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An ultrastructural study of C-type virion assembly in mouse cells.

The Eveline strain of STU mouse embryo cells, which are chronically infected with Friend leukemia virus, produces on the average 6.1 mg of virus protein per liter of cell supernatant fluids. Mouse L-cells, which produce endogenous C-type virions, yield about 0.13 mg of virion protein. Similarly, by thin-section studies, about 20 times more virions were found to be associated with Eveline cells as compared to L-cells. This 20- to 50-fold greater number of virions, observed both in the harvest from supernatant fluids and associated with the cells, cannot be attributed to an increase in the number of particles budding from the outer surface membrane because both cell lines showed a similar average of 0.7 to 0.8 budding, “immature” C-type particles per cell per thin section. However, there were morphologically quantifiable differences between the cell lines. Single thin sections of Eveline cells, as compared to L-cells, exhibited about a 3-fold higher yield in: ( a ) the total amount of apparent cytoplasmic “vacuoles;” ( b ) the number of virions budding into or contained in these vacuoles; and ( c ) the number of virions present in the cytoplasm. These increases probably account in part for the relatively greater production of Friend leukemia virus in Eveline mouse cells.

A further characterization of the vacuoles by serial thin-section studies showed that, in many instances, they represented intracytoplasmic invaginations of the outer surface. We still could not rule out the possibility that, in some instances, the vacuoles were true cytoplasmic structures. In both instances, however, an increased membrane surface area has been provided for the budding of C-type viruses.