Cell preparation. (A) Unlabeled fourth-passage MSCs in culture. (B) Fluorescence microscopy images of MSCs labeled with magnetic nanoparticles and DiR stain (Mag-Dir-MSCs) expressing strong red fluorescence. Blue indicates the DAPI nuclear stain. (C) Prussian blue staining of Mag-Dir-MSCs on glass coverslips. The magnified image shows blue magnetic particles distributed around cell nuclei as a spherical shell. (D) Transmission electron microscopy of Mag-Dir-MSCs. The black compact particles are magnetic nanoparticles that were successfully internalized into the cells. (E, F) Proliferation rate and viability rate of MSCs. No significant differences were detected among unlabeled MSCs and labeled MSCs with and without exposure to magnetic fields.