Cell Injury Mechanisms

1. Causes of Cell Injury

• Robbins & Kumar Basic Pathology, p. 10-11

2. General Principles

1. Dose- and time-dependence: Low doses of a toxin or brief ischemia cause reversible injury; larger doses or prolonged ischemia lead to irreversible injury and necrosis.
2. Cell-type specificity: Skeletal muscle survives complete ischemia for 2-3 hours; cardiac muscle dies after only 20-30 minutes. Neurons are even more sensitive.
3. Multiple overlapping pathways: Any single injurious stimulus often triggers several biochemical mechanisms simultaneously, which is why targeting a single pathway rarely prevents cell death completely.
4. Genetic variation matters: Individuals with polymorphisms in cytochrome P-450 genes, for example, metabolize the same toxin at different rates, yielding different outcomes.

• Robbins & Kumar Basic Pathology, p. 11-12; Robbins, Cotran & Kumar Pathologic Basis of Disease, p. 62

3. Key Biochemical Mechanisms

3a. Mitochondrial Dysfunction and ATP Depletion

• ATP depletion: Failure of oxidative phosphorylation reduces ATP, which is required for virtually every enzymatic and biosynthetic activity in the cell.
  • Na+/K+-ATPase pump fails → intracellular Na+ accumulates → osmotic water influx → cell swelling and ER dilation
  • Anaerobic glycolysis increases → lactic acid accumulates → intracellular pH drops → enzymatic activity impaired
  • Ribosomes detach from rough ER → protein synthesis falls
  • Lipid deposition occurs (fatty change) due to defective lipoprotein assembly
• Mitochondrial permeability transition pore (MPTP): Opens under injury conditions → loss of mitochondrial membrane potential → failure of oxidative phosphorylation. MPTP opening is a key step in both necrosis and the intrinsic (mitochondrial) apoptosis pathway.
• Release of pro-apoptotic proteins: Cytochrome c, Smac/DIABLO, and apoptosis-inducing factor (AIF) leak into the cytoplasm when the outer mitochondrial membrane is permeabilized, activating caspases and triggering apoptosis.
• Robbins & Kumar Basic Pathology, p. 10; Pathologic Basis of Disease, p. 62-63

3b. Oxidative Stress and Reactive Oxygen Species (ROS)

• Normal mitochondrial respiration (low-level "leak")
• Ischemia-reperfusion injury (major source - see below)
• Activated phagocytes (respiratory burst)
• Ionizing radiation
• Reactions with redox-active metals (Fe2+ + H2O2 → •OH, Fenton reaction)
• Metabolism of drugs/chemicals (e.g., CCl4)

• Lipid peroxidation of membrane phospholipids → membrane disruption
• Protein oxidation → cross-linking, fragmentation, enzymatic inactivation
• DNA damage → strand breaks, base modifications

• Robbins & Kumar Basic Pathology, p. 11-12

3c. Membrane Damage

• Direct injury: Toxins (e.g., complement, bacterial toxins) form pores or disrupt bilayer structure
• Phospholipase activation: Elevated cytosolic Ca2+ activates phospholipases that degrade membrane phospholipids
• Lipid peroxidation by ROS: Oxidative attack on polyunsaturated fatty acids
• Cytoskeletal damage: Protease activation detaches the plasma membrane from the cytoskeleton, making cells fragile and prone to rupture under mechanical stress
• Lysosomal membrane rupture: Leakage of acid hydrolases into the cytoplasm causes digestion of cellular components, contributing to irreversible damage and necrosis

• Robbins & Kumar Basic Pathology, p. 13

3d. Disturbance in Calcium Homeostasis

1. Releasing it from intracellular stores (ER and mitochondria) early on
2. Increasing influx across the dysfunctional plasma membrane later

• Protease activation → cytoskeletal and membrane protein degradation
• Phospholipase activation → membrane phospholipid breakdown
• ATPase activation → accelerated ATP depletion
• Endonuclease activation → chromatin fragmentation
• MPTP opening → mitochondrial dysfunction and apoptosis
• Robbins & Kumar Basic Pathology, p. 13; Pathologic Basis of Disease, p. 64

3e. Endoplasmic Reticulum (ER) Stress and the Unfolded Protein Response

• Mild ER stress (adaptive UPR): Sensors like IRE1 detect misfolded proteins → ↑ chaperone synthesis, ↓ protein translation, ↑ misfolded protein degradation → reduces the load of misfolded proteins
• Severe ER stress (terminal UPR): If the load cannot be cleared → activation of BH3 proteins and caspases → apoptosis via the intrinsic (mitochondrial) pathway

