4. Patterns of coagulation type necrosis. Organ examples

Last updated on November 19, 2018 at 17:16

Necrosis (on the macroscopical level) can be divided into four types, coagulative type, liquefactive type, caseation type and adiponecrosis.

In the coagulative type of necrosis, the rough tissue structure is upheld and can be recognized. Four subtypes of coagulative necrosis exist, infarct, decubitus, pseudomembrane formation and gangrene.

Infarct
Anemic infarction of the kidney. Note the well-defined border and the triangle shape.

An infarct is an area of ischemic necrosis in an organ that is caused by occlusion (blockage) of the supplying artery or draining vein. There are two types of infarct, anemic and haemorrhagic infarct.

The name anemic means “without blood”, showing how this type of infarct usually is not accompanied by bleeding. Four organs are at risk for anemic infarct, the heart, kidney, spleen and liver. This is because these organs have a relatively “simple” circulation system: They have an artery that goes in, a capillary network inside the parenchyme, and a vein that drains it. They are also solid organs, compared to the spongy lung and intestines for example.

The infarction happens when there is occlusion of the supplying arteries (usually) or draining veins (less frequently), due to a thrombus formation in the vessel, an occlusive atherosclerotic plaque, an embolus or due to vasculitis (rare).

Usually only a small artery inside the organ is blocked, meaning that a part of the organ dies and not the whole organ. Because these organs are solid, there is a limit to how easily blood can “flow” inside the organ. This makes so that there is little overlap between the blood supply of different arteries, meaning that a certain slice of the parenchyme is often supplied by only one artery. If this one artery is blocked, the parenchyme supplied by the artery will receive no blood and will therefore die. The infarcted area is often greyish, dry and fragile, with a well-defined border, meaning there is a clear border between the dead and healthy tissue. The infarcted area is also commonly found near the surface of the organ, and often has a triangle or dome-like shape. The second and third macropreparations are good examples for what it looks like.

Haemorrhagic infarct of the lung. The black part is blood.

The haemorrhagic infarct is characteristic for the lung and gastrointestinal tract and can occur either when the artery or the vein is blocked. The word haemorrhagic means “bleeding”, which refers to how this type of infarct is accompanied by bleeding. Both the lung and the GI tract have dual circulations, as the lung has both the pulmonary and bronchial circulations and the GI tract has an anastomosis between the two mesenteric arteries. Haemorrhagic infarct in the lung is usually due to a combination of different factors, like a pulmonary embolism, congestive heart failure and insufficient perfusion from the bronchial circulation.

Let’s say that a thrombus forms in the right atrium due to atrial fibrillation, and this thrombus travels to the pulmonary circulation and gets stuck in a small artery there. Because the pulmonary circulation doesn’t provide nutritional blood for the lungs, this usually isn’t enough to cause an infarction, but if the bronchial circulation for some reason is inadequate, or there is congestion in the pulmonary circulation in case of a left-sided heart failure, there may be an infarction. The “bleeding” blood will then come from the bronchial circulation, which is not blocked.

Haemorrhagic infarct of the intestines. You can see the bleeding.

Hemorrhagic infarction of the GI tract has a 90% mortality. Reasons for infarct can be occlusion of mesenteric artery by thrombus, embolus or atherosclerotic plaque, occlusion of portal vein or mechanical causes like incarcerated hernia, volvulus, invagination or strangulation of the intestine. It is so fatal because infarction causes ileus (no peristaltic activity) and perforation of the intestine, leaking the contents into the peritoneum, which will eventually cause sepsis.

 

Decubitus

In the cases where patients lie on their beds over extended periods of time, several parts of their bodies will be compressed by the weight of the rest of the body. These parts, like the sacrococcygeal (most frequent), calcanei, scapulae and occiput regions have their blood supplies compressed, which will eventually result in a type of ischemic cell death called decubitus.

Gangrene

When we talk about ischemic cell death in the extremities we call it gangrene. Dry gangrene occurs when the extremities (mainly the toes, feet and legs) have inadequate circulation due to atherosclerosis, hyperlipidemia or diabetes mellitus. The tissue loses its normal color, becoming darker and darker until falling off completely if not treated. Wet gangrene occurs when there is an infection on top of the dry gangrene, which causes the tissue to rot. The infection is usually by saprophytic pathogens which are bacteria and fungi that thrive in dead tissue. Wet gangrene can also occur in the lung and the intestine.

Pseudomembrane formation

Pseudomembrane formation usually occurs in the colon as part of a Gram-positive bacterium called clostridium difficile infection in antibiotic-treated patients. The bacterium produces two toxins, enterotoxin A and enterotoxin B, which cause necrosis of the tissues. This eventually causes formation of a kinda-membrane called pseudomembrane, which consists of leukocytes, fibrin and necrotic cell debris. The colonic crypts are distended by pus. The clinical features are pain, fever and diarrhea. Pseudomembrane formation can also be seen in diphteria.

C. difficile infection. The yellow parts are the pseudomembranes, the red parts are ulcerations.

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3. Necrosis. Ultrastructural, light microscopical and gross changes

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5. Patterns of liquefactive type necrosis. Organ examples

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