Incidence and size of SR stacks. (A and B) Representative EM images of cross sections of EDL muscle fibers from WT (A) and CASQ1-null (B) mice (empty arrows point to stacks of SR membranes). (C and D) Representative EM images of longitudinal sections of EDL muscle fibers showing stacks of SR membranes in proximity to triads (SR-TT-SR). (E–H) Quantitative analyses of the percentage of muscle fibers exhibiting SR stacks (E), the number of SR stacks per 100 µm² of cross-sectional area (F), the average SR stack length (G; labeled by dots in C and D), and the frequency of the number of SR stack elements (H; numbered in C and D) in EDL muscles WT and CASQ1-null mice. Data are shown as mean ± SEM; *, P < 0.05. Numbers in bars (n) indicate the number of fibers analyzed. Number of mice: n = 4 WT; n = 4 CASQ1-null. Data for EDL muscles from WT mice shown in E–G are the same as those reported in Boncompagni et al. (2017) and Michelucci et al. (2019). Scale bars: 0.2 µm, A and B; 0.1 µm, C and D.