Comparative models of CEUs in EDL and soleus fibers from WT mice and representative EM images of CEUs in EDL and soleus fibers from dCASQ-null mice. (A and B) In adult mammalian fibers, the triad is placed approximately at the A-I band transition of the relaxed sarcomere. CEUs localize within the I band in close proximity to the triad, extending toward the Z-line. In soleus fibers, SR surface and volume are reduced compared with those of EDL fibers, and this difference is reflected in smaller triadic jSR, as well as in a lower number of longitudinal SR stack elements forming CEUs. (A’ and B’) In EDL muscle fibers from dCASQ-null mice, SR stacks of CEUs usually contain multiple SR membrane layers (four in the representative image in A’), while in soleus they typically contain one or two layers at the most (one in the representative image in B’). Within the junctional gap between both SR/SR membranes in the stack (dashed box and inset in A’) and SR/T-tubule interface (dashed box and inset in B’), numerous and small electron-dense strands, likely reflecting STIM1 aggregates (Boncompagni et al., 2017), are visible. TT, T-tubule. Scale bar: 0.1 μm.