Figure 1. Electron microscopy of pandemic (H1N1) 2009 virus. A) Negatively stained virions grown in MDCK cells showing spherical particles with distinct surface projections. Scale bar = 100 nm. B) Filamentous and ovoid particles assembling at the plasma membrane. Scale bar = 100 nm. C) Extracellular particles showing internal nucleocapsids, seen in cross-section, surrounded by an envelope with prominent spikes. Note all 8 nucleocapsids present in 1 virion (arrow). Scale bar = 100 nm. D) Dense tubules (arrow), which were found in the nuclei of some MDCK-infected cells. Arrowhead, nuclear envelope. Scale bar = 100 nm. E) Immunogold labeling of the nuclear tubules by using an antibody against the matrix protein. Scale bar = 100 nm.