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Programmed cell death – the body's normal method of disposing of damaged, unwanted, or unneeded cells. Apoptosis is important for sculpting tissue and organ structure during development of the embryo, but may occur at any time even in adult cells when a tissue needs to be remodeled.

Signals to trigger apoptosis may come from within the cell or from outside, by stimulating suicide receptors in the cell's external membrane. Internal signals producing apoptosis depend on interactions of several proteins and may serve to protect the organism from cancer by killing cells that have undergone pre-cancerous changes.

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