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Fluorescent activated[edit]
Main article: Flow cytometry

Fluorescent Activated Cell Sorting, or FACS, utilizes Flow cytometry to provide a fast, objective and quantitative measurement of intra- and extracellular properties, not including morphology, for sorting a heterogeneous mixture of cells. This is the most common method of currently separating cells and involves encapsulating cells into small liquid droplets and selectively labeled with electric charges and sorted by an external electric field. FACS has several systems that work together to achieve successful sorting of events of interest. These include fluidics, optics, sorting systems. Utilizing fluorescence, users can identify a particular population of events within a sample containing many differently characterized cells. This technology is used extensively in hematology labs. However, researchers can use a variety of fluorescent dyes to design multi-color panels to achieve successful, simultaneous sorting of multiple, precisely defined cell-types.

Fluorescent Dyes in Cell Sorting
Fluorescent dyes can act very differently. Generally, a fluorescent dye will be excited by a coherent light source (a laser) at a particular wavelength and emit light at a lower energy and longer wavelength. The most common dyes act by binding to antigens presented on cells. Common antigens targeted are Clusters of Differentiation (CDs). These are specific to a certain type of cell. If you can identify which CD is presented on your cells of interest, then you can stain your sample with a fluorescent dye specific to it and using FACS, sort out only those cells. However, there are many other mechanisms by which fluorescent dyes can act.

Some dyes are able to diffuse across membranes. By taking advantage of this property of the dye, users can characterize intracellular activity as well as surface-expression of proteins. For example, in dead cells, Propidium Iodide (PI) can penetrate the nucleus where it binds to DNA. The fluorescent signal of PI can be used to quantify DNA content for cell cycle analysis or to identify dead cells in a sample.

Certain fluorescent dyes can be used to characterize kinetic intracellular activity rather than fixing cells in formaldehyde and losing viable cells. The table below outlines dyes that can be used to measure several parameters of cytotoxicity caused by oxidative stress. This experimental setup is just one example of the capability of flow cytometry. In FACS systems, these characterized cells can then be sorted and purified for further experiments.

Cell Sorting


 * 1) Fluorescence-Activated Cell Sorting Analysis of Heterotypic Cell-in-Cell Structures
 * 2) Fluorescence-Activated Cell Sorting
 * 3) Flow Cytometry: First Principles
 * 4) Flow Cytometry and Cell Sorting
 * 5) Table of Fluorescent Dyes used for Common CD markers
 * 6) Kinetic Characterization of Intracellular Activity