• Robbins & Kumar Basic Pathology, p. 13-14

3f. DNA Damage

• If damage is repairable → cell cycle arrest to allow time for repair
• If damage is too great → p53 activates pro-apoptotic genes (e.g., BAX) → apoptosis via the intrinsic pathway

• Robbins & Kumar Basic Pathology, p. 14

4. Reversible vs. Irreversible Injury

5. Clinicopathologic Examples

Hypoxia / Ischemia

Ischemia-Reperfusion Injury

• Burst of ROS generation (from xanthine oxidase, neutrophils, mitochondria) damages membranes and proteins
• Calcium overload into reversibly injured cells pushes them to irreversibility
• Complement activation and neutrophil recruitment amplify tissue damage
• This mechanism is relevant in myocardial infarction, stroke, and organ transplantation
• Robbins & Kumar Basic Pathology, p. 15

Chemical/Toxic Injury

• Direct toxins (e.g., HgCl2): bind to membrane proteins and sulfhydryl groups → ↑ membrane permeability → loss of ion homeostasis
• Indirect toxins (e.g., CCl4): converted by cytochrome P-450 in the ER to CCl3• free radical → lipid peroxidation → membrane destruction → liver cell necrosis

6. Cell Death Pathways Summary

• Robbins & Kumar Basic Pathology (10th ed.), Chapter 1 - Cellular Responses to Stress and Toxic Insults
• Robbins, Cotran & Kumar Pathologic Basis of Disease (10th ed.), Chapter 2 - Cellular Responses to Stress and Toxic Insults

Cell Injury Mechanisms - Quick Summary

1. ATP Depletion (Mitochondrial dysfunction) - Loss of energy → Na+/K+ pump fails → cell swells → proteins stop synthesizing → eventually irreversible damage
2. Oxidative Stress (ROS) - Free radicals (O2•-, H2O2, •OH) damage membrane lipids, proteins, and DNA; major player in ischemia-reperfusion injury
3. Calcium Overload - Elevated cytosolic Ca2+ activates destructive enzymes: proteases, phospholipases, endonucleases → membrane breakdown + DNA fragmentation
4. Membrane Damage - From phospholipases, ROS, toxins, or complement → loss of ionic gradients → cell rupture (necrosis)
5. ER Stress / Misfolded Proteins - Overwhelms the unfolded protein response → apoptosis via the intrinsic (mitochondrial) pathway
6. DNA Damage - Activates p53 → either cell cycle arrest (for repair) or apoptosis if damage is irreparable

• Mild/brief injury → Reversible (cell recovers)
• Severe/prolonged → Irreversible → Necrosis (inflammatory, passive) or Apoptosis (programmed, non-inflammatory)

Inflammation

Acute Inflammation

1. Vascular Changes

• Vasodilation (main mediator: histamine) → increased blood flow → redness and warmth
• Increased vascular permeability → protein-rich fluid leaks out of postcapillary venules into tissues → exudate forms → swelling (edema)
  • Mechanism: endothelial cell contraction opens interendothelial gaps (triggered by histamine, bradykinin, leukotrienes)
• Slowed blood flow + increased viscosity → stasis

2. Leukocyte Recruitment (Key steps)

3. Phagocytosis and Destruction

• Leukocytes recognize, engulf (phagocytosis), and destroy microbes/debris
• Killing occurs via:
  • ROS (superoxide, H2O2, hypochlorous acid - from the respiratory burst via NADPH oxidase)
  • Nitric oxide (NO)
  • Lysosomal enzymes (elastase, cathepsins, myeloperoxidase)

Inflammatory Mediators

Key drug targets: Aspirin/NSAIDs block COX → reduce prostaglandins. Steroids block phospholipase A2 → reduce all arachidonic acid metabolites. Anti-TNF biologics (e.g., adalimumab) block TNF directly.

