Electron tomography of a 250 nm thick section. A tilt series (three stills are shown in A, B and C) between +/- 60° with an increment of 1° provides the information for a volume reconstruction by weighted backprojection (D) of an in vitro grown alveolar epithelial cell (cell line A549), exposed to polystyrene NP prior to chemical fixation. The 2 nm thin slice (E, at a depth of 58 nm in the section) reveals a crisper and clearer depiction of the polystyrene NP than the 250 nm thick Epon section (F). Arbitrary digital slices can be made (G, position shown by the two arrowheads in D) in order to provide unquestionable recognition of the NP. Bars = 250 nm in A-D, 50 nm in E-G. Software based 3D rendering offers a way for segmentation according to the interpretation of the user and allows full perspective freedom (H and I). Moreover, clipping planes can partially dissect the scene (J), segmented objects can be omitted (the membrane surrounding the NP in K) and specific quantitative information on rendered objects obtained. Blue: nanoparticle, shades of green: rough endoplasmic reticulum (RER), orange: plasma membrane, transparent white: membrane surrounding the NP, red: ribosomes (Ri) on the RER. Me = membrane, NP = nanoparticle, PM = plasma membrane.