Cell Death: Apoptosis and Necrosis

Cell Death: Apoptosis

• Apoptosis is controlled cell death, or programmed cell death.
• Apoptosis eliminates worn-out cells, cells produced in excess, cells that develop improperly, or cells with damaged DNA.
• Apoptosis is involved in normal physiological processes like embryonic development of organs, separation of webbed fingers/toes, removal of white blood cells in inflammation, removal of self-reacting lymphocytes during immune system development, loss of liver cells during hepatitis C infection.

Cell Death: Necrosis

• Necrosis is an unnatural, pathological form of cell death, resulting from cell injury.
• Necrosis is characterized by cell swelling, rupture of the cell membrane, and inflammation.
• There is an unregulated enzymatic digestion of cell components within the cell.
• Different types of necrosis exist: coagulative, liquefactive, caseous, fat, and gangrenous.

Coagulative Necrosis

• The most common form of necrosis.
• Results usually from hypoxia (lack of oxygen).
• Proteins in the tissues coagulate (clump together), forming a solid mass.
• Most often seen in the kidney, heart, and adrenal glands.

Liquefactive Necrosis

• Results from ischemia (lack of blood flow) to neurons and glial cells (support cells in the central nervous system).
• Enzymes (hydrolases) digest cells, making them soft and liquid.
• Often occurs due to bacterial infections, where neutrophils release hydrolases to kill bacteria and create an area of pus.

Caseous Necrosis

• Combination of coagulative and liquefactive necrosis.
• Dead cells disintegrate, but the debris is not completely digested, leaving a cheese-like appearance.
• Often seen in tuberculosis infection in the lungs.
• Necrotic tissue is walled off from the rest of the tissue.

Fat Necrosis

• Describes the death of fat cells.
• Commonly occurs in breast tissue, the pancreas, and abdominal organs after injury.
• Caused by powerful lipases (fat-digesting enzymes) breaking down fat cells.
• Results in the formation of opaque, chalky white lumps due to the combination of free fatty acids, calcium, magnesium, and necrotic tissue.

Gangrenous Necrosis

• A large mass of tissue undergoes necrosis.
• Often due to severe hypoxic injury (lack of oxygen) due to blockage of major arteries, like arteriosclerosis.
• Hypoxia followed by bacterial invasion.
• Includes wet, dry, and gas gangrene.

Dry Gangrene

• A form of coagulative necrosis.
• The affected area becomes dry and shrinks.
• Skin wrinkles, and color changes to dark brown or black.
• Usually occurs in the extremities.

Wet Gangrene

• A form of liquefactive necrosis.
• The affected area becomes cold, swollen, and moist.
• Foul odor from bacterial invasion.
• Can occur on extremities or internal organs.
• Release of bacterial toxins into bloodstream could be life-threatening.

Gas Gangrene

• Caused by the anaerobic bacteria (bacteria that don't need oxygen), such as Clostridium.
• These bacteria are commonly found in soil.
• Often occurs during a compound fracture (break in a bone) where dirt/debris are embedded into the wound.
• Gas bubbles form in the tissue, potentially fatal.

Apoptosis vs Necrosis

• Apoptosis: controlled, programmed cell death; cell shrinks, clean-up.
• Necrosis: uncontrolled cell death, resulting from injury; cell swells, messy clean-up.

Study Questions

• What's the difference between apoptosis and necrosis?
• In apoptosis, does the cell shrink or swell?
• Which type of necrosis is most common?
• In fat necrosis, what happens to the fat cells?
• Which type of necrosis resembles a delicious hors d'oeuvre?
• What type of gangrene is associated with anaerobes?
• Why does wet gangrene smell so bad?
• What type of gangrene is best associated with frostbite?

Explore the different mechanisms of cell death, focusing on apoptosis and necrosis. Learn how apoptosis serves as programmed cell death essential for various physiological processes, while necrosis represents a pathological response to injury. Delve into the characteristics and types of necrosis, including coagulative necrosis.