Abstract
Sections of tissue culture cells infected with visna virus were studied in the electron microscope. The ultrastructure of the nuclei and the cytoplasm of infected cells was found to look much like that of uninfected control cells. In the former, however, a number of spherical particles was seen on the external cell surface. The particles averaged 85 mμ in diameter, were bounded by a single membrane, and contained a centrally located electron-dense core. They were found in three types of tissue cultures infected with various strains of the virus. The particles appeared 2–3 days after inoculation, concurrently with the typical cytopathic changes and the rise in virus titer of the fluid. They seemed to be formed by a budding process in the cell membrane and to be released as double-walled bodies which later developed to form the characteristic particles. It is suggested that the particles represent visna virus, and their apparent similarity to certain cancer viruses is pointed out.
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