🔬 NECROSIS - Complete Guide for MBBS 2nd Year

📌 FIRST - What is Cell Death?

📊 Look at this important diagram from Robbins - it shows that cell function fails FIRST, then cell death occurs, and only much later do we see visible changes under the microscope:

• A heart muscle cell stops contracting within 1-2 minutes of ischemia (lack of blood)
• But it only dies after 20-30 minutes
• And changes only appear under the microscope after 6-12 hours
• This is why we can do CPR - the cells are not dead yet!

🔴 WHAT IS NECROSIS?

• Loss of oxygen supply (ischemia) - most common
• Microbial toxins (bacteria, fungi)
• Burns and chemical injury
• Enzymes leaking out (as in pancreatitis)
• Physical trauma

⚙️ MECHANISM - How does Necrosis happen?

1. Severe injury hits the cell (e.g., artery blockage stops blood supply)
2. Mitochondria fail → no ATP (energy) produced
3. Cell membranes get damaged - the cell can no longer control what goes in/out
4. Lysosomal enzymes (the cell's own "digestive juices") leak into the cytoplasm
5. These enzymes digest the cell from within
6. Cell contents spill into the extracellular space
7. Spilled contents (called DAMPs - Damage Associated Molecular Patterns) include:
   • ATP from damaged mitochondria
   • Uric acid (DNA breakdown product)
   • Other normally-hidden proteins
8. These DAMPs trigger macrophages and cause INFLAMMATION
9. Inflammatory cells come in, produce more enzymes, and clean up the dead tissue

💡 Clinically important: When cells undergo necrosis, their contents leak INTO THE BLOOD. This is how we diagnose:

• Heart attack - Cardiac troponin rises in blood within 2 hours of myocardial necrosis
• Liver damage - Transaminases (ALT, AST) rise
• Bile duct damage - Alkaline phosphatase rises

🔵 MORPHOLOGY OF NECROSIS (What it looks like under the microscope)

A) Changes in the Cytoplasm:

B) Changes in the Nucleus (THE MOST TESTED PART!):

💡 Memory trick: The sequence is often: Pyknosis → Karyorrhexis → Karyolysis (the nucleus shrinks, then breaks, then dissolves). After 1-2 days, the nucleus completely disappears.

🟠 TYPES OF NECROSIS (MOST IMPORTANT FOR EXAMS!)

1. 🟡 COAGULATIVE NECROSIS

• Most common type
• What happens: The dead tissue keeps its shape/architecture for days (it looks "solid" like cooked egg white). Proteins are denatured, which also inactivates the digestive enzymes, so the structure is preserved temporarily.
• Appearance: Firm, pale, intensely eosinophilic "ghost cells" with preserved outlines but no nuclei
• Cause: Ischemia (blocked blood supply)
• Where: ALL organs EXCEPT the brain
• Classic example: Myocardial infarction (heart attack) - the heart muscle dies in a solid, pale pattern
• A localized area = called an INFARCT

2. 🟢 LIQUEFACTIVE NECROSIS

• What happens: Dead cells are completely digested/liquefied into a thick fluid. It's the OPPOSITE of coagulative necrosis - no structure is preserved.
• Appearance: Viscous liquid, often creamy yellow (called pus when due to bacteria)
• Causes:
  • Bacterial or fungal infections (microbes bring lots of leukocytes which release digestive enzymes)
  • Brain infarction (for unknown reasons, brain ischemia causes liquefaction, not coagulation)
• Where: Brain and sites of bacterial infection (abscesses)
• Classic example: Brain stroke causes a fluid-filled cavity; abscess (pus collection)

3. 🧀 CASEOUS NECROSIS

• What happens: Dead tissue looks like soft, crumbly white cheese (caseum = cheese in Latin)
• Appearance: Structureless, granular, "cheesy" debris; surrounded by epithelioid macrophages = called a GRANULOMA
• Cause: Tuberculosis (TB) - almost exclusively
• Where: TB-infected lymph nodes, lungs
• Classic example: TB lymphadenitis - the node is full of cheesy white material

• 📝 "Caseous necrosis = TB until proven otherwise" - remember this for exams!

4. 🔥 FAT NECROSIS

• What happens: Pancreatic enzymes (lipases) leak out and digest fat cells. The free fatty acids released combine with calcium to form white chalky deposits (a process called saponification = soap-making reaction)
• Appearance: Chalky white patches in the abdomen; on microscopy - shadowy fat cell outlines + blue calcium deposits
• Cause: Acute pancreatitis (enzyme leak)
• Where: Pancreas and surrounding peritoneal fat
• Classic example: Surgeon opens the abdomen in acute pancreatitis and sees white chalk-like spots on the fat

5. 🩸 FIBRINOID NECROSIS

• What happens: Damage to blood vessel walls by immune complexes (antigen-antibody deposits). Plasma proteins including fibrin deposit in the vessel wall.
• Appearance: Bright pink, smudgy (amorphous) deposits in vessel walls on H&E stain - looks "fibrin-like"
• Cause: Immune reactions - vasculitis, malignant hypertension
• Where: Walls of arteries/arterioles
• Classic example: Autoimmune diseases like SLE, polyarteritis nodosa

6. 🦵 GANGRENOUS NECROSIS

• Note: This is NOT a separate microscopic pattern - it is a clinical term
• What happens: Limb (usually lower leg) loses blood supply → undergoes coagulative necrosis
  • Dry gangrene = just coagulative necrosis (dry, mummified limb)
  • Wet gangrene = bacterial infection is superimposed → liquefactive changes added (smelly, wet, black)
• Classic example: Diabetic foot gangrene

📊 SUMMARY TABLE - Types of Necrosis (Exam Revision)

⚡ KEY CONCEPTS TO REMEMBER FOR EXAMS

1. Necrosis vs Apoptosis: Necrosis is always pathological (disease-related) and causes inflammation. Apoptosis is "programmed" cell death that does NOT cause inflammation.
2. Two signs of irreversible injury:
   • Inability to restore mitochondrial function (no ATP)
   • Severe membrane damage
3. Nuclear changes = Pyknosis → Karyorrhexis → Karyolysis
4. Coagulative = everywhere EXCEPT brain (brain = liquefactive)
5. Caseous = TB, Fat necrosis = Pancreatitis, Fibrinoid = Autoimmune vasculitis
6. DAMPs - substances released from necrotic cells that trigger inflammation; clinically important because they raise serum biomarkers (troponin, transaminases)
7. Dystrophic calcification - necrotic tissue not cleaned up can become calcified (calcium deposits in dead tissue)

🩺 CLINICAL CORRELATION (Exam favourite!)

Q: Why does troponin rise in a heart attack? Because myocardial cells undergo necrosis → membrane breaks → troponin leaks into blood → detectable within 2 hours. This is why we measure troponin to diagnose MI!

Q: Why is brain infarction liquefactive but kidney infarction is coagulative? The brain is rich in lipids and has fewer structural proteins. The abundant lipases and proteases of leukocytes liquefy the tissue. Solid organs (kidney, heart) have more structural proteins that resist digestion, preserving the architecture.

• Go deeper into any specific type (e.g. caseous, coagulative)?
• Move on to Apoptosis (the other type of cell death)?
• Cover differences between Necrosis and Apoptosis (very common exam question)?
• Start a new topic like Inflammation?