Impact of particle exposure on hematopoietic progenitor proliferation and mesenchymal stem cell proliferation. (A) CD34⁺ human progenitor cells were exposed to a range of concentrations of the iron oxide fluorescent particle for 18 hours and then plated in methylcellulose. Ten to 12 days later, macroscopic colonies (CFUs) were enumerated. The x-axis gives the concentration of particle added following plating of either 500 or 1000 cells and the y-axis the number of CFUs present. There was no difference in the average size or composition of the individual colonies. The colony number data are derived from 3 independent experiments. These data are from 1 representative experiment; error bars show standard deviations. Similar results were obtained in 3 independent experiments using different CD34⁺ cell donors. (B) MSCs were exposed to a range of particle concentrations and growth assessed using an MTT assay. No effect on cell proliferation was observed after overnight labeling with 1 or 10 μL/mL. Proliferation was mildly impaired if particles were not removed from the media for the duration of the growth assay. The y-axis shows cell proliferation in absorbance units at 560 nm and the x-axis days in culture; error bars show standard deviations. (C) MSCs were exposed to 10 μL/mL particles for 4 hours, and the particles were removed. The next day, and then serially for up to 10 days, aliquots of the cells were collected and analyzed by flow cytometry for the presence of the fluorescent particles. The histograms show the uniform very bright initial labeling, with gradual decline over the next 10 days with ongoing cell proliferation.