Apoptosis, or programmed cell death, refers to both the initiation and execution of the events whereby a cell commits suicide. This process is important in development and its deregulation is found in many diseases (1–6), including cancer (6–10). Apoptosis is distinct from other ways in which cells may lose viability (e.g., necrosis, senescence). Apoptosis is an active process triggered by a variety of stimuli, which induces closely comparable structural changes (Fig. 1). These morphological changes are especially evident in the nucleus where the chromatin condenses to compact and apparently simple, globular, crescent-shaped figures (11). Other typical features include cytoplasmic shrinkage, zeiosis, and the formation of apoptotic bodies within the nucleus. The earliest definitive changes in apoptosis that have been detected by electron microscopy are compaction of the nuclear chromatin into sharply circumscribed, uniformly-dense masses that abut on the nuclear envelope and condensation of the cytoplasm. Continuation of condensation is accompanied by convolution of the nuclear and cellular outlines, and nucleus often break up at this stage to produce discrete fragments. The surface protuberances then separate with sealing of the plasma membrane, converting the cell into a number of membrane-bounded apoptotic bodies of varying size in which the closely packed organelles appear intact; some of these bodies lack a nuclear component, whereas others contain one or more nuclear fragments in which compacted chromatin is distributed either in peripheral crescents or throughout cross-sectional area.

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