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In all organisms, cells die for a variety
of reasons, both intentional and unintentional. For example,
during the early stages of development in mammals, an intricate
program of cell proliferation and death is required to create
organs of normal size and function. This also applies to the
generation of an immune system that only recognizes foreign antigens
and not those that are "self". Unintentional cellular
insults may also trigger cell death such as those caused by ultraviolet
light or chemical agents. This ordered destruction of a cell
is referred to as programmed cell death or apoptosis and it is
distinguishable from death by necrosis which is a considered
a random event. The morphological features of apoptosis consist
of chromatin condensation, cell shrinkage and membrane blebbing,
which can be clearly observed by light microscopy. The biochemical
features include DNA fragmentation, protein cleavage at specific
locations, increased mitochondrial membrane permeability, and
the appearance of phosphatidylserine on the cell membrane surface. Apoptosis consists of a cascade of events leading to the ordered dismantling of critical cell survival components and pathways. This can prevent undesirable effects to the organism such as inflammation. There are two main pathways: an extrinsic pathway, which relays apoptotic messages via receptors and an intrinsic (or mitochondrial) pathway, which transmits death signals received intracellularly. These pathways are depicted in the figure below. In addition to providing an overview of apoptosis, this site provides links to a variety of resources to aid investigators that are interested in measuring apoptosis in their own model systems.  |
