Cell death is irreversible loss of the integrated functions that sustain a biological cell. It can follow catastrophic injury or regulated processes such as apoptosis. Apoptosis typically packages cellular material for removal by other cells; uncontrolled membrane rupture can release intracellular contents. A transient fall in ATP or a single altered marker does not by itself establish irreversible cell death. In development, selective cell death helps shape tissues, illustrating that death of individual cells can serve the organism. [ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 17, Programmed Cell Death (Apoptosis)
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