Electron microscopy was carried out as described previously (Agbulut et al., 2001 (link); Joanne et al., 2013 (link)). Briefly, the calf muscles of mice were fixed in 2% glutaraldehyde and 2% paraformaldéhyde in 0.2 M phosphate buffer at pH 7.4 for 1 h at room temperature. After 1 h, the plantaris muscle was dissected and separated in three by a short-axis section, then fixed overnight at 4°C in the same fixative. After washing, specimens were post-fixed for 1 h with 1% osmium tetroxide solution, dehydrated in increasing concentrations of ethanol and finally in acetone, and embedded in epoxy resin. The resin was polymerized for 48 h at 60°C. Ultrathin sections (70 nm) were cut with an ultramicrotome (Leica UC6, Leica Microsystems), picked-up on copper rhodium-coated grids and stained for 2 min with Uranyl-Less solution (Delta Microscopies, France) and 2 min with 0.2% lead citrate before observation at 80 kV with an electron microscope (912 Omega, Zeiss) equipped with a digital camera (Veleta 2kx2k, Emsis, Germany).
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