Figure 1.

Visualization of filamentous particle formation in MDCK cells. (a and b) Immunofluorescence (a) and phase-contrast microscopy (b) at 9 h.p.i., revealing the presence of numerous influenza virus filaments emanating from the cell surface. The immunofluorescence shows double staining with HA-specific antibody (red) and antibody to vimentin (green). The latter did not colocalize to viral filaments. (c) Low-magnification electron micrograph showing the presence of many long viral filaments and filament bundles projecting from the surface of A/Udorn-infected MDCK cells. (d and e) Electron micrographs of negatively stained filamentous particles released into the medium. The viral filaments depicted in d are 5–7 μm in length. The characteristic viral spike-like surface projections are readily visible at higher magnification (e). (a and b = ×1,000; c = ×15,000; d = ×10,000; e = ×85,000.)