Measuring apoptosis by microscopy and flow cytometry

Apoptosis is a programmed mode of cell death that is accompanied by numerous morphological as well as biochemical changes to the cellular architecture. This results not only in cell death but also in the efficient removal of apoptotic cells by phagocytes. Apoptotic cells display a range of common characteristics that include cell shrinkage, plasma membrane blebbing, cell detachment, nuclear condensation, DNA fragmentation, externalization of phosphatidylserine (PS) and activation of caspases. In contrast, necrotic cell death is characterised by rapid plasma membrane, organelle swelling and plasma membrane rupture with none of the features of apoptosis. Apart from severe physical stresses, necrotic cell death often betrays the activities of viral infection and the activities of bacterial toxins. While necrotic cell death is characterized by the release of endogenous 'danger signals' and subsequent inflammation, apoptosis is largely tolergenic. Therefore, care must be taken when assessing whether cells are dying via apoptosis or necrosis. Here, we highlight a number of assays, utilizing both microscopy and flow cytometry, to determine whether cells have undergone apoptosis or alternative modes of cell death.