TUNEL Assay for Apoptotic Cells

Cell death may occur via two different mechanisms: apoptosis or necrosis. Necrosis is caused by the cell’s inability to maintain homeostasis and is characterized by loss of plasma membrane integrity, cell swelling and lysis, random degradation of DNA, and lack of an energy requirement or macromolecular synthesis. Necrotic cell death may be observed in localized areas of a tissue, probably as the result of physical damage. An inflammatory response is usually elicited. In contrast, apoptosis is genetically driven and is characterized by chromatin condensation, nuclear shrinkage and eventual loss of nuclear membrane, membrane blebbing that produces apoptotic bodies containing cellular organelles and chromatin, DNA degradation into distinct nucleosomal units, an energy and protein synthesis requirement, and a lack of an inflammatory response. Whereas it is common to observe localized clusters of cells in a tissue dying via necrosis (for example, as a result of tissue injury), usually only individual or isolated cells are engaged in the apoptotic death pathway (for example, as a genetically programmed step in normal development of a tissue) (1 ,2 ).