Outcomes of Acute Inflammation

1. Complete resolution - injury removed, tissue regenerates fully (most common with mild, limited injury)
2. Scarring/fibrosis - when tissue destruction is extensive or regeneration is impossible
3. Progression to chronic inflammation - when the cause persists or healing is impaired

Chronic Inflammation

• Mononuclear infiltrate: lymphocytes, macrophages, plasma cells (not neutrophils)
• Tissue destruction by activated macrophages and lymphocytes
• Fibrosis (scarring) as repair attempts continue

Causes

• Persistent infections (mycobacteria, fungi, viruses, parasites) - organisms resist eradication
• Autoimmune/hypersensitivity diseases (rheumatoid arthritis, multiple sclerosis, asthma)
• Prolonged toxic exposure (silica → silicosis; cholesterol → atherosclerosis)

Key Cells

• Macrophages - central orchestrators; activated by IFN-γ → secrete TNF, IL-1, ROS, proteases → cause tissue damage; also drive fibrosis via PDGF, TGF-β
• Lymphocytes (T and B cells) - amplify and sustain macrophage activation; B cells produce antibodies
• Plasma cells - antibody production
• Eosinophils - allergic reactions and parasitic infections
• Mast cells - IgE-mediated reactions

Granulomatous Inflammation

Systemic Effects (Acute Phase Response)

• Fever (prostaglandins act on hypothalamus)
• Leukocytosis (WBC count rises)
• Acute phase proteins - CRP, fibrinogen, serum amyloid A rise (useful as clinical markers)
• Malaise, anorexia, weight loss in severe/chronic cases

Inflammation — Simple Language

What is it?

Why does it happen?

What you see (and feel)

Two types

🔴 Acute (Short-term)

• Starts fast (minutes to hours)
• Lasts days
• Main soldier: Neutrophil (arrives first, kills germs, then leaves)
• Example: a pimple, a sprained ankle, a sore throat

🟠 Chronic (Long-term)

• Lasts weeks to months
• Happens when the problem won't go away (e.g., TB, autoimmune disease)
• Main soldier: Macrophage + Lymphocytes
• Can quietly damage your own tissues over time
• Example: rheumatoid arthritis, asthma, atherosclerosis

How does it end?

1. Full recovery - problem solved, tissue goes back to normal ✅
2. Scarring - damage was too much to fully repair
3. Becomes chronic - the cause sticks around

The key players (simplified)

When inflammation is the problem

• Autoimmune disease - attacks your own body (e.g., lupus, rheumatoid arthritis)
• Allergy - overreacts to harmless things (pollen, food)
• Chronic disease - low-grade inflammation quietly damages organs (atherosclerosis, diabetes)

Acute vs. Chronic Inflammation

Key Cellular Difference

• Short-lived, arrive fast, kill microbes via ROS and enzymes
• Die after ~24-48 hours at the site

• Macrophages are activated by IFN-γ (from T cells)
• They release TNF, IL-1, proteases, ROS → prolonged tissue damage
• T cells and B cells sustain the immune response
• Plasma cells produce antibodies

Key Mediator Difference

Examples

Special Form of Chronic: Granulomatous Inflammation

Events of Acute Inflammation

Vascular Changes → Leukocyte Recruitment → Phagocytosis & Destruction

EVENT 1: Vascular Changes

Step 1 - Transient Vasoconstriction

Step 2 - Vasodilation

• Arterioles and then capillaries dilate → increased blood flow to the area
• Mediator: Histamine (primary), NO, prostaglandins
• Result: Redness (rubor) and Heat (calor)

Step 3 - Increased Vascular Permeability

• Inflammatory mediators cause endothelial cells to contract → interendothelial gaps open in postcapillary venules
• Protein-rich fluid leaks into tissue → Exudate forms
• Mediators: Histamine, bradykinin, leukotrienes, substance P
• Occurs within 15-30 minutes of stimulus
• Result: Swelling (tumor)

Exudate = high protein, cellular debris (inflammation) Transudate = low protein, no cells (heart failure, hypoalbuminemia)

Step 4 - Stasis

• Fluid loss + vasodilation → blood flow slows
• Red cells concentrate centrally; leukocytes shift to periphery (margination)
• Vessels become engorged → vascular congestion

Lymphatics also respond:

• Increased lymph flow drains excess fluid
• Lymphangitis (red streaks) and lymphadenopathy may develop

EVENT 2: Leukocyte Recruitment (Cellular Events)

Step-by-step:

Key concept: Selectins handle rolling. Integrins handle firm adhesion. PECAM-1 handles transmigration. These can be blocked by drugs (e.g., anti-integrin therapies in IBD).

Which cells arrive when?

• Neutrophils - first (within 6-24 hours); rapid, short-lived, major killers
• Monocytes → Macