The ability to isolate cells is important in both clinical and research settings. There are many available techniques for cell separation. These techniques differ in specificity of cell selection, cost of equipment, time to complete, technology needed, and skill required. Cell separation based on cell density is rapid and inexpensive, but is unspecific. Still, it is a fundamental technique that is commonly used in a variety of settings for general cell separation.
Specific vs. unspecific cell separation
Cell separation based on cell density is very general as it is only based on one physical characteristic. It is not good for isolating a single cell type, but it is useful for rough bulk separation as a precursor to other more specific cell separation techniques. It is commonly used toseparate whole blood into plasma, mononuclear cells, centrifugation medium, and red and white blood cells. Specific cell separation techniques such as magnetic separation or fluorescent activated cell separation use ligands to specifically bind to unique surface markers on the target cells.
A centrifuge is used to separate cells based on density
Most commonly, density-based cell separation requires a centrifuge to spin the sample in a circular motion at an appropriate angular velocity to cause the cells in the sample to separate into distinct layers. The circular rotation around a fixed axis creates a force perpendicular to the direction of spin. This radial force vector points out from the center to outer edge of the circle. A laboratory centrifuge has a circular rack with holes drilled at an angle that hold the sample tubes. The angle is such that the tops of the tubes point inward toward the axis of rotation while the bottoms of the tubes are furthest from the axis of rotation. The centripetal acceleration causes the denser cells to move away from the axis of rotation. During this process the denser cells displace the less dense cells. The result is that the densest cells are forced to the bottom of the container, and the least dense cells are displaced to be near the top. The different populations can then be collected and used for further analysis.
Advanced biomagnetic separation vs. cell density separation
If the cells are spun in a centrifuge at an inappropriate speed they can shear due to extreme centripetal force. Therefore, it is very important to optimize the centrifuge conditions for each cell type. However, advanced magnetic separation techniques are gentle. With a properly engineered biomagnetic separation rack there is a low risk of cell lysis or shearing. Once a magnetic separation protocol is optimized, it can be performed in a similar amount of time as density separation, and can result in the isolation of a specific cell type. This could potentially save time down the road and could open doors to advanced diagnostic techniques in the clinical setting, and new discoveries in basic research.
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