Necrosis (in Greek Νεκρός = "dead") is the name given to unnatural death of cells and living tissue. It begins with cell swelling, chromatin digestion, and disruption of the plasma membrane and organelle membranes. Late necrosis is characterized by extensive DNA hydrolysis, vacuolation of the endoplasmic reticulum, organelle breakdown, and cell lysis. The release of intracellular content after plasma membrane rupture is the cause of inflammation in necrosis.
In contrast to apoptosis, cleanup of cell debris by phagocytes of the immune system is generally more difficult, as the disorderly death generally does not send cell signals which tell nearby phagocytes to engulf the dying cell. This lack of signaling makes it harder for the immune system to locate and recycle dead cells which have died through necrosis than if the cell had undergone apoptosis.
There are many causes of necrosis including prolonged exposure to injury
, and inflammation
. Severe damage to one essential system in the cell leads to secondary damage to other systems, a so-called "cascade of effects." Necrosis can arise from lack of proper care to a wound site. Necrosis is accompanied by the release of special enzymes
, that are stored by lysosomes
, which are capable of digesting cell components or the entire cell itself. The injuries received by the cell may compromise the lysosome membrane, or may initiate a disorganised chain reaction which causes the release in enzymes. Unlike apoptosis
, cells that die by necrosis may release harmful chemicals that damage other cells. In biopsy
, necrosis is halted by fixation
or freezing. Many species of viper
) produce venom
which causes severe necrosis in snake-bite victims. Additionally, some spiders (eg the brown recluse
) contain venom which may cause significant cutaneous injuries with tissue loss and necrosis.
There are seven distinctive morphologic patterns of necrosis:
are cited as causing necrosis in some areas. These claims are widely disputed. In the US at least, only the bites of spiders in the genus Loxosceles
or brown recluse have been proven to consistently cause necrosis. Many other spider species are claimed to cause necrosis but in most cases firm evidence is lacking, partially because the early bite is often painless and the spider species seldom identified and because a common reaction by doctors to a possible necrotic spider bite is to remove the flesh pre-emptively.
Several species of spiders possess toxins proven to cause necrosis:
Spiders suspected of, but not shown to cause necrosis:
The standard therapy of necrosis (wounds, bedsores, burns etc.) is surgical debridement of necrotic tissue. Another option for removal of necrotic tissue would be use of an enzymatic debriding.
In selected cases special maggot therapy has been utilized with good results.