Assembly of enveloped respiratory syncytial virus particles within the cytoplasm of infected Vero cells.

Electron microscopic examination of syncytia induced by a bovine respiratory syncytial virus strain in Vero cell cultures revealed the presence in the cytoplasm of assembled enveloped virus particles within inclusions of variable sizes. Moreover, budding virus particles were shown occasionally in the intracytoplasmic vesicles. These particles were filamentous in form, about 80-120 nm in diameter, variable in length, containing 12 nm diameter nucleocapsids, and looked like the extracellular particles budding at the plasma membranes. This is the first report on assembly of intracellular virus particles in cells infected by a member of the family Paramyxoviridae. Vero cell-dependent variations appear to be the factor leading to the defect in the late virus replication cycle.