What is apoptosis and why is it beneficial for cells to clean themselves up?

Apoptosis is the genetically programmed mechanism by which a cell cleans itself up in a controlled way. The cell shrinks, the nucleus breaks apart into fragments, and the whole thing is neatly packaged so that neighbouring cells are not harmed. This is very different from what happens when a cell 'swells and bursts' due to oxygen deprivation or another acute overload: the latter causes far more damage to the surrounding tissue.

This controlled clean-up is indispensable for a healthy body. During embryonic development, apoptosis literally helps shape organs by removing surplus cells. After birth, it keeps your tissues in balance: cells that are damaged or simply no longer needed are removed before they can cause problems. Apoptosis also plays a role in infections: cells that have been taken over by a virus or bacterium are eliminated through this mechanism before the infection can spread further.

Balance is everything here. Too little apoptosis and damaged cells can keep growing, contributing to cancer as tumour cells build up resistance to cell death. Too much apoptosis is also dangerous, especially in the brain: adult brain cells can barely renew themselves, so if too many neurons die off, this plays a major role in conditions such as stroke or Alzheimer's disease.

Apoptosis is not the only way in which cells can die in a programmed manner. Other pathways also exist, which cause more inflammation and play a role in different diseases. Which pathway a cell takes has consequences for how the surrounding tissue responds. In addition, the cell can sometimes prevent apoptosis by breaking down its own damaged energy factories (mitochondria) in time through a separate internal recycling process. When that goes wrong, the risk of cell death increases.