Incidence of SR stacks and CEUs, T-tubule extensions, and T-tubule/SR stack contact length in soleus muscle fibers. (A) Representative EM images of cross sections of soleus muscle fibers from WT (left), CASQ1-null (middle), and dCASQ-null (right) mice and higher-magnification EM images (vertical panels) showing T-tubules stained in black with ferrocyanide precipitate. Color labeling: jSR, false labeled in green; SR cisternae element of a CEU, false labeled in yellow and pointed to by black arrows. Scale bars: horizontal panels (left), 1 μm; vertical panels (right) 0.1 μm. (B–E) Bar plots showing quantitative analyses of the number of SR stacks (B), T-tubule length (C), the number of CEUs (D), and T-tubule/SR stack contact length (E) in the I band (expressed as μm/100 μm² of cross-sectional area) in soleus muscle fibers from WT (n = 109; n = 158; n = 158; n = 158 fibers), CASQ1-null (n = 32; n = 148; n = 148; n = 148 fibers), and dCASQ-null (n = 84; n = 125; n = 109; n = 125 fibers) mice. Data are shown as mean ± SEM. Statistical significance: B: *, P = 0.015, WT vs. dCASQ-null; C: *, P = 0.00000004, WT vs. CASQ1-null; *, P = 0.00000003, WT vs. dCASQ-null; D: *, P = 0.00000004, WT vs. CASQ1-null; *, P = 0.00000002, WT vs. dCASQ-null; *, P = 0.00082, CASQ1-null vs. dCASQ-null. Number of mice used: WT, n = 3; CASQ1-null, n = 3; dCASQ-null, n = 3.