Recovery of renal function after acute nephrotoxic or ischemic insult is dependent on the replacement of necrotic tubular cells with functional tubular epithelium (86). This cellular regeneration originates from resident cells or from extrarenal cells. Recently, the possibility of differentiation of bone-marrow-derived mesenchymal stem cells (MSC) into mesangial cells (87,88) and of hematopoietic stem cells into tubular cells was demonstrated in vivo (89), bringing great therapeutic promise for the future. Noninvasive imaging techniques allowing in vivo assessment of the location of stem cells could be of great value for experimental studies in which these cells are transplanted. It provides a tool to immediately verify if the grafted cells have reached the target organ, to estimate the number of cells that were seeded, and to assess the permanence of these cells over time with sequential imaging. Using SPIO preparations to magnetically label the cells, several groups have demonstrated the feasibility of grafting and subsequent visualization of progenitor different organs (90-92). The renal distribution of SPIO-labeled MSC following intravascular administration has been investigated recently. Labeled MSC were observed in vivo within the kidney cortex as long as seven days after injection into the renal artery, with 1.5 T MRI (Fig. 16) (93). In a model of acute glomerulopathy, with an intravenous administration of magnetically labeled mesenchymal stem cells, no renal uptake could be observed in vivo at 4.7 T, but imaging at very high field strength (9 T) showed that the cells had reached the diseased areas of the cortex (specific homing effect) (Fig. 17) (94).
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