Target organs for translocation
Studies observed that the liver had a prominent role in extra pulmonary organ translocation, although other organs like the spleen and pulmonary lymph nodes were also target organs for translocated particles. This may take place through interstitial pathway clearance via blood circulation. When uptake in other organs was assessed, only the liver showed a time-dependence pattern. The fractions found in the pulmonary lymph nodes, although low showed an inverse relationship to particle size (Oberdörster et al, 2002; Kreyling et al, 2002; van Ravenzwaay, 2009).
Uptake into the lymph node is different from other secondary organ uptake as it has been suggested that penetration of particles into lung lymph nodes appears to be highly dependent on the accumulated dose and inflammatory response. Two types of TiO2 (a 20 nm ultrafine and a 250 nm fine TiO2) at similar concentrations were compared with regard to their inflammogenic potential and their impact on clearance behaviour. The ultrafine sample exerted much more inflammation accompanied by a significantly larger fraction migrating to the interstitial space and the lung lymph nodes. (Section 2 on surface area, Oberdörster et al, 1994). In the study by Morfeld et al, 2012; TiO2 (20-25 nm) was only detected in the lung and the mediastinal lymph nodes, which was considered a consequence of an inflammatory response and not of size characteristics of the test material. The inflammation potential was higher for quartz than for TiO2 (either in pigmentary or nano-sized form), which was also reflected in the amount found in the mediastinal lymph nodes (van Ravenzwaay 2009).
Taken together, the translocation of PSPs from the deposition site into the lung lymph nodes appears to be a function of inflammation rather than of particle size per se, suggesting that lung lymph nodes should probably not be considered as secondary organs of translocation as they serve as lung draining lymph nodes and thus a clearance mechanism.