Immuno-Electron Microscopy of the Morphogenesis of Mumps Virus

The fine structure of mumps virus-infected chick embryo fibroblastic cells was examined sequentially after viral inoculation. Intracytoplasmic nucleoprotein strands, similar to those described for parainfluenza viruses, were detectable in small aggregates between 36 and 48 hr. The peripheral strands of this viral component lie beneath and along an antigenically altered bulging portion of the cell membrane. The outermost strands are consistently parallel to the differentiated segment of the plasma membrane, which is invariably associated with surface projections. As has been found with other myxoviruses, mumps virus replicates by budding from the cell surface. The virus particle, roughly spherical in shape, has a size ranging from 1,000 to 8,000 A. Filamentous forms are rarely observed in the present culture system. Ferritin-conjugated antibody specifically labels the cytoplasmic nucleoprotein, the modified cell membrane, and the virus particle. Intranuclear inclusions of low electron density and morphologically different from those described in measles virus-infected HeLa and amnion cells were observed in the nucleus of several infected cells. Immuno-electron microscopic observations suggest that the nucleoprotein synthesis rate exceeds that of cell membrane differentiation into viral envelope. This difference results in the accumulation of viral nucleoprotein in large intracytoplasmic masses which can be demonstrated by electron microscopy.