The surface of healthy cells is composed of lipids that are asymmetrically distributed on the inner and outer leaflet of the plasma membrane. One of these lipids, phosphatidylserine (PS), is normally restricted to the inner leaflet of the plasma membrane and is, therefore, only exposed to the cell cytoplasm. However, during apoptosis lipid asymmetry is lost and PS becomes exposed on the outer leaflet of the plasma membrane. Annexin V, a 36-kDa calcium-binding protein, binds to PS; therefore, fluorescently labeled Annexin V can be used to detect PS that is exposed on the outside of apoptotic cells. Annexin V can also stain necrotic cells because these cells have ruptured membranes that permit Annexin V to access the entire plasma membrane. However, apoptotic cells can be distinguished from necrotic cells by co-staining with propidium iodide (PI) because PI enters necrotic cells but is excluded from apoptotic cells. This protocol describes Annexin V binding and PI uptake followed by flow cytometry to detect and quantify apoptotic and necrotic cells.

The nuclei of healthy cells are generally spherical, and the DNA is evenly distributed. During apoptosis the DNA becomes condensed, but this process does not occur during necrosis. Nuclear condensation can therefore be used to distinguish apoptotic cells from healthy cells or necrotic cells. Dyes that bind to DNA, such as Hoechst 33342 or 4',6-diamidino-2-phenylindole (DAPI), can be used to observe nuclear condensation. These dyes fluoresce at 461 nm when excited by ultraviolet light and can therefore be visualized using conventional fluorescent microscopes equipped with light sources that emit light at ∼350 nm and filter sets that permit the transmission of light at ∼460 nm. This protocol describes staining and visualization of cells stained with Hoechst 33342, but it can be adapted for staining with DAPI or other dyes.

Propidium iodide (PI) is a small fluorescent molecule that binds to DNA but cannot passively traverse into cells that possess an intact plasma membrane. PI uptake versus exclusion can be used to discriminate dead cells, in which plasma membranes become permeable regardless of the mechanism of death, from live cells with intact membranes. PI is excited by wavelengths between 400 and 600 nm and emits light between 600 and 700 nm, and is therefore compatible with lasers and photodetectors commonly available in flow cytometers. This protocol for PI staining can be used to quantitate cell death in most modern research facilities and universities.