Two distinct modes of cell death, apoptosis and necrosis, can be recognized based on differences in the morphological, biochemical, and molecular changes of the dying cell. The assays of cell viability presented in this chapter are discussed in light of their applicability to differentiate between these two mechanisms.
The terms “apoptosis,” “active cell death,” “cell suicide,” and “shrinkage necrosis” are being used, often interchangeably, to define a particular mode of cell death characterized by a specific pattern of changes in nucleus and cytoplasm. Because this mode of cell death plays a role during the programmed cell death, as originally described in embryology, the term “programmed cell death” is also being used synonymously (albeit incorrectly, in the context of denoting the mode of cell death) with apoptosis.
The role of apoptosis in embryology, endocrinology, and immunology is the subject of several reviews.The wide interest in apoptosis in oncology, so apparent in recent years, stems from the observations that this mode of cell death is triggered by a variety of antitumor drugs, radiation, or hyperthermia and that the intrinsic propensity of tumor cells to respond by apoptosis is modulated by expression of several oncogenes such as Bcl-2, c-myc, or tumor suppressor gene p53 and may be prognostic of treatment. Extensive research is underway in many laboratories to understand the mechanism of apoptosis. Knowledge of the molecular events of this process may be the basis for new antitumor strategies. Apoptosis affecting CD4+ lymphocytes of HIV-infected patients also appears to play a pivotal role in pathogenesis of AIDS. The most common feature of apoptosis is active participation of the cell in its self-annihilation. The cell mobilizes a cascade of events that lead to its disintegration and the formation of the “apoptotic bodies” which are subsequently engulfed by the neighboring cells without invoking inflammation. Increased cytoplasmic Ca2+ concentration, cell dehydration, increased lipid peroxidation, chromatin condensation originating at the nuclear periphery, activation of endonuclease which has preference to DNA at the internucleosomal (linker) sections, proteolysis, fragmentation of the nucleus, and fragmentation of the cell are the most characteristic events of apoptosis. On the other hand, even during advanced stages of apoptosis, the structural integrity and the transport function of the plasma membrane are preserved. Also preserved and functionally active are the mitochondria and lysosomes. Thus, regardless of cell type, or the nature of event which triggers apoptosis, this mode of cell death has many features in common. Some of these features can be analyzed by image or flow cytometry, and several methods have been described to identify apoptotic cells.
Mitotic death, also termed delayed reproductive death, shows some features of apoptosis and thus may represent delayed apoptosis; it occurs as a result of cell exposure to relatively low doses of drugs or radiation, which induce irreparable damage, but allow cells to complete at least one round of division.
Necrosis is an alternative to the apoptotic mechanism of cell death. Most often it is induced by an overdose of cytotoxic agents and is a cell response to a gross injury. However, certain cell types do respond even to pharmacological concentrations of some drugs or moderate doses of physical agents by necrosis rather than apoptosis and the reason for the difference in response is not entirely clear. While apoptosis requires active participation of the involved cell, often even in terms of initiation of the de novo protein synthesis, necrosis is a passive and degenerative process. In vivo, necrosis triggers the inflammatory response in the tissue, due to a release of cytoplasmic constituents to intercellular space, often resulting in scar formation. In contrast, remains of apoptotic cells are phagocytized not only by the “professional” macrophages, but also by other neighboring cells, without evoking any inflammatory reaction. The early event of necrosis is swelling of cell mitochondria, followed by rupture of the plasma membrane, and release of the cytoplasmic content.