How do you identify stages of apoptosis?

Apoptosis can be seen as a stage-dependent process from its induction to early, intermediate and late-stage apoptotic events. Methods and reagents have been developed to identify the various apoptotic stages.

• Early stage: Early stage apoptosis is represented by changes to, and ultimate loss of, the mitochondrial membrane potential, which can be detected by mitochondria selective dyes such as JC-1 and rhodamine 123.
• Intermediate stage: The exposure of phosphatidylserin (PS) to the extracellular portion of the membrane is a sign for intermediate stage. Annexin V staining is used for PS detection.
• Late stage: DNA fragmentation is seen as a marker for late stage apoptosis, where TUNEL assay can be used to detect the cleaved DNA strand.