*4.3. Electron Microscopy*

For electron microscopic investigation, samples were fixed with 3% glutaraldehyde solution (pH 7.4) for 2 h at 4 ◦C, over-fixed with 1% osmium tetraoxide solution for 1.5 h, and then dehydrated in alcohol series with increasing alcohol concentrations (70% alcohol was saturated with uranyl acetate). The samples were embedded in Epon-812 epoxy resin. Serial ultrathin sections were made with a Leica ULTRACUT UCT microtome and stained by lead according to Reynolds [61]. The resulting preparations were scanned and photographed using a JEM-1400 electron microscope (JEOL, Japan).

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4409/9/3/712/s1: Video S1. 3D reconstruction of the mitochondrial network from rat ventricular cardiac myocyte stained with the tetramethyl rhodamine methyl ester (TMRM). Video S2. Timeseries of the mitochondrial network from rat ventricular cardiac myocyte stained with tetramethyl rhodamine methyl ester (TMRM). Video S3. 3D reconstruction of the mitochondria from ultrathin sections from serial sections of a cardiomyocyte nucleus of a 3-month-old Wistar rat. Video S4. 3D reconstruction of the mitochondrial network from rat ventricular cardiac myocyte stained TMRM. Table S1. The frequency of mitochondrial appearance in nuclei of cardiomyocytes of Wistar, Oxys and naked mole rats of di fferent ages.

**Author Contributions:** C.M.E., I.M.V., V.B.V., L.E.B., and D.B.Z. designed the experiments; S.H., T.B.H., and N.G.K. provided the animals for the experiments; I.M.V., V.B.V., and L.E.B. prepared the samples for electron microscopy; C.M.E., I.M.V., V.B.V., and L.E.B. obtained electron-microscopic images; D.B.Z. ran experiments using confocal microscope; C.M.E., I.M.V., V.B.V., E.V.S., V.A.P., E.Y.P., and L.E.B. analyzed the data; L.E.B., D.B.Z., and V.P.S. wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by the Russian Science Foundation gran<sup>t</sup> 19-14-00173 (confocal microscopy) and the Russian Foundation of Basic Research gran<sup>t</sup> 19-04-00578 (electron microscopy).

**Acknowledgments:** The authors thank Dagmar Viertel of the laboratory of electron microscopy and her supervisors Alex Greenwood and Gudrun Wibbelt for their kind support by providing reagents and laboratory space.

**Conflicts of Interest:** The authors declare that they have no competing interests.

**Data Availability:** Typical results of this study are included in this published article.
