FIG. 6.

Viral forms produced in human cells infected with strain WR as visualized by electron microscopy. Monolayers of HeLa cells infected at 5 PFU of virus/cell were chemically fixed at 6 and at 12 hpi and then processed for conventional embedding in an epoxy resin. Ultrathin sections were studied by transmission electron microscopy at both low and high magnifications. (A) At 6 hpi small viroplasm foci and forming viruses (asterisks) were detected near the nucleus (N) of infected cells. DNA crystalloids of variable size (double arrow) were also frequent in the assembly areas. (B) At 12 hpi the IMVs (arrowheads) were the predominant viral form. A minor population of IVs was also observed (arrow). (C to J) The hypothetical sequence of morphogenesis of the WR strain of VV is shown at higher magnifications. (C) Viral crescent (c) with associated vesicular (v) elements; (D) characteristic IV with DNA spot; (E) previously proposed spherical and dense transition form IV→IMV. This viral form is occasionally seen being wrapped by two membrane units, as shown in panel F (arrow). (G) Potential intermediate in the process of core assembly (arrowhead). Characteristic mature forms are shown in panels H to J: IMV (H), IEV (I; the arrow points to the additional double membrane), and EEV (J; the arrowhead points to the thick external coat). Bars: 0.5 μm, A and B; 100 nm, C to J.