Phagocytosis of asbestos fibers and MWCNTs and cell aggregation in 3D cultures. A-D) Evidence of frustrated phagocytosis after 6 hrs of exposure to 1.25 μg/ml of asbestos fibers or MWCNTs. At the times specified, 3D cultures were transferred to a glass-coverslip, incubated for 15 min to allow re-attachment and fixed for examination by scanning electron microscopy. A) Crocidolite asbestos fibers. B) MWCNT 1. C) MWCNT 2. D) MWCNT 3. E) Time course of cell aggregation after exposure to 1.5 μg/ml of MWCNT 1 in 3D cultures.