Cells of each sample were collected, washed twice, and fixed with 2.5% glutaraldehyde at 4°C overnight. After being washed with PBS, cells were fixed with 1% osmic acid, underwent a second round of washes, and dehydrated with gradient alcohol. Before being embedded, each sample was treated with a mixture of acetone and embedding medium, in which concentration was gradually elevated. Then, the sample was sliced to several pieces, stained with uranyl acetate and lead citrate orderly, and imaged.