Critical aspects in the analysis of apoptosis and necrosis

Despite that large number of methods to analyze cell death, in particular apoptosis, that have been developed, identification of the mode of cell death and its quantitation is not always simple or straightforward. The difficulties, potential pitfalls and traps in quantitation of dead cells, whether apoptotic or necrotic are reviewed. The following are common flaws in the measurement of cell death, which include incorrect assumptions, erroneous data interpretation and shortcomings of the methodology: 1) Misclassification of apoptotic bodies or chromatin fragments as individual apoptotic cells based on cellular DNA content analysis by flow cytometry; 2) Assumption that the quantity of fragmented DNA extracted from cells represents the frequency of apoptosis; 3) Conjecture that the apoptotic index represents the cell death rate; 4) Assumption that apoptotic cells must exhibit classical features of apoptosis e.g. internucleosomal DNA fragmentation; 5) Inadequacy of methods that presume to discriminate between late apoptotic and necrotic cells; 6) Possibility of a selective enrichment or loss of apoptotic cells during cell separation on density gradients, during trypsinization or other procedures of cell collection; and 7) Inability to distinguish between live, nonapoptotic cells phagocytizing apoptotic bodies and genuine apoptotic cells by flow cytometric methods. Many of the problems stem from the difficulty in identifying apoptotic or necrotic cells. Because apoptosis and necrosis have been originally defined based on morphological criteria it is essential to confirm the mode of cell death by microscopy. Laser scanning cytometry (LSC), which combines the advantages of flow and image cytometry, offers the possibility of morphological examination of apoptotic cells. By virtue of this attribute LSC appears to be the instrument of choice for analysis of apoptosis.