AN ELECTRON MICROSCOPIC STUDY OF SINGLE-CYCLE INFECTION OF CHICK EMBRYO FIBROBLASTS BY INFLUENZA VIRUS

Cells infected with the WSN strain of influenza virus under single-cycle conditions were examined with the electron microscope. With a stain which enhances the contrast of membranes, it was shown that the viral envelope contains a unit membrane like that of the host cell, and an additional inner leaflet not found in the cell membrane. Virions formed at the plasma membrane or at vacuoles which arose at the plasma membrane. A striking alteration was found in the nucleolus, which contained dense spots at 4 hours, and thereafter became distended and gave rise to numerous discrete dense masses by 12 hours. Small inclusions were associated with ribosomes in the cytoplasm as early as 4 hours, and larger inclusions of similar appearance were found at later times. Vesicles 500-1500 Å in diameter were often associated with cell surfaces where virions emerged, and Thorotrast labeling demonstrated that they arose at the cell surface. Distinct spikes were visible on the surfaces of emerging virions, but were not seen on adjacent cell membrane; despite this, erythrocytes appeared to adsorb firmly to such membrane. A hypothesis is presented for the sequential stages in the assembly of virus particles which takes account of these and other observations. © 1969.