Conventional TEM of polystyrene nanoparticles. This figure demonstrates the impossibility to distinguish between NP and cellular structures by conventional TEM unambiguously. In A, five polystyrene NP (NP!) with a mean diameter of 78 nm are observed next to an A549 cell. Once taken up by the cells, they may have an appearance as shown in B. It is very likely that the spherical structures in B (NP?) are not NP but vesicular structures like caveolae. CC = Clathrin coated vesicle; PM = Plasma membrane; AJ = Adherens junction. Chemical fixation, Epon embedding, 40–70 nm sections. Bar = 1 μm (A and B are at identical magnification).