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I recommend imaging spheroids by confocal microscopy in a low background fluorescence media such as FluoroBrite. If you are wanting to image spheroids in suspension, you can try coating the surface of the imaging slide or well with a substrate such as poly-lysine that will adhere to the spheroids. If the spheroids are embedded in an extracellular matrix (ECM), you could fix, cryopreserve, cryosection and then immunolabel the spheroids. This approach produces beautiful, crisp images but does not provide information on the spheroid 3 dimensional context. If you are wanting to obtain 3D contextual information on spheroids that are embedded in ECM, you can partially dissolve the ECM with PBS-EDTA, smear the spheroids along with the residual ECM onto a slide, fix and then immunolabel them.