Necrosis is a type of cell death. When a normal cell undergoes irreversible cell injury, the end result is death of the cell by Necrosis. When acell is necrosed, it cannot return back to its original form or health.
The article covers the following contents:
- Definition
- causes
- Morphological changes
- More about Necrosis
- patterns/types of tissue necrosis
Definition/What is Necrosis?
Necrosis is a diseased process, in which the shape, structure and form of a normal cell gets changed and ultimately the cell dies. It is an “accidental” and unregulated form of cell death.
It results due to severe, irreversible injury to the cell.
Causes:
- Loss of oxygen supply (Ischemia)
- Exposure to bacterial toxins
- Burns
- In case of leakage of proteases(enzymes that breaks down proteins) out of the cells and damage surrounding tissue.
The following general changes can be Noticed in a necrotic cell:
- Denaturation or breakdown of proteins that are present inside the cell.
- Leakage of contents of the cell because the membrane enveloping the cell is damaged.
- Local inflammation
- Enzymatic damage of lethally injured cell.
Morphological changes in a necrotic cell:
Cytoplasmic changes:
- When viewed under a microscope, necrotic cells show dark pink or reddish pink colour upon H and E staining. This is due to loss of cytoplasmic RNA and collection of broken down proteins.
- Necrotic cells have a glassy appearance due to loss of glycogen particles.
- As the enzymes digest the cell’s organelles, the cell becomes vacuolated.
Nuclear changes:
Necrotic cells exhibit or show one of the three patterns of nuclear changes:
- Karyolysis: when the cell is viewed under a microscope, the purple colour(basophilic) of the nucleus gradually fades away.
- Pyknosis: Shrinkage of nucleus and deeply basophil(dark bluish- purple under microscope)
- Karyorrhexis: Here, the pyknotic nucleus undergoes fragmentation and breaks into many small parts.
Fate of Necrotic cells:
- As the cell membrane is severely damaged, lysosomal enzymes enter the cytoplasmic and digest the cell.
- Leakage of cellular contents through damaged plasma membrane into the space outside the cells also takes place which stimulate inflammation.
- There are particular substances released from injured cells like ATP, uric acid etc. These molecules are recognised by macrophages(scavenging cells) which triggers phagocytosis (process of eating or engulfing up) of scattered remains of the cell (debris).
More about Necrosis:
The leakage of intracellular proteins through damaged plasma membrane in a necrotic cell ultimately enters the circulation and is the basis for blood tests that detect tissue specific cellular injury.
For example ; if Cardiac muscle cells are undergoing necrosis, there will be increased serum levels of Cardiac specific variant of the contractile protein Troponin.
Similarly, if bile duct is undergoing necrosis, there will be increased serum levels of specific isoform of the enzyme Alakaline phosphatase.
In liver necrosis, increased serum levels of transaminases will be noticed.
In this way, these substances serve as biomarkers that are used clinically to assess and quantify tissue damage.
Patterns/Types of tissue necrosis:
- Coagulative type
- liquefactive type
- Caseous type
- Fat necrosis
- Fibrinogen necrosis
- Gangrene (Gangrenous Necrosis)
Coagulative type:
It is a form of necrosis in which basic structure of dead tissue is preserved for some days. It is often seen in infarcts (areas of necrosis caused by decreased oxygen supply) in all solid organs except brain.
Causes: Decreased oxygen supply due to obstruction in a blood vessel.
Organs affected: All organs except for brain. More frequent in organs like heart, kidney, spleen and limb ( dry gangrene)
Appearance: Involved region appears dry, pale, yellow and firm. It is wedge shaped in organs like kidney and spleen.
Liquefactive type:
It is a type of necrosis where dead tissue rapidly undergoes softening and transforms into a liquid thick mass.
Causes: – Ischemic injury to CNS
Infections by bacteria or fungus
Mechanism: Liquefaction is due to digestive action of hydrolytic enzymes released from dead cells and WBC’s.
Organs affected are:
- Brain
- Abscess everywhere: The necrosis of tissue produces liquid thick material which is frequently creamy, yellow and is called pus or purulent exudate. The localised collection of pus is termed abscess. Pus consists of liquefied necrotic cell debris, dead WBC and macrophages.
- It is also seen in wet gangrene and pancreatic necrosis.
Caseous type:
It shows combined features of both coagulative and liquefactive necrosis.
Causes: Often seen in tuberculosis.
Organs affected: TB may involve any organ, most common in lungs and lymph nodes.
Appearance: Yellow-white, soft, granular and resembles drg, clumpy cheese, Hence the name Caseous ( cheese like) necrosis.
Caseous necrosis may undergo dystrophic calcification ( deposition of calcium salts in dying/injured or dead tissues)
Fat type:
Refers to focal areas of fat destruction, which affects adipose tissue (fat storing). Occurs at locations rich in fat.
Types:
- Enzymatic fat necrosis: Occurs in adipose tissue in acutely inflamed pancreas (acute pancreatitis). In pancreatic, the enzymes leak into the substances of pancreas and peritoneal cavity and causes tissue damage. Lipase destroys fat cells and liberates free fatty acids which combine with calcium and forms calcium soaps. The involved area appears to be chalky white.
- Traumatic fat necrosis: Occurs in tissues with high fat content (like in heavy, pendulous breasts, thigh) following severe injury.
Fibrinoid type:
It is aspecial a special form of necrosis. Characterised by pink staining protein acoustic material in the tissue matrix.
Causes: Usually seen in immune mediated vascular injury/vasculitis, malignant hypertension, placenta in preeclampsia, or hyperacute transplant rejection. Appears brightly pink in the vessel wall.
Gangrene (Gangrenous necrosis):
Gangrene: massive necrosis with superadded putrefaction (process of decay or rotting in a body or other organic matter)
Types: Dry gangrene and wet gangrene
Dry gangrene:
Causes: obstruction of arteries ( atherosclerosis)
Sites: usually involves a limb (leg, foot and toe)
Affected part is dry, shrunken and dark brown or black resembling the foot of a mummy. Black co.our is due to iron sulfide. A line of demarcation is seen between gangrenous and adjacent normal area.
Here the necrosis is coagulative type.
Wet gangrene:
Causes: Due to blockage in veins.
Sites: Occurs in moist tissues or internal organs ( bowel, lung, mouth etc)
The affected area is swollen, cold, putrid and black/dark. No clear line of demarcation.
liquefactive type of necrosis.