100 μL of exosomes and 4 μL of PKH26 were respectively diluted with 200 μL of diluent (Sigma, St. Louis, MO, USA). The two diluted solutions were immediately mixed and incubated for 10 min at room temperature, and then an equal volume of serum was added to the mixture to stop the reaction. Afterward, the exosomes were re-extracted by centrifugation at 120000 g for 90 min at 4°C. The exosomes were added to the cells in 6-well plates and cultured for 4 h. The nucleus was stained by DAPI (Beyotime Biotechnology Co. Ltd., Shanghai, China) for 10 min and observed under a fluorescent microscope (Nikon, Tokyo, Japan).