Enhanced darkfield image of CNT-exposed lungs. Figure 2A shows the general distribution of MWCNTs in the lungs 7 days after aspiration (40 ug dose) with the section oriented with the pleural space running along the top. CNTs scatter light with high efficiency and thus produce the bright, white structures in these enhanced darkfield images, while nuclei (red) and other tissues (green) produce a significantly duller image. Arrow points to an individual MWCNT penetrating into the mesothelial cell layer forming the boundary between the alveolar tissues and pleural space. While alveolar macrophages are foci for MWCNTs, scattered, submicron MWCNT structures can be found in the alveolar interstitium throughout the section. Figure 2B gives a comparison image from a mouse lung exposed to a highly dispersed preparation of SWCNTs (aspiration 10 ug dose, 7 day). In the case of dispersed SWCNTs, the majority of CNT structures are rapidly incorporated into the alveolar interstitium (arrows).