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Necrosis (from the Greek νεκρός, "dead", νέκρωσις, "death, the stage of dying, the act of killing") is a form of cell injury that results in the premature death of cells in living tissue. Necrosis is caused by factors external to the cell or tissue, such as infection, toxins, or trauma that result 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.

Cells that die due to necrosis do not follow the pathway but rather various receptors are activated that result in the loss of cell membrane integrity and an uncontrolled release of products of cell death into the intracellular space. This initiates an inflammatory response in the surrounding tissue, nearby phagocytes are prevented from locating and engulfing the dead cells. The result is a build-up of 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 process known as debridement.

Classification
Necrosis is the sum of cell changes after localized cellular death through a process known as autolysis. Structural signs that indicate irreversible cell injury and the progression of necrosis include: dense clumping and progressive disruption of genetic material: and disruption to membranes of cells and organelles. There are five distinctive morphological patterns of necrosis:

1.	Coagulative necrosis is characterized by the formation of a gelatinous (gel-like) substance in dead tissues in which the architecture of the tissue is maintained and can be observed by light microscopy. Coagulation occurs as a result of protein denaturation, causing the albumin in protein to form a firm and opaque state. This pattern of necrosis is typically seen in hypoxic (low-oxygen) environments, such as infarction. Coagulative necrosis occurs primarily in tissues such the kidney, heart and adrenal glands. Severe ischemia most commonly causes necrosis of this form

2.	Liquefactive necrosis (or colliquative necrosis) in contrast to coagulative necrosis, liquefactive necrosis is characterized by the digestion of dead cells to form a viscous liquid mass. This is typical of bacterial or sometimes, fungal infections because of their ability to stimulate an inflammatory response. The necrotic liquid mass is frequently creamy yellow due to the presence of dead leukocytes and is commonly known as pus. Hypoxic infarcts in the brain presents as this type of necrosis, because, the brain contains little connective tissue and high amounts of digestive enzymes and lipids and therefore can be readily digested by their own enzymes.

3.	Caseous necrosis can be considered a combination of coagulative and liquefactive necroses. Typically caused by mycobacteria (e.g. tuberculosis), fungi and some foreign substances. The necrotic tissue appears as white and friable, like clumped cheese. Dead cells disintegrate but are not completely digested, leaving granular particles. Microscopic examination will show amorphous granular debris enclosed within a distinctive inflammatory border. Granuloma has this characteristic.

4.	Fat necrosis is specialized necrosis of fat tissue resulting from the action of activated lipases on fatty tissues such as the pancreas. In the pancreas it leads to acute pancreatitis and. In acute pancreatitis the enzymes leak out into the peritoneal cavity and liquefy the membrane by splitting the triglyceride esters into fatty acids through fat saponification Calcium, magnesium or sodium may bind to these lesions to produce a chalky-white substance .The calcium deposits are microscopically distinctive and may be large enough to be visible on radiographic examinations. To the naked eye, calcium deposits appear as gritty white flecks.

5.	Fibrinoid necrosis is a special form of necrosis usually caused by immune-mediated vascular damage. It is marked by complexes of antigen and antibodies, sometimes referred to as “immune complexes” deposited within arterial walls together with fibrin.

Other clinical classifications of necrosis
1.	There are also very specific forms of necrosis such as gangrene (term used in clinical practices for limbs which have suffered severe hypoxia)s, gummatous necrosis (due to spirochaetal infections) and haemorrhagic necrosis (due to the blockage of venous drainage of an organ or tissue). 2.	In the United States, only spider bites from the brown recluse spider (genus Loxosceles) have been proven to cause necrosis. Other spiders of the same genus such as the Chilean recluse in South America, have similarly been shown to cause necrosis in other countries. While both the yellow sac spiders and Hobo spider are often claimed to possess necrotic venom, these claims have been challenged.

Causes
Necrosis may occur due to external or internal factors. External factors may involve mechanical trauma, physical damage to the body (that causes cellular breakdown), any damage to blood vessels (which may disrupt the blood supply to that area); and ischemia. Thermal effects (extremely high or low temperature) can result in necrosis due to the disruption of cells. In frostbite, crystals form, increasing the pressure of remaining tissue and fluid causing the cells to burst. Under extreme conditions tissues and cells die through an unregulated process of destruction of membranes and cytosol. Internal factors causing necrosis include trophoneurotic disorders; injury and paralysis of nerves cells. Pancreatic enzymes (lipases) are the major cause of fat necrosis. Necrosis can be activated by; bacterial toxins and components of the immune system; such as the complement system, activated natural killers cells and peritoneal macrophages. Pathogen-induced necrosis programs in cells with immunological barriers (intestine mucosa) may alleviate invasion of pathogens through surfaces affected by inflammation. Toxins and pathogens may cause necrosis; toxins such as snake venoms may inhibit enzymes and cause cell death.

