SR stack incidence, number/area, and TT extension in FDB muscle fibers. (A–C) Quantitative analyses of the percentage of muscle fibers exhibiting SR stacks (A), number of SR stacks per 100 µm² of cross-sectional area (B), and TT length in the I band (μm/100 µm² of cross-sectional area; C) determined from longitudinal sections of FDB muscle fibers from WT and CASQ1-null mice. Data are shown as mean ± SEM; *, P < 0.05. Numbers in bars (n) indicate the number of FDB fibers analyzed. Number of mice: n = 3 WT; n = 3 CASQ1-null.