Bromodeoxyuridine (BrdU) immunohistochemistry showing a cell proliferative responses 21 days post-exposure in the lung alveolar region (panels A, C, E, G) and in the bronchus-associated lymphoid tissue (panels B, D, F, H). Saline vehicle containing 0.1% Pluronic F-68. (A, B). Carbon black treatment (C, D). SWCNT treatment (E, F). Vanadium pentoxide (V2O5) treatment (G, H). Solid arrows in panels C and E indicate aggregates of carbon black and SWCNT, respectively. Inset shows two macrophages joined by a carbon bridge within a nearby alveolar space. Open arrows indicate representative BrdU-positive nuclei. Original magnification for all panels, 40×.