Apoptosis is a form of programmed cell death triggered by various stimuli and regarded as an auto-destruct mechanism. This mechanism is well-conserved against mutation through natural selection. Important as a stop against cancers and essential in development.   Affecting apoptosis may be of use in treatment of some cancers 
Once triggered, apoptosis involves a co-ordinated and well-regulated dismantlement of the cellular apparatus, leaving cell remnants that are easily phagocytosed by neighbouring cells. The apoptotic cell vanishes without a trace, leaving no chaos in the aftermath of its demise.
In contrast, cell necrosis occurs when toxic stimuli, e.g. heat, mechanical damage and extremes of pH, irretrievably damage the cell, causing uncontrolled cell death, catastrophic cell lysis and inflammation. A high volume of apoptosis can overwhelm phagocytic capacity and can lead to secondary necrosis.
Apoptosis is an energy requiring process, necrosis is not. Leaf fall is an active process. Leaves tend to stay on a tree branch broken off in summer.
- Extrinsic pathway(s) through cell membrane receptors that can act directly through cytoplasmic caspase or through mitocondrial cytochrome C associated pathways. Both are mediated by a family of enzymes called caspases .
- Intrinsic pathway through mitochondrial stress signalling leading to DNA fragmentation
- DNA damage activating the P53 pathway
Some of the genes involved will be tumour suppressors.
- ↑ Kerr JF, Wyllie AH, Currie AR. Apoptosis: a basic biological phenomenon with wide-ranging implications in tissue kinetics. Br J Cancer. 1972 Aug;26(4):239-57. Review.
- ↑ Sjostrom J, Bergh J. How apoptosis is regulated, and what goes wrong in cancer. BMJ. 2001 Jun 23;322(7301):1538-9. Direct link: http://bmj.bmjjournals.com/cgi/content/full/322/7301/1538
- ↑ Renehan AG, Booth C, Potten CS. What is apoptosis, and why is it important? BMJ. 2001 Jun 23;322(7301):1536-8.
- ↑ DOI: 10.1002/ptr.2185
- ↑ http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/Apoptosis.html