A cautionary note on the use of the TUNEL stain to determine apoptosis

Detection of DNA fragments in situ using the terminal deoxyribonucleotidyl transferse (TdT)-mediated biotin-16-dUTP nick-end labelling (TUNEL assay) is now commonly used to investigate apoptosis. Previous reports suggested that physiologically appropriate death is due to apoptosis and that pathological mechanisms involve necrosis. Strong evidence of apoptosis, following ischaemia and epilepsy, has been recently provided by combining genomic DNA gel electrophoresis, light and electron microscopy and in situ, DNA-break labelling. However, only an observation in light microscopy with high magnification permits the detection of chromatin condensation and apoptotic bodies. We report that a positive TUNEL assay reaction should not be considered as a specific marker of apoptosis but can also indicate necrotic cell death.