Frog virus 3 replication: electron microscope observations on the terminal stages of infection in chronically infected cell cultures.

An examination of BHK, CEF, and FHM cells chronically infected with frog virus 3 has been made by scanning and transmission (thin section, freeze fracture, and surface replica) electron microscopy. With minor differences the pattern of virus development is similar in all three cell line. Virus particles were detected in cell nuclei which subsequently became degenerate very late in infection. Three inclusions were associated with frog virus 3 cytoplasmic foci of infection; lamella structures, extensive microtubule formation (in BHK and FHM cells), and linear crystalline structures. The last two structures may play a role in creating or maintaining the cell rounding c.p.e. revealed by scanning electron microscopy. Very late in infection most BHK and FHM, but not CEF, cells are stripped of the plasma membrane. Replicas of frozen fractured BHK cells featured cytoplasmic foci of infection, budding at the plasma membrane, and showed that at early times when virus is detected in the nucleus, the nuclear membranes are intact and morphologically unaltered. Budding at the plasma membrane was better resolved by scanning and as surface replicas. This demonstrated that sparse to profuse localized budding occurred. Frequently virus particles were located singly, or as multiples, at the end of, or along, cytoplasmic protrusions which occur both on the body of the cells and at the cytoplasmic/coverslip 'interface'.