We followed the OTO fixation protocol after Deerinck et al. (2010) to enhance the contrast of the tissue for SEM. Courtesy Frank Assen, Ludek Lovicar, Vanessa Zheden, IST Austria Reference NCMIR methods for 3D EM: A new protocol for preparation of biological specimens for serial block face scanning electron microscopy Thomas J. Deerinck, Eric A. Bushong, Andrea Thor and Mark H. Ellisman (2010) Center for Research in Biological Systems and the National Center for Microscopy and Imaging Research, University of California, San Diego, La Jolla, CA, USA. Available at: https://ncmir.ucsd.edu/sbem-protocol