Defining apoptosis.

A great deal of interest and effort has been recently focused on understanding the inner workings of a particular style of cell death that occurs in variety of physiological settings. As carefully documented in this issue of The American Journal of Pathology by Majno and Joris,1 physiological cell deaths have been noted for over 100 years without attracting much scientific attention. The nomenclature that is now well accepted for these cell deaths, apoptosis, was devised by Kerr et a12 in 1972 after their important recognition that morphologically similar cell deaths were found in many pathological conditions as well as normal tissue. Interest in this area advanced when a requirement for RNA and protein synthesis was demonstrated for many apoptotic deaths, as had previously been noted for "programmed cell deaths" in developmental biology.3 Apoptosis was thus conceptualized as a self-directed cellular "suicide. " The rapid increase in publications on apoptosis over the last 5 years followed the identification of several genes which govern this cellular process, providing a molecular foothold from which to analyze the key steps in the activation and execution of apoptosis.67 Many questions arise in thinking about apoptosis and differentiating this process from other types of cell death, including necrosis or the cell death process defined as "oncosis" by Majno and Joris. 1 The current state of knowledge in this field is rapidly evolving, but complete answers to many of these questions are not yet available. Apoptosis is currently best identified by a series of morphological changes in dying cells. The morphological features of apoptosis recognizable by electron or light microscopy are remarkably similar across cell types and species and constitute the most unequivocal experimental proof of the underlying process.2,8,9 Cell shrinkage and loss of normal contacts, dense chromatin condensation, cellular budding and fragmentation, and rapid phagocytosis by professional phagocytes or adjacent cells are the typical events of apoptosis that occur in fixed sequence. A universal biochemical or antigenic marker for apoptosis is still lacking, although sensitive assays for DNA fragmentation may be applicable. DNA digestion at internucleosomal sites was recognized in 1980 as a characteristic feature of apoptosis, producing small double-stranded fragments of DNA that migrate in a ladder pattern (multiples of 185 bp) after electrophoresis in agarose gels.10 It is now clear that DNA degradation resulting in oligonucleosome-sized fragments does not occur in every cell death with classic apoptotic morphologies.11 However, less extensive digestion of DNA to sizes of 50 to 300 kb occurs in several examples of apoptosis without progressing to oligonucleosome-sized fragments and can be recognized by fractionation of genomic DNA on density gradients or pulse field gel electrophoresis.12 In situ end-labeling of DNA strand breaks has been shown to mark apoptotic cells in a distinctive pattern prior to morphological changes and may be sufficiently sensitive to detect the limited number of DNA strand breaks in these apoptotic deaths.13 Random DNA cleavages occur as a late event in necrosis, but these