CEMOVIS reveals regular cytosolic intermembrane distances within OSER stacks. A. A typical projection view of a 50 nm-thick vitreous section of the OSER shows a regular spacing between OSER membranes on their cytosolic side (indicated by black arrowheads). HEK293 cells transfected with calnexin-YFP construct were processed for CEMOVIS analysis, as described in detail under "Materials and methods". Standard low-dose imaging conditions with 26,000-30,000× magnification were used to perform cryo-EM experiments; images were acquired on film. Open arrow indicates the cutting direction; asterisk indicates the fold of the peripheral OSER membrane. Scale bar corresponds to 100 nm. B. Intermembrane distances on cytosolic and luminal sides of OSER membranes were sampled from 13 micrographs in which membrane stacks were aligned along the cutting direction, as described in "Materials and Methods"; n values are 86 and 81 for cytosolic and luminal intermembrane distances, respectively; asterisks indicate P < 0.0001. C. As in B, but the distances were measured from 4 micrographs in which the membrane stacks were aligned perpendicular to the cutting direction; n = 17 for cytosolic and luminal distances. Asterisks indicate P < 0.05. Data in C-D are shown as mean ± S.D.