Detecting Gold-Labeled Cells

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Colloidal gold particles bind tightly but not covalently to proteins at pH values that are around the protein's pI. Colloidal gold particles conjugated with a wide range of anti-immunoglobulin antibodies, Protein A, or streptavidin are available commercially. Gold labels were developed originally for electron microscopic studies, but they also work well at the level of the light microscope. They give higher resolution than enzyme-based methods and avoid the problems of substrate preparation and endogenous enzyme activity. Until recently, the gold labels lacked sensitivity at the level of light microscopy, but the recent development of the photochemical silver method of amplification, described here, has overcome this problem. Unamplified gold labels can be detected under the light microscope using bright-field illumination in which the label ranges from pale pink to deep red, depending on the strength of the reaction. Nomarski differential interference contrast microscopy makes the label appear dark red to black. With the silver enhancement method, the gold particles become coated in metallic silver and yield a black-brown label, best visualized by bright-field optics. Gold labeling methods are compatible with many histochemical stains. Gold labeling reactions are very readily controlled, because the appearance of staining can be monitored directly and continuously under the microscope.