Histochemical methods used to detect apoptosis are based on in situ labeling of cleaved nuclear DNA.13
The TUNEL (TdT-mediated dUTP nick end labeling )method utilizes the ability of the TdT (terminal deoxynucleotidyl transferase) enzyme to incorporate deoxyuridine (dUTP) at the 3′-OH sites of DNA breaks.14
The dUTP is conveniently labeled with a tag which is then detected, depending on the chemical nature of the tag. The final detection can be using a color development reagent such as DAB (3, 3′-diaminobenzidine) or by fluorescence.
Double-labeling techniques incorporating the TUNEL method have alsoproved helpful in associating apoptosis with the expression of various proteins in the cell.15
Finally, the old standby method of detection of fragmented DNA by gel electrophoresis or DNA laddering is discussed. 16