Pathological conditions are characterized by inadequate secretion of cytokines. Nitrogen oxide (NO) and reactive oxygen species (ROS) are also accompanied by intense necrotic death of cells. A classic example of necrotic condition is ischemia that leads to a drastic depletion of oxygen, glucose and other trophic factors and evokes massive necrotic death of endothelial cells and non-proliferating cells of surrounding tissues (neurons, cardiomyocytes, renal cells, etc)"Proskuryakov"/>. Recent cytological date indicates that necrotic death occurs not only during pathological events but it is also a component of some physiological process.

Activation induced death of primary T-lymphocytes, and important constituents of the immune response, are caspase independent and necrotic by morphology; hence current researchers have demonstrated that the occurrence of necrotic cell death can not only occur during pathological processes but also during normal processes such as tissue renewal, embryogenesis and immune response.

Pathogenisis
Until recently, necrosis was thought to be a process that was unregulated in its stages of development. There are two broad pathways in which necrosis may occur in an organism.

] The first of these two pathways initially involves oncosis, where swelling of the cells occur. The cell then proceeds to blebbing, and this is followed by pkynosis, in which nuclear shrinkage transpires. In the final step of this pathway the nucleus is dissolved into the cytoplasm, which is referred to as karyorrhexis.

The second pathway is a secondary form of necrosis that is shown to occur after apoptosis and budding. Cellular changes of necrosis occur in this secondary form of apoptosis, where the nucleus breaks into fragments, which is known as karyolysis.

Cellular changes
The nucleus changes in necrosis and characteristics of this change are determined by the way in which the DNA is broken down, as shown in figure 3. There are three different ways in which the DNA can be broken down, which are; Karyolysis, Pkynosis and Karyorrhexis.

Karyolysis is a process where the chromatin of the nucleus fades due to the loss of the DNA by degradation. Pkynosis is where the nucleus shrinks and the chromatin in the nucleus condenses. Karyorrhexis follows on from the process of Pkynosis, and involves the shrunken nucleus proceeding to fragmentation until the nucleus completely disappears. Plasma alterations are also seen in necrosis. Plasma membranes appear discontinuous when viewed with an electron microscope. This discontinuing membrane is caused by cell blebbing and the loss of microvilli.

Treatment
There are many causes of necrosis, and as such treatment is based upon on how the necrosis came about. Treatment of necrosis typically involves two distinct processes. Usually, the underlying cause of the necrosis must be treated before the dead tissue itself can be dealt with.

•	In the case of Ischemia, which includes myocardial infarction, the restriction of blood supply to tissues causes hypoxia and the creation of [|reactive oxidant species] (ROS) that react with, and damage proteins and membranes. Antioxidant treatments can be applied to scavenge the ROS.

•	Wounds caused by physical agents, including direct physical trauma and injury, can be treated with antibiotics and anti-inflammatory drugs to prevent bacterial infection and inflammation. Keeping the wound clean from infection also prevents necrosis.

•	Chemical and toxic agents (e.g. pharmaceutical drugs, acids, bases) react with the skin leading to skin loss and eventually necrosis. Treatment involves identification and discontinuation of the harmful agent, followed by treatment of the wound, including prevention of infection and possibly the use of immunosuppressive therapies such as anti-inflammatory drugs or immunosuppressant’s .In the example of a snake bite, the use of anti-venom halts the spread of toxins whilst receiving antibiotics to impede infection.

Even after the initial cause of the necrosis has been halted, the necrotic tissue will remain in the body. The body's immune response to apoptosis, which involves the automatic breaking down and recycling of cellular material, is not triggered by necrotic cell death due to the apoptotic pathway being disabled. The standard therapy of necrosis is surgical removal (debridement) or non-surgical debridement. Depending on the severity of the necrosis, this may range from removal of small patches of skin, to complete amputation of affected limbs or organs. Chemical removal of necrotic tissue is another option in which enzymatic debriding agent’s, categorised as proteolytic, fibrinolytic or collangenases, are used to target the various components of dead tissue. In select cases, special maggot therapy using Lucilia sericata larvae has been employed to remove necrotic tissue and infection.

In plants
If calcium is deficient, pectin cannot be synthesized, and therefore the cell walls cannot be bonded and thus an impediment of the meristems. This will lead to necrosis of stem and root tips and leaf edges.