The replication of frog virus 3 in primary chick embryo fibroblasts has been studied by examination of thin sections with the electron microscope and the assay of infectious virus. Uptake of frog virus 3 by the cells was observed to occur by pinocytosis and this may be the method of entry. Early in infection (1 h p.i.) marked margination of the nuclear chromatin occurred and the chromatin remained in this condition throughout the infection. Foci of infection were first detected in the cytoplasm of cells 24 h p.i. when production of infectious virus commenced. These foci appeared as electron translucent areas containing fine grains, surrounded by degenerate mitochondria. The foci usually contained virus particles. At this time budding of virus particles at the plasma membrane occurred. Later in infection at 36 and 48 h p.i. large numbers of virus particles were detected in the cytoplasm of cells either scattered loosely throughout the cell, arranged as clusters or in paracrystalline arrays. Extensive budding at the plasma membrane then took place. Virus particles were detected in the nucleus of the cells at these late stages and it is possible that the virus may infect and replicate at this site. Throughout the productive stages of infection aberrant forms of the virus, namely particles devoid of cores, incompletely assembled particles and elongated bacilliform particles were noticed.


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