Viability assay by staining for apoptotic cells with the fluorescent dye ethidium bromide is also based on the principle of dye exclusion. The ethidium bromide homodimer can permeate dead cells and fluorescently stain the nuclei as it chelates DNA; intact cells remain unstained. 7,20
When viewed under a fluorescence microscope, the nuclei of dead cells have a bright red fluorescence. 21 The other fluorescent dye frequently used in similar experiments is propidium iodide (PI). PI is also a membrane impermeable nuclear dye, under a fluorescence microscope the stained dead cells emit red fluorescence upon green excitation. 6,22,23
The advantage of using both these dyes for viability assay is that in addition to distinguishing between live and dead cells based on membrane integrity, they also stain the nucleus, and thus
differentiate a normal nucleus from a condensed, apoptotic one. One advantage of using fluorescent dyes for cell viability assay over trypan blue is that the count of stained cells remains constant with time. 24
However, while counting several hundred cells under a fluorescence microscope, the fluorescent dye tends to bleach. This problem is overcome by photographing various fields and counting stained cells from the photographs.