Tannic acid-mediated osmium impregnation after freeze-substitution: A strategy to enhance membrane contrast for electron tomography

In this technical note we report a tannic acid-mediated osmium impregnation method that, applied after freeze-substitution, increases membrane contrast in cells for transmission electron microscopy and tomography studies. The general staining that is achieved allows visualization of organelles, plasma membrane and associated specializations (e.g. caveolae) in non-post-stained plastic sections by conventional transmission electron microscopy. In combination with electron tomography it results in membranes with a proper contrast and equal staining pattern through the depth of the tomograms. The protocol that we contribute can serve as starting point for those willing to improve the membrane contrast of their specimens or to make 3D studies on the architecture of membranous compartments by electron tomography.