FESEM and light micrographs of fibrotic granulomas following MWCNT aspiration. The FESEM image of Figure 4A shows a granulomatous lesion which is essentially filling an alveolus of the lung (40 ug dose, 56 days). Arrows indicate MWCNTs in the section of the granuloma. The light micrograph in Figure 4B illustrates the extensive collagen network (red) developed to encircle a small mass of MWCNTs in the airspace (black, 20 μg dose, 56 days). Collagen fibers are red in the section due to staining with Sirius Red. TB-terminal bronchiole.