RESUMEN
OBJECTIVE: To evaluate the feasibility of in vivo magnetic resonance imaging (MRI) with 1.5T system tracking of the survival, migration and differentiation of magnetically labeled seed cells-bone marrow-derived mesenchymal stem cells (MSCs) injected into the articular cavity. METHODS: Rabbit MSCs were isolated, purified, expanded, and then coincubated in vitro with supermagnetic iron oxide particles (SPIO) and 5-bromo-2-deoxyuridine (BrdU). Prussian blue staining and transmission electron microscopy were performed to observe the intracellular iron. Some labeled MSCs were subjected to chondrogenic differentiation and the phenotype was examined to assess their chondrogenic differentiation capacity. MSCs colabeled with SPIO nanoparticles and (BrdU were suspended in chitosan and glycerophosphate (C-GP) gel. Eighteen rabbits underwent damage to the femoral trochlea to create cartilage defect models, and randomly divided into 3 groups 1 week later: Group A (n=6) undergoing injection of the MSC suspension in C-GP gel into the intra-articular space of knee joints, Group B (n=6), injected with un-labeled MSC suspension in C-GP gel, and Group C (n=6), without injection. MRI of the knee was performed 1, 4, 8, and 12 weeks after the injection respectively on a certain numbers of rabbits. and then the rabbits were killed with their knee joints taken out to undergo immunohistochemistry. The MR imaging findings were compared with the histological findings. RESULTS: Prussian blue staining and transmission electron microscopy showed intracytoplasmic nanoparticles in the SPIO-labeled cells. Safranin-O staining showed deposition of proteoglycan and type II collagen outside both the labeled and unlabeled MSCs, showing chondrogenesis. GRE T2-weighted MR image showed marked hypointense signal void areas, representing the implanted MSCs, in the intra-articular space after the MSC injection in Group A that persisted for 12 weeks at least; 2 week after the MSC injection hypointense signal could be seen in the defect, which peaked in the signal intensity about 4 weeks later, and then gradually decreased in the signal intensity; and 12 weeks after the injection no recognizable hypointense signal in the defect was detected. Immunohistochemical staining demonstrated the presence of Prussian blue-positive cells and BrdU-positive cells in the tissue sections in the areas corresponding well to the signal intensity loss regions in the MRI images. Group B and Group C showed no signal intensity loss in the intra-articular spaces by GRE T2-weighted MR imaging. Histological observation showed that the defects were repaired with fibrocartilage in Groups A and B, and with fiber tissue in Group C. CONCLUSION: Labeled with SPIO, the MSCs remains their ability of chondrogenic differentiation. It is feasible to track the fate and dynamic redistribution of magnetically labeled MSCs, the seed cells, injected into the articular cavity by 1.5T MRI, an efficient noninvasive technique.