Light microscopy images of collagen fibers in clean-air control and MWCNT-exposed lungs. Left micrograph shows normal clean-air control section of the alveolar region. Right micrograph is from fibrotic region of a MWCNT-exposed lung at 168 days post-exposure. Arrows indicate some of the MWCNT structures within the alveolar interstitium in this region. Fibrillar collagen, stained with Sirius Red. Nuclei counterstained with Hematoxylin.