Figure 3: Elastica van Gieson stain for the collagen bundles and the interstitium of alveolar duct region. (Left) The filtered air group. Note the centriacinar regions of alveolar ducts were generally straight and less collagen bundles. (Right) The HONO exposure group. Note the similar architecture to the filtered air group in the alveolar duct centriacinar regions. However, we observed the replication of elastic fibers, and the terminal bronchial epithelial cells extended to the alveolar ducts. The collagen bundles were indistinct in both the HONO exposure group and the filtered air group.