Morphological Observations on the Replication of Herpesvirus saimiri in Monkey Kidney Cell Cultures

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Abstract
Owl and African green monkey kidney cell cultures were infected with 1 p.f.u.[plaque-forming unit]/cell of herpesvirus saimiri, and sample cultures were taken for examination by EM at 3-6 h intervals over a period of 7 days; the experiments were repeated several times. The peculiarly slow replication cycle of Herpesvirus saimiri enabled distinct cytoplasmic and nuclear phases in virus maturation to be clearly distinguished; the overall fine structural features were similar in both cell types. Immature particles were 1st detected in the nucleus and cytoplasm 63 h after infection. Thereafter, abundant cytoplasmic immature particles matured by budding through cytoplasmic membranes until about 100 h, whereas nuclear immature particles budded through the inner nuclear membrane or intranuclear invaginations of it later, from about 100 h until cytolysis was complete at 160 h. Morphological differences were also observed between particles budding at cytoplasmic membranes and the nuclear envelope. At the former site the membrane overlying the bud showed an electron opaque thickening which imparted to the mature particle an asymmetrical appearance. Such thickenings of the envelope were not observed in mature particles of nuclear origin. Unusual tubular and laminated nuclear structures were seen towards the end of the replicative cycle corresponding with the phase of nuclear virus maturation by budding; the morphology of the latter structures is described.