Electron microscopical study of initial and final stages of fowl plague virus-replication in chick embryo cells

Cellular uptake of fowl plague virus occurs 10-30 minutes after inoculation of chick embryo cells. The penetration of the virions is by pinocytosis (viropexis); fusion with the cellular membrane has not been observed. After pinocytosis the virions become gradually disintegrated. Budding of newly formed virions from the cellular membrane starts 3 hours post inoculation (p.i.) and reaches its maximum 8 hours p.i. At the same time budding takes place into electron microscopically empty and autophagic vacuoles. Eight hours p.i. about 3 per cent of the infected cells show budding of virions from the surface and into cytoplasmic vacuoles. Labelling of the cellular membrane with ruthenium red demonstrated that these cytoplasmic vacuoles are not simple cross-sections of invaginations of the cellular membrane. Cluster-like structures were found at 6 hours p.i. in the nuclei of infected cells; however, the suggestion that the clusters develop from nucleoli could not be confirmed.