[MR imaging of embryonic stem cells labeled by superparamagnetic iron oxide]

To evaluate the labeling efficiency of superparamagnetic iron oxide (SPIO) nanoparticles and its toxicity to mouse embryonic stem cells (ESCs) and (embryoid body (ES)-derived cardiomyocytes.

Mouse ESCs of the line CGR8 were cultured and induced to differentiate into ES-derived cardiomyocytes. The EB-derived cardiomyocytes were coincubated with SPIO contrast agent at different concentrations (1, 8, 9.3, 14, 28, and 56 mg/L) and transfection agent for 24 and 48 hours for cell labeling. Cells not labeled by SPIO and cells labeled by SPIO without transfection agent were used as controls. Spectrophotometer was used to detect the iron concentration in the cells. Confocal microscopy was used to test the intracellular calcium levels ([Ca(2+)] i). The ultrastructure of the cells was observed by electron microscopy. Ex vivo MRI was used to observe the signals of the cells.

Iron-containing intracytoplasmic vesicles could be observed clearly with electron microscopy. The intracellular iron concentration was higher in the cells treated with transfection agent than in the cells not treated with transfection agent. The iron concentration of the cells treated with the SPIO at the concentration of 9.3 microg/ml for 24 hours was the highest. There were no differences in the morphology, contractile areas (chi(2) = 1.32; P = 0.25), and the beating frequency (t = 1.73; P = 0.10) between the EBs from iron-labeled ESCs and from the control ESCs. The rhythmic intracellular free Ca(2+) fluctuation in the labeled cardiomyocytes was similar to that of the controls. The MR images with T(2)WI and T(2)WI sequences, especially those with T(2)WI sequence, of the ESCs showed that the signals of the SPIO labeled cells were lower than those of the SPIO-labeled cells.

SPIO labeling of ESCs and ES-derived cardiomyocytes does not influence the cell viability and proliferation. The standard 1.5T MR equipment can image the labeled cells, thus offering the possibility of cell tracking and migration monitoring in MRI.