Comparison of the incidence of SR stacks and CEUs, T-tubule extensions, and T-tubule/SR stack contact length between EDL and soleus muscle fibers. (A–D) Quantitative analyses of the number of SR stacks per 100 μm² of cross-sectional area (A), T-tubule length in the I band (μm/100 μm² of cross-sectional area; B), the number of CEUs per 100 μm² of cross-sectional area (C), and length of T-tubule/SR stack contacts in the I band (expressed as μm/100 μm² of cross sectional area; D) determined from cross sections of EDL (left bars) muscle fibers from WT (n = 60; n = 60; n = 112; n = 60 fibers), CASQ1-null (n = 150; n = 100; n = 100; n = 53 fibers), and dCASQ-null (n = 104; n = 123; n = 145; n = 103 fibers) mice. Right bars in each plot represent the soleus muscle fibers from WT, CASQ1-null, and dCASQ-null mice shown in Fig. 2. Data are shown as mean ± SEM. Statistical analysis for EDL muscle fibers: A: *, P = 0.000000038, WT vs. CASQ1-null; *, P = 0.000000029, WT vs. dCASQ1-null. B: *, P = 0.000000028, WT vs. CASQ1-null. *, P = 0.000000024, WT vs. dCASQ-null. C: *, P = 0.000000027 WT vs. CASQ1-null; *, P = 0.000000013 WT vs. dCASQ-null. D: *, P = 0.0000064, WT vs. CASQ1-null; *, P = 0.000000027, WT vs. dCASQ-null. Number of mice used: WT, n = 3; CASQ1-null, n = 3; dCASQ-null, n = 3. Note: quantitative EM analyses of SR stacks per 100 μm² of cross-sectional area (A), T-tubule length in the I band (μm/100 μm² of cross-sectional area; B), and length of T-tubule/SR contacts in the I band in EDL muscles from WT and CASQ1-null mice (D) were replotted from Michelucci et al. (2020).