH₂O, L_seg averaged 46 µm. We hyperinflated the lung to P_alv of 20 cmH₂O and identified the same optical section as referenced against morphological landmarks. Hyperinflation increased mean L_seg 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.

alveolar distension; alveolar fluorescence; heterogeneity; lung hyperinflation; type 2 cell

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