Necrosis
Necrosis (in
Greek Νεκρός =
Dead) is the name given to unprogrammed or accidental
death of
cells and living
tissue. It is less orderly than
apoptosis, which are part of
programmed cell death. In contrast with apoptosis, cleanup of cell debris by
phagocytes of the
immune system is generally more difficult, as the disorderly death generally does not send "eat-me"
cell signals which tell nearby phagocytes to
engulf the dying cell. This lack of signalling 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. The release of intracellular content after cellular membrane damage is cause of inflammation in necrosis.
There are many causes of necrosis including
injury,
infection,
cancer,
infarction,
invenomation 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 is caused by special
enzymes that are released 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 set off an unorganized chain reaction which causes the release in enzymes. Unlike in apoptosis, cells that die by necrosis may release harmful chemicals that damage other cells. Biopsy material necrosis is halted by
fixation or freezing.
There are seven distinctive morphologic patterns of necrosis:
*
Coagulative necrosis – typically seen in
hypoxic environments (e.g.
myocardial infarction, infarct of the
spleen). Cell outlines remain after cell death and can be observed by light
microscopy.
*
Liquefactive necrosis – is associated with cellular destruction and
pus formation (e.g.
pneumonia)
*
Gummatous necrosis – is restricted to necrosis involving spirochaetal infections (e.g.
syphilis)
*
Haemorrhagic necrosis – is due to blockage of the venous drainage of an organ or tissue and occurs in
testicular torsion.
*
Caseous necrosis – is a mix of coagulative necrosis and liquefactive necrosis (e.g.
tuberculosis)
*
Fatty necrosis – results from the action of
lipases on fatty tissues (e.g.
acute pancreatitis,
breast tissue necrosis)
*
Fibrinoid necrosis – caused by
immune-mediated
vascular damage. It is marked by deposition of
fibrin-like
proteinaceous material in
arterial walls, which appears smudgy and
acidophilic on light microscopy.
Several histological features convey the gradual, irreversible transformation into necrosis from
sub-lethal damage. The following sequential structural changes can be observed under a light microscope:
* An
Eosinophilic Cytoplasm*
Pyknosis*
Karyorrhexis*
Karyolysis* Eventually
complete karyolysis is attained. The dead cell appears as an anucleate and eosinophilic mass.
Spider bites are cited as causing necrosis in some areas. These claims are widely disputed, but experts say the possibility is still there. Examples include:
*
Brown recluse spiders in the midwestern United States
*
White-tailed spiders in
Australia*
Peripheral vascular disease*
Aseptic bone necrosis*
Gangrene*
Ischemic cascade* Necrotoxins are
toxins that cause necrosis
*
Cystic medial necrosis
