TT extension and TT/SR stack contact lengths. (A–D) Representative EM images of longitudinal sections of EDL muscle fibers from WT (A and B) and CASQ1-null (C and D) mice showing TTs stained in black with ferrocyanide precipitate. (E and F) Quantitative analyses of total TT length (E) and TT-SR contact length (F) per 100 µm² of cross-sectional area in EDL muscle from WT and CASQ1-null mice. Data are shown as mean ± SEM; *, P < 0.05. Numbers in bars (n) indicate the number of EDL fibers analyzed. Number of mice: n = 4 WT; n = 4 CASQ1-null. Data for EDL muscles from WT mice shown in E and F are the same as those reported by Boncompagni et al. (2017) and Michelucci et al. (2019). Scale bars: 1 µm, A and C; 0.5 µm, B and D.