ABSTRACT
Cellular senescence and apoptosis (or programmed cell death) are evolutionarily conserved processes that occur throughout the life span of complex organisms such asmammals (Chapter 5). Apoptosis is essential for embryonic development, whereas it is likely (although not certain) that cellular senescence does not contribute to embryogenesis. In addition, apoptosis is important for tissue homeostasis in adult organisms-for example, during the development of immunity to certain infections (Chapter 14). Both apoptosis and cellular senescence are crucial for preventing the development of cancer (malignant transformation or tumorigenesis). Thus, both processes are beneficial. On the other hand, cellular senescence and apoptosis are thought to contribute to aging and/or certain age-related pathologies. At first glance, it may seem paradoxical that the same processes can be both beneficial (by preventing cancer) and detrimental (by promoting aging). However, the idea that genes or processes can equally promote early-life fitness and drive aging phenotypes is consistent with the evolutionary theory of antagonistic pleiotropy (discussed in Chapter 5).
