Background: Autologous mesenchymal stem cells (MSC) have been shown to improve the functional outcome after severe skeletal muscle trauma. The reasons for this improvement have yet not been revealed. Up to now insufficient techniques of cell labelling, which could only be used for histologic analysis ex vivo, have been a problem.
The development of iron oxide nanoparticles, which are taken up and endosomally stored by stem cells, allows the evaluation of cellular behaviour in the muscle with the use of magnetic resonance imaging (MRI). Previous work has shown that labelling does not affect the proliferation and neurogenic differentiation capacity of embryonic stem cells. In the present study we are currently investigating the in vivo distribution and migration of locally transplanted MSC after blunt muscle trauma in a rat model.
Methods: MSC cultures are derived from tibial biopsies of Sprague Dawley rats via plastic adherence. A standardized open crush injury of the left soleus muscle is performed in each animal. 24 hours before transplantation cells are labelled with very small superparamagnetic iron oxid particles (VSOP-C200, Ferropharm, Teltow, Germany) and Green Fluorescent Protein (GFP). One week after trauma different amounts of stem cells (5×105, 1×106 and 5×106) are transplanted into the soleus muscle by local injection. Distribution and migration of the cells are evaluated over time by the repeated performance of high resolution-MRI at 7 Tesla (Bruker, Rheinstetten, Germany). At the endpoint of the study, three and six weeks after transplantation, the muscles are harvested and histologically and immunohistochemically analysed.
Results: Cells could be visualised inside the soleus muscle in the MRI 24 hours after transplantation showing characteristic signal extinctions in T2*-weighed images. The hypointense signal could be followed over the longest investigated time of six weeks and could be easily discriminated from the structures of the injured muscle. Preliminary results show that the cell pool changed its shape over time with the loss of an initially depicted injection canal and an increase in the surface/volume ratio. First histologic Prussian Blue stained sections showed co-localisation of the respective MRI signal and nanoparticle labelled cells. Fusion events of marked cells with regenerating myofibers could be observed.
Conclusion: Magnetic labelling of MSC is a powerful tool to analyse the in vivo behaviour of the cells after transplantation into a severly injured skeletal muscle. For the first time the observation of an intraindividual time course of the distribution of the transplanted cells is possible. Our preliminary results are promising and the ongoing work will further characterise migration processes and the correlation of the MRI results with muscle function evaluated by contraction force measurements.
