Effects of 0.25 PPM Nitrogen Dioxide on the Developing Mouse Lung. Part 1: Quantitation of Type 2 Cells and Measurements of Alveolar Walls

Abstract

Weanling Swiss-Webster male mice were exposed intermittently to 0.25 ppm NO₂. Lactate dehydrogenase stained frozen sections of gelatin-inflated lungs were used for image analysis measurements of Type 2 cells and alveolar walls at 3 test periods, immediately after 6 wk of exposure, 10 wk postexposure, and 32 wk postexposure, for a total of 196 control and 196 exposed animals. NO₂ exposure resulted in a trend toward Type 2 cell hyperplasia and hypertrophy at all 3 test periods, but significant increases in number and size (mean area) of Type 2 cells did not occur until the 32-wk postexposure test (p = .002 and p <.04, respectively). Alveolar wall area, perimeters, and linear intercepts were also consistently greater for the exposed animals at all three test periods, with perimeters and intercepts at borderline levels of significance at the first test period (p = .09 and p = .08, respectively). An increase in the ratio of Type 2 cell number to alveolar wall area (excluding Type 2 cells within the walls) for the NO₂-exposed animals approached significance at the 32-wk postexposure period (p = .08). The delay in the detection of significant Type 2 cell alterations until the 32-wk postexposure test seems best explained by an adverse effect of NO₂ exposure on lung development and maturation during the 3–9 wk of age growth period when exposure took place. Since significant Type 2 cell alterations were present 32 wk after exposure (41 wk of age), it seems likely that injury to Type 1 cells of the alveolar epithelial lining (implicated by the Type 2 cell hyperplasia) has occurred and was not completely reversible. The concomitant occurrence of an increase in Type 2 cell size is in accord with reports of NO₂ -induced edematous swelling but may also reflect a functional hypertrophy. The data add to cumulative evidence that an ambient level of NO₂ has an adverse effect on the mammalian lung.