Quantum of dots

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Abstract

QUANTUM dots or fluorescent semiconductor nanoparticles are very valuable tools in multiplexed cell labeling and cell tracking (1). With their unique features of extremely long stoke shift, their fluorescence signal can be more easily distinguished from autofluorescence. Additionally, they can be combined with organic fluorochromes adding new colors to polychromatic panels (2). Because of their high photostability, they are hardly bleachable; they are ideal tools for long-term observation of cells in cell cultures. However, cell division dilutes out the cell’s fluorescence after several cell cycles, making it difficult to distinguish its signal from autofluorescence. Brown and colleagues (this issue, page 925) demonstrate that time-series flow cytometry measurement of quantum dot labeled U-2OS osteosarcoma cells can be used to model the fluorescence signal and distinguish it from autofluorescence by deconvolution. This method provides an opportunity to quantify cell division and identify daughter cells for up to eight generations. Summers and colleagues (this issue, page 933) also performed cell tracking, albeit with peripheral blood mononuclear cells loaded with quantum dots. In their study, the authors focused on long-term stability of nanoparticles within loaded cells. They found a biphasic fluorescence intensity decay that was steeper in cells than in culture medium alone. The formula they developed compensates for this fluorescence loss. Both methods in combination should further improve quantitative cell tracking experiments by flow and image cytometry. Multiparametric cell characterization is the playground of cytometry (3). In parallel, the demand arises for further analysis of different subpopulations in enriched form for surface intracellular or genetic markers. There are two main technological platforms for sorting of cells. One is magnetic, whereas the other is cytometric. Cytometric sorting has the advantage that, during sorting, we get quantitative data about the cytome of cell populations simultaneously with sorting. Cell sorting flow cytometers sort cells by applying electrical charges to a stream that forms electrostatically loaded liquid droplets containing the cells at a set time after sample interrogation. Cytometric sorting methods were developed for separation of spherical human and animal cells, however, some important plant and bacteria types possess filamentous structures with a possible length up to 1,000 lm or longer. This morphology makes their sorting in drops difficult. Here, van Dijk and colleagues (this issue, page 911) worked out an effective system of nozzle size, drop delay and plate charge for analysis and improved sorting of natural phytoplankton communities. Identification and sorting of adipose stem cells is feasible by eight-color immunofluorescence techniques (4). However, this approach is expensive and technically demanding. Schaedlich and colleagues (this issue, page 990) concentrated on a simpler and cheaper way to isolate embryonic stem cellderived adipocytes. Adipocyte markers used with increased granularity (higher SSC values) and enhanced Nile Red dye uptake (lipid specific dye). Fluorescence in the 585/42 nm spectral range were used for sorting definition. After sorting light microscopy, examination of the obtained cells detected that positively sorted cells showed a larger cell diameter than cells in the initial unsorted culture, as well as Nile red fluorescence emission and accumulation of multiple lipid droplets inside the cells. Technical details of electrostatic sorting were further studied by Osborne (this issue, page 983). HEK293T cells were loaded with Accudrop beads and were used for the experiments. Cell size was related to the number of loaded beads