Light micrograph of fibrotic response to interstitial MWCNTs. Comparision of Sirius red staind collagen fibers in PBS treated (5A) and MWCNT treated lungs (5B, 80 ug dose at 56 days post-aspiration). Figure 5A illustrates the typical long and thin collagen fibers which traverse within the alveolar interstitium of a normal lung. MWCNTs incorporated into the alveolar interstitium were found to be associated with significant interstitial development of collagen fibers and accompanying interstitial cells. The region of the alveolar wall containing the MWCNTs (arrows) is substantially thicker than the average alveolar wall thickness of 4 to 5 um which is typical of a normal mouse. Sirius Red-Hematoxylin stained sections.