Thin sections of virus and infected cells were examined on a FEI Morgagni 268 TEM equipped with a 1K side mounted CCD camera (SIS, Muenster, Germany). Quantification of cell morphology and cell-associated virus morphology was carried out on 150 nm resinembedded sections in a systematic random sampling manner [52] . Starting points for sampling were chosen randomly on each grid, and all grids were examined in the same systematic manner. At least three grid squares per EM grid and three EM grids per time point were sampled. Morphology of viruses from cell culture supernatants was quantified by EM in the same systematic random sampling manner on thawed 70 nm cryosections of virus pellets, purified and embedded as described above. At least 200 particles from three different grids were evaluated per sample. ET was carried out essentially as described elsewhere [53] . Dual axis tilt series from 300 nm sections were recorded on a FEI TECNAI TF30 microscope operated at 300kV (4K FEI Eagle camera; binned pixel size 0.77 nm or 1 nm on the specimen level) over a 260u to 60u tilt range (increment 1u) and at a defocus of 20.2 mm. Tomograms were reconstructed using the IMOD software package (version 3.12.20) [54] . 3D measurements of virus end distance, analysis of virus end morphology and 3D surface renderings were carried out using the AMIRA Visualisation Package (version 5.2.0, Visage Imaging, Berlin, Germany). In total, 68 3D reconstructions from three independent infection experiments were analysed, of which 48 of high image quality were used for detailed analysis and measurements. NCs in different samples displayed variable contrast, which resulted from differential access of electron-dense stain to individual cells and different subcellular regions during sample preparation. NC representations in Figures 2 and 4 and Video S1 and Video S2 were generated and displayed using the AMIRA EM package [55] and MatLab (version 7.4.287).