Effects of 0.25 PPM Nitrogen Dioxide on the Developing Mouse Lung. Part 2: Quantitation of Elastic Tissue and Alveolar Walls

Abstract

A colony of 3-wk-old mice (Swiss-Webster) was divided into young adult mice exposed to intermittent 0.25 ppm NO₂ for 6 wk and a control group. The left lungs of 186 control and 186 exposed animals were perfused-inflated with glutaraldehyde at 6, 10, and 32 wk post-exposure, and elastic fibers in paraffin sections were stained with aldehyde fuchsin. Image analysis quantitation of elastin content at the first test period showed significant increases in lung elastin for the NO₂-exposed animals, that is, fiber number (p = .001), elastin area (p = .007), and elastin alveolar perimeters (p <.008). Internal surface area (alveolar wall perimeters) decreased (p = .05) and lung volume increased slightly but not significantly. There were also nonsignificant trends toward a larger fiber size, and higher elastic fiber/ wall area ratios. A reversal occurred at the 10-wk postexposure test. All elastic fiber measurements for the exposed animals were significantly less than those for the control group, namely, elastic fiber numbers (p < .02), area per field (p < .005), intercepts (p = .006), perimeters (p < .02), and mean size (p < .04). There was also a decrease in nonelastin and total alveolar wall area, but at borderline levels of significance (p = .1, p < .07, respectively). At the 32-wk postexposure test, the findings reversed in part, with the exposed animals showing a trend toward an increase in elastin fiber number (p = .09) and elastin area (p = .1), but with nonelastin alveolar wall area still less than control values (p < .06). A highly significant finding was an interaction between the increase in elastin and the fall in nonelastin alveolar wall area, namely, increases for the exposed animals in ratios of elastic fiber number to alveolar wall area and alveolar fiber area to alveolar wall area, p < .003 and p < .02, respectively. The altered relationship of alveolar wall components may in part account for the Type 2 cell hyperplasia noted in Part 1 of the study, that is, an intact lung scaffolding is essential for complete restoration of an injured epithelial lining. The sequence of events over the three test periods suggest that an ambient level of NO₂ exposure has injured both elastin and nonelastin components of the alveolar wall. The presence of alterations at 32 wk postexposure, including data from Part 1 of the study, further suggests that both the epithelium and connective tissue have sustained some irreversible damage.