localized death of tissue in a living organism.

Merriam-Webster Online Dictionary
necrosis (noun)
usu. localized death of living tissue
necrosis (Wikipedia)
Structural changes of cells undergoing necrosis and apoptosis

Necrosis (from the Greek νέκρωσις "death, the stage of dying, the act of killing" from νεκρός "dead") is a form of cell injury which results in the premature death of cells in living tissue by autolysis.

Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma which result in the unregulated digestion of cell components.

In contrast, apoptosis is a naturally occurring programmed and targeted cause of cellular death.

While apoptosis often provides beneficial effects to the organism, necrosis is almost always detrimental and can be fatal.

Cellular death due to necrosis does not follow the apoptotic signal transduction pathway, but rather various receptors are activated, and result in the loss of cell membrane integrity and an uncontrolled release of products of cell death into the extracellular space.

This initiates in the surrounding tissue an inflammatory response which attracts leukocytes and nearby phagocytes which eliminate the dead cells by phagocytosis. However, microbial damaging substances released by leukocytes would create collateral damage to surrounding tissues. This excess collateral damage inhibits the healing process. Thus, untreated necrosis results in a build-up of decomposing dead tissue and cell debris at or near the site of the cell death. A classic example is gangrene. For this reason, it is often necessary to remove necrotic tissue surgically, a procedure known as debridement.

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