Ultrastructure of SOs in cells processed by high-pressure freezing and freeze-substitution. HBMECs were adsorbed with reovirus T1L M1-P208S at an MOI of 1 PFU/cell and imaged by EM at 18 hpi. (A) Ultrathin section showing a VI with mature virions (arrows) and empty viral particles (arrowheads). Two SOs with mature virions and lamellae are apparent near the VI. Scale bar, 500 nm. A higher magnification of an SO from a different cell is shown in the panel on the right. Mature virions and membranes fill the SO, while empty viral particles are only seen in the VI. Scale bar, 200 nm. (B) Enlarged image of the VI–SO interface from the cell shown in A on the right. Empty particles (arrowheads) and mature virions (arrows) are detected at the VI periphery in apposition with the SO, which exclusively contains mature virions. Scale bar, 200 nm. (C) Maturation stages of SOs. The image on the left shows an SO near a VI incorporating mature virions (arrows). An SO with dense, membranous content and virions is apparent in the center image. The image on the right shows an SO with many virions and an adjacent MC. MCs are electron dense and contain tightly packed virions. Scale bars, 200 nm. (D) The percentage of mature virions and empty viral particles present in VIs, SOs, and MCs. Data are presented as absolute values from 25 cells with a total of 23 VIs, 10 SOs, and 7 MCs.