Distribution of PCNA-stained cells in all treatment groups. The actively proliferating cells are stained with PCNA. Positively stained cells can be identified by the deep brown nuclei. In control animals (a), positively stained cells are infrequent among the alveolar cells. Exposure to the allergen (OVA and alum) increased the number of positively stained cells in the injury group (b), indicating that the cells are actively dividing and proliferating to combat infiltration of the sensitizer into the lung. The cells are observed at the basal area of the epithelium (E), smooth muscle (SM), and alveolar cells. After treatment with VCO, positively stained cells are also observed among the airway epithelial cells and cell infiltrates in the submucosa area (c and d). C, cartilage.