Part Fibre Toxicol

BackgroundInhalation exposure studies of mice were conducted to determine if multi-walled carbon nanotubes (MWCNT) distribute to the tracheobronchial lymphatics, parietal pleura, respiratory musculature and/or extrapulmonary organs. Male C57BL/6\uc2\ua0J mice were exposed in a whole-body inhalation system to a 5\uc2\ua0mg/m3 MWCNT aerosol for 5\uc2\ua0hours/day for 12\uc2\ua0days (4 times/week for 3\uc2\ua0weeks, lung burden 28.1 ug/lung). At 1\uc2\ua0day and 336\uc2\ua0days after the 12\uc2\ua0day exposure period, mice were anesthetized and lungs, lymph nodes and extrapulmonary tissues were preserved by whole body vascular perfusion of paraformaldehyde while the lungs were inflated with air. Separate, clean-air control groups were studied at 1\uc2\ua0day and 336\uc2\ua0days post-exposure. Sirius Red stained sections from lung, tracheobronchial lymph nodes, diaphragm, chest wall, heart, brain, kidney and liver were analyzed. Enhanced darkfield microscopy and morphometric methods were used to detect and count MWCNT in tissue sections. Counts in tissue sections were expressed as number of MWCNT per g of tissue and as a percentage of total lung burden (Mean\ue2\u20ac\u2030\uc2\ub1\ue2\u20ac\u2030S.E., N\ue2\u20ac\u2030=\ue2\u20ac\u20308 mice per group). MWCNT burden in tracheobronchial lymph nodes was determined separately based on the volume density in the lymph nodes relative to the volume density in the lungs. Field emission scanning electron microscopy (FESEM) was used to examine MWCNT structure in the various tissues.ResultsTracheobronchial lymph nodes were found to contain 1.08 and 7.34 percent of the lung burden at 1\uc2\ua0day and 336\uc2\ua0days post-exposure, respectively. Although agglomerates account for approximately 54% of lung burden, only singlet MWCNT were observed in the diaphragm, chest wall, liver, kidney, heart and brain. At one day post exposure, the average length of singlet MWCNT in liver and kidney, was comparable to that of singlet MWCNT in the lungs 8.2\ue2\u20ac\u2030\uc2\ub1\ue2\u20ac\u20300.3 versus 7.5\ue2\u20ac\u2030\uc2\ub1\ue2\u20ac\u20300.4 um, respectively. On average, there were 15,371 and 109,885 fibers per gram in liver, kidney, heart and brain at 1\uc2\ua0day and 336\uc2\ua0days post-exposure, respectively. The burden of singlet MWCNT in the lymph nodes, diaphragm, chest wall and extrapulmonary organs at 336\uc2\ua0days post-exposure was significantly higher than at 1\uc2\ua0day post-exposure.ConclusionsInhaled MWCNT, which deposit in the lungs, are transported to the parietal pleura, the respiratory musculature, liver, kidney, heart and brain in a singlet form and accumulate with time following exposure. The tracheobronchial lymph nodes contain high levels of MWCNT following exposure and further accumulate over nearly a year to levels that are a significant fraction of the lung burden 1\uc2\ua0day post-exposure