The cell samples were prepared for transmission electron microscopic analysis as previously reported (Huang et al., 2014a (link)). Briefly, after washing with pre-warmed PBS, the cells were next fixed in 2% glutaraldehyde for 1 h. Subsequently, the fixed cells were postfixed with 1% osmium tetroxide for 1 h after briefly washing with PBS. The samples were rinsed in distilled water, stained with 1% tannic acid, dehydrated in a gradient of ethanol, and embedded in Epon. Sections of 70 nm were generated with an ultramicrotome (Ultracut 7, Leica) and post-stained with 2% aqueous uranyl acetate and Reynold’s lead citrate for 10 min each. The specimens were examined under a transmission electron microscope operating at 200 kV (JEM-2100, JEOL, Japan). Mitochondrial Feret’s diameter (maximum and minimum), the distance between two parallel tangential lines within the selected mitochondrion, was determined using Image J (Demeter-Haludka et al., 2018 (link); Lomash et al., 2019 (link)).
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