Magnets, magnetic fields and formation of human MSCs labeled with SPIO. 3D-visualization of the magnetic fields by cuttings of iron in 75 cm2 culture flasks (A) and 6-well plates (B). The magnets (C) were positioned under the culture flasks and 6-well plates. SPIO-labeled MSCs immediately exposed to the magnetic fields at the time of seeding immediately accumulated in the area over the magnets as identifiable by the red-brown particles (= SPIO) by light microscopy (D) and additional blue colour after cell staining with Coomassie blue (E). Microphotographs show the orientation of the SPIO-labeled MSCs according to the magnetic flux lines under exposition to magnetic fields (F), whereas unlabeled MSCs with or without exposition to magnetic fields and labeled MSCs without exposition to magnetic fields showed the usual clonogenic growth clusters without general order in the culture flasks (G). Scale bars indicate 50 μm.