Although its importance as a counterbalance to mitosis has been recognized for some time and the broad features of the apoptotic process were known for nearly 20 years (Raff, 1992), it is only over the last decade that apoptosis has attracted extensive research of an intensity previously devoted to cell proliferation. It is now apparent that most, if not all, human cells express the proteins necessary to undergo suicide by apoptosis and that this may be the default pathway in the absence of survival signals from surrounding cells (Raff, 1992; Thompson, 1995). Apoptosis can occur very rapidly, with the entire process from initiation to phagocytosis being complete within an hour (McCarthy, 2002). It is also a stochastic process, i.e. not all cells in a given tissue die at the same time so histological assessment of the magnitude/rate of apoptosis can be difficult (McCarthy, 2002). The apoptotic process can be considered from the perspective of the molecules involved in four distinct phases: (i) survival signals; (ii) death signals; (iii) regulatory genes and proteins; and (iv) effector molecules. An excellent review of the molecular biology of apoptosis is given by Jacobson and McCarthy (2002) and the processes are summarized in Fig. 6.1.
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