Fig. 7. Tomographic study of A549 cells immunolabelled with anti-RNA polymerase I antibodies. Contrast was inverted so that silver particles appear white. A 500 nm thick section observed using a STEM working at 250 kV is displayed: several independent clusters, 270 nm in diameter, are seen (A). (B-D) Different projections of the tomogram were calculated after tomographic reconstruction of the cluster framed in (A). At + 15° (D), five 60 nm coils are seen, as indicated by brackets numbers 1 to 5. The large circle shows the area where the coils are fused together. The arrows point to twines, 20 nm in thickness. (E) A stereopair of the tomogram presented in the same orientation as in (D) was calculated thanks to a surfacic rendering mode. (F-I) Four successive 30 nm thick sections were achieved with a coronal orientation within the tomogram presented in (D). Asterisks (G,H) indicate the internal part of the cluster, devoid of labelling, and arrows (G,I) refer to twines. Bar, 200 nm (A) or 100 nm (B-I).