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Recent advances in stem cell research have shown the promising nature of mesenchymal stem cells as plausible candidates for cell-based regenerative medicine. Many studies reported the use of human dental pulp stem cells (hDPSCs), which possess self-renewal capacity, high proliferation potential, and the ability to undergo multilineage differentiation. Together with this therapeutic approach, development of effective, noninvasive and nontoxic imaging techniques for visualizing and tracking the cells in vivo is crucial for the evaluation and improvement of stem cell therapy. Magnetic resonance imaging (MRI) is one of the most powerful diagnostic imaging techniques currently available for in vivo diagnosis and has been proposed as the most attractive modality for monitoring stem cell migration. The aim of this study was to investigate the labeling efficiency of hDPSCs using superparamagnetic iron oxide (SPIO) particles in order to allow visualization using in vitro and in vivo MRI without influencing cellular metabolism. MRI and transmission electron microscopy (TEM) showed optimal uptake with low SPIO concentrations of 15 μg/ml in combination with 0.75 μg/ml poly-l-lysine (PLL) resulting in more than 13 pg iron/cell and an in vitro detection limit of 50 labeled cells/μl. Very low SPIO concentrations in the culture medium resulted in extremely high labeling efficiency not reported before. For these conditions, tetrazolium salt assays showed no adverse effects on cell viability. Furthermore, in vivo MRI was performed to detect labeled hDPSCs transplanted into the brain of Rag 2-γ C immune-deficient mice. Transplanted cells did not show any signs of tumorgenecity or teratoma formation during the studied time course. We have reported on a labeling and imaging strategy to visualize human dental pulp stem cells in vivo using MRI. These data provide a solid base to allow cell tracking in future regenerative studies in the brain longitudinally.

The high mortality in the final stage of certain liver diseases and the lack of whole organ donors for liver transplantation require alternative therapeutic approaches. Cell based therapy has been one of the promising approaches for treating liver diseases. Hereby, the utilization of hepatocytes or stem cells differentiated towards functional hepatocytes has been studied intensively. In order to validate the success of those approaches, it is necessary to monitor the location, homing and functionality of engrafted cells longitudinally and non-invasively. This has been possible with the help of imaging modalities like magnetic resonance imaging, positron emission tomography and optical imaging methods. All those methods require contrast agents, tracers or imaging reporters that track not only the location of transplanted cells but also visualize their functional status in vivo. In this review, we provide an overview of possible contrast agents and tracers for liver imaging, an outline of imaging approaches for tracking transplanted cells in the liver and discuss imaging strategies to assess liver function non-invasively. Hereby, a strong focus will be on the assessment of imaging markers for the different modalities in experimental models for normal and diseased liver. We conclude the review with the possible advantages of such markers for diagnosis and monitoring of disease progression and therapy assessment over currently available imaging modalities.