Model of a metaphase Huh-7 cell imaged with the SBF-SEM technique. A whole Huh-7 cell transiently expressing ssHRP-KDEL was chemically fixed, cytochemically stained, and imaged with SBF-SEM (Supplemental Video S6A), providing a voxel size of 27 × 27 × 50 nm, and the ER was modeled. (A) An image of one block-face overlaid with the modeled ER. For a clear view, a small part of the cell and ER model (A–D, Supplemental Video S6B) is shown starting from the metaphase plate of the chromosomes (A, B), including the spindle region (A, D) and the cortical parts (B, C). Cutting surface is marked with red color. Organization of the ER in cortical layers is visible in the block-face image (A) and views of the 3D model (C, D). The cortical layers are composed of fenestrated sheets and short tubules (C; tubules are marked with arrows). Long tubules originating from the cortical ER extend toward the middle of the cell (D, E). A view covering the complete middle part of the cell shows that this tubular part is quite extensive (E). Bars, 5 μm (A, B, and inset of E) and 1 μm (C–E).