Page 169 - Cellular Imaging in Regenerative Medicine, Cancer and Osteoarthritis

Summary, Discussion, Concluding remarks
MRI resulting in local signal loss in MR images. The efficacy of these particles for cell labelling and imaging studies was investigated by variations in iron particle labelling protocols, and the influence thereof on incorporation, distribution and retention of iron oxide nanoparticles as described in Chapter 2. Cultured human umbilical cord cells (HUVECs) were labelled with iron particles. The average iron load of SPIO increased when incubation time was extended from 4h to 24h. The optimal labelling protocol in this study resulted in 12.0 pg iron/per cell. For MPIO labelling, incubation time was of less importance, since most of the particles were already incorporated in the cell within 4 h, with a 100% labeling efficiency and an intracellular iron load of up to 626 pg iron/cell. MPIO were taken up more efficiently, were better tolerated without causing significant cell death and had a much more pronounced effect on cell appearance in an MR image than SPIO. A drawback of the use of the bigger MPIO particles is that there are fewer particles per cell. When cells divide, not every daughter cell will contain a MPIO particle. This results in the inability to track these cells. Optimal label incorporation required different protocols for SPIO and MPIO. No standard labelling protocol could be used for different cell types growing in vitro. Cell lines differ from each other for example in ease of spontaneous particle uptake. They also can differ in particle uptake rate and in tolerated load of iron particles in the cell. Other studies showed that iron labelled cells can be followed for several weeks in vivo (8, 9). The iron particles inside the cells can be visualized with MRI and the iron particles have an effect on different MRI parameters (eg. R2' (R2*-R2) = relaxivity). So, cell labelling with iron particles offers a promising method for in vivo human and animal model cell tracking by MRI. Different processes can occur when iron labelled cells are injected in vivo, like cell death, cell migration and cell division, all influencing signal intensity (10, 11). Mimicking these biologically relevant processes was executed by labelling cells with small (SPIO) and larger (MPIO) iron particles; the performed experiments and the results are described in Chapter 3. For the MR measurements phantoms were prepared. As a model system for dividing cells in vivo, Brown Norway 175 sarcoma cells were labelled with SPIO and injected subcutaneously in the flank of a rat. The effects of spatial distribution and compartmentalization of paramagnetic iron-oxide particles on relaxivity were analyzed. We demonstrated that relaxometry does not allow labelled cell quantification when multiple physiological processes such as cell division and cell migration coexist. Not only we, but also other groups (12-14), concluded
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