Introduction

Irreversible cell injury and eventual cell death due to pathological processes are termed necrosis. It is an uncontrolled cell death that results in swelling of the cell organelles, plasma membrane rupture and eventual lysis of the cell, and spillage of intracellular contents into the surrounding tissue leading to tissue damage.[1] Unlike programmed cell death known as apoptosis which generates from intrinsic signals, necrosis occurs due to overwhelming noxious stimulus from outside the cell and is almost always associated with inflammatory responses due to the release of heat shock proteins, uric acid, ATP, DNA, and nuclear proteins, which cause inflammasome activation and secretion of proinflammatory cytokine interleukin-1 beta (IL1).[2] Typically, necrosis is not associated with caspase activation or normal development, but different types of regulated necrosis have been described, such as necroptosis, pyroptosis, and ferroptosis. like apoptosis, enzymatic pathways such as caspases, kinases, and the polyubiquitin system, have the main role in necroptosis and pyroptosis.[3][4][5] Necroptosis, especially, shares several key processes with both apoptosis and autophagy.[6][7] Synchronized regulated necrosis is the result of specific lipid peroxidation in ferroptosis.[5]

Causes

Cell injury can range from external injury to internal abnormalities. The most common causes of injurious stimulus include[1]:

1. Hypoxia: This can occur due to ischemia, shock, or respiratory failure.
2. Physical agents: These include external injuries such as trauma, extremes of temperature, radiation exposure, or electric shock 
3. Chemical agents: These include poisons, occupational exposure, drug toxicities, or recreational drugs. 
4. Biological agents: bacteria, viruses, or fungi 
5. Immunologic reactions: autoimmune responses

Anatomical Pathology

The cellular mechanism that leads to necrosis is the loss of cell membrane integrity as a result of exposure to a noxious stimulus; this allows extracellular ions to move inside the cell, followed by fluid leading to eventual swelling of the cell and its organelles. Another cellular mechanism is the disruption of the lysosomal membrane, which leads to the release of proteolytic enzymes into the cell, such as proteases, RNAase, DNAases, and phosphatases. These, when activated in the cytosol, leading to damage to DNA, RNA, and proteins.[8] These enzymes cause the digestion of the cellular components causing cell destruction. Both these mechanisms lead to disruption of the plasma membrane leading to the spilling of intracellular contents into the surrounding tissue.[1]

Histopathological features[9] - hematoxylin and eosin stain:

There are different patterns of necrosis based on the nature of the insult and the tissue:

1. Single-cell - single, noncontiguous cells in a tissue that are characterized by swelling of the cell and nucleus and pale cytoplasm
2. Focal - Contain contiguous cells
3. Multifocal
4. Diffuse - Contain contiguous cells
5. Centrilobular
6. Zonal

The main features of necrosis are:

• Swelling of the cell
• Swelling of the nucleus
• Karyolysis (nuclear dissolution)
• Karyorrhexis (nuclear fragmentation)
• Nuclear pyknosis
• Pale eosinophilic cytoplasm
• Cytoplasmic vacuoles may be present in areas of necrosis
• Adjacent cellular debris and inflammatory cells may be present if there is leakage of the cell membrane

Clinical Pathology

The classic “dead red” neuron has been described as the most common morphologic manifestation of neuronal death in a necrotic cell in the central nervous system.[10] But it has been proven that the same cell can occur as a result of either apoptosis or necrosis.[11] Since there are considerable difficulties in differentiating apoptosis from necrosis in the kidney and brain, it is recommended that either term be used if there is confidence (based on experience or special studies) in the type of cell death present.[9]

Morphology

When cells die of necrosis, they exhibit different microscopic and macroscopic appearances.[5] These different patterns of necrosis appear below:

1) Coagulative necrosis: Ischemia in most organs except the brain can lead to coagulative necrosis. In this type of necrosis, the cell architecture remains preserved. Under the microscope, the cells appear anucleate, eosinophilic, with preserved structure. Eventually, the dead cells are cleared by phagocytosis and leukocytes. [12]

2) Liquefactive necrosis: This morphology is most commonly observed in the central nervous system.[13] The dying cells are digested by hydrolytic enzymes and hence lose their structural integrity and turn into a viscous mass. This morphology also occurs in most bacterial infections, and the accumulation of such necrotic material is called pus. 

3) Caseous necrosis: The term caseous means "cheese-like," which refers to the whitish appearance of the necrotic area. This necrosis takes place in tuberculous infection, and the necrotic area is referred to as a granuloma. 

4) Gangrenous necrosis: This is not a morphological pattern but rather a clinical term for ischemic necrosis of the limbs. It has two types i) dry (ischemia leading to coagulative necrosis), and ii) wet (ischemia with superimposed bacterial infection leading to liquefactive necrosis).[14]

5) Fat necrosis: This type of necrosis occurs in acute pancreatitis. The release of pancreatic enzymes leads to liquefaction of the fat cells in the peritoneal cavity. These liquified fat cells then combine with calcium and are identified as chalky white areas. This process is referred to as saponification. Under a microscope, this is visible as basophilic calcium deposits on the outlines of the necrotic fat cells. Fat necrosis also occurs in breast tissue due to fat saponification.[15]

6) Fibrinoid necrosis: This type of necrosis occurs in blood vessels due to the deposition of immune complexes in blood vessel walls leading to leakage of fibrin. This observed staining appears as a bright pink amorphous material.[16]

Mechanisms

Morphologically, necrotic cells characteristically demonstrate by swelling of organelles, such as the endoplasmic reticulum and mitochondria, the rupture of the plasma membrane, and the lysis of the cell.[17] These changes cause cells to be more eosinophilic, glassy, and vacuolated. Loss of cell membrane integrity and compromise of organelle membranes adds to this. ATP depletion or decreased synthesis is the first biochemical change observed in injury. ATP is produced by oxidative phosphorylation in mitochondria in the presence of oxygen. In necrosis associated with hypoxia or chemical injury, there is a lack of supply of oxygen to cells, leading to decreased ATP production. This lack of ATP results in the failure of the energy-dependent sodium pump in the plasma membrane, causing an influx of calcium and water, leading to cell swelling and detachment of ribosomes from the endoplasmic reticulum. Increased cytosolic calcium and oxidative stress lead to mitochondrial damage. Cytosolic calcium can also lead to the activation of several cytosolic enzymes, including phospholipases and proteases, which lead to the breakdown of both membranes(including lysosomal membranes) and proteins. 

Necrotic death is almost always associated with an inflammatory response. Necrotic cells release factors like high mobility group box 1 protein (HMGB1), and hepatoma-derived growth factor (HDGF).[18][19] These factors are sensed by a nod-like receptor protein 3 (NLRP3), which is a core protein of the inflammasome.[20] This results in inflammasome activation and causes the release of the pro-inflammatory cytokine IL1β. NLRP3 inflammasome activation is triggered mainly through ATP produced by mitochondria released from damaged cells.[21] Necrosis does not typically correlate with activation of caspases, and it appears that it causes cell demise in response to damage or pathology, but not during normal development. Despite this, it turns out that a programmed form of necrotic death (termed necroptosis) is very common in vivo, mainly in diverse forms of neurodegeneration and death inflicted by ischemia or infection. Unlike unordered necrosis, necroptosis is a more physiological and programmed type of necroptotic death and shares several key processes with apoptosis. It occurs due to the activation of the kinase domain of the receptor-interacting protein 1 (RIP1) and the assembly of the RIP1/RIP3-containing signaling complex. It is triggered by tumor necrosis factor (TNF) family members, needs caspase 8 inhibition, and assembly of necrosome (RIPK1-RIPK3 complex IIb).[9][19]

Clinicopathologic Correlations

Necrosis is a hallmark of various diseases and the different patterns of necrosis observed in different organs help to identify the mechanism of cell injury and possible noxious stimuli causing them. Acute tubular necrosis observed in the kidney is one such example. Various drugs have been linked to kidney injury including phenylbutazone, ibuprofen, and mefenamic acid.[22] Similarly, alcohol consumption has been studied to lead to hepatic inflammation, necrosis, and steatosis. inflammation has been proposed to be a progression event in the development of alcoholic steatohepatitis.[23][24] Ischemia of the heart leading to myocardial injury, ischemia of the brain leading to stroke, and ischemia of the limbs leading to gangrene are all clinical examples of necrosis. Necrosis hence helps to describe the pathological mechanism of most commonly encountered diseases.[25]

Clinical Significance

Identifying the various types of necrosis and the underlying cause of necrosis can help to target treatment for multiple diseases. Most of the time, identifying the cause of necrosis and treating it is more important than removing the dead tissue. In the case of myocardial infarction, we are aware that necrosis occurs due to hypoxia due to the occlusion of coronary vessels. Therefore treatment is targeted at opening the coronary vessels either by thrombolysis or PCI to restore blood supply.[25]

An inflammatory response accompanies necrosis, so studying the effects of anti-inflammatory drugs on necrosis suppression would be valuable.[26]