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menstruation (Kokawa et al. 1996), death of epidermal cells during their migration from
the basal germinal layer to the surface in squamous epithelia (Weil et al. 1999), the death
of neutrophils during an acute inflammation (Sendo et al. 1996) and deletion of autoreactive
T cells in developing thyrnus (King and Ashwell 1994). There are two specific
pathways involved in the initiation and execution of apoptosis namely; the death receptor
pathway and the mitochondrial pathway.
III.a.1. Death receptor pathway: The death receptor pathway, also known as
extracellular or extrinsic pathway, is one of the best characterized pathways involved in
the induction and the execution of apoptosis. Cellular decision to undergo apoptosis can
be initiated by the cessation of extracellular signals that negatively regulate apoptosis.
These signals are also known as survivins (Ruoslahti and Reed 1994). Apoptosis may
also be induced by the binding of extracellular death proteins to specif,rc membrane
receptors termed death receptors. Death receptors belong to the tumor necrosis factor
(T¡W') receptor gene super-family and are defined by the existence of characteristic
cysteine rich extracellular domains (Porter et al. 1997). Another characteristic of death
receptors is the existence of homologous cytoplasmic sequences termed death domains
(Cryns and Yuan 1998; Enari et al. i998). The death domains are considered to be a
crucial elements responsible for the pro-apoptotic function of death receptors.
IlI,a.Z. Mitochondrial Pathway: In addition to the extrinsic apoptotic pathway,
apoptosis can be initiated in the cells by a multitude of stressful stimuli and metabolic
disturbances. These factors will result in the activation of apoptosis through the intrinsic
mitochondrial pathway, Mitochondrial pathway is found to play an important role in
apoptosis of cardiac myocytes, due to their high mitochondrial content (Roucou et al.
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