Araldite 502 resin

The induced‐hPSC‐derived cardiomyocytes were directly scraped off from the dish and then fixed with 2% glutaraldehyde overnight at 4°C.26 These samples were postfixed with 0.25% osmium/0.25% K₄Fe(CN)₆, 1% tannic acid, followed with 50 mmol/L uranyl acetate. Then specimens were washed 3 times and dehydrated with a series of ethanol. Finally, the cell samples were embedded in araldite 502 resin (Polysciences Inc, Warrington, PA), and polymerization proceeded at 65°C for several days. The ultrathin sections (≈60 nm) obtained by ultramicrotome (Leica EM UC7; Leica, Wetzlar, Germany)) were mounted in EM‐grids, stained with lead citrate, and then observed by FEI Tecnai G2 Spirit TEM (FEI, Hillsboro, OR).

hPSC spheres were chemically fixed with 2% glutaraldehyde in NaHCa buffer (100 mM NaCl, 30 mM HEPES, and 2 mM CaCl₂ at pH 7.4). The specimens were postfixed with 0.25% OsO4/0.25% K₄Fe(CN)₆, dehydrated with a graded ethanol series, and embedded in Araldite 502 resin (Polysciences). After polymerization at 65°C for a few days, ultrathin sections, obtained using a Ultramicrotome (Leica FC6), were mounted on EM grids, stained with lead citrate, and then observed with a conventional transmission electron microscope (JEOL JEM1400). Bright-field light microscopy images were obtained from thick sections (∼1 μm) that had been stained with toluidine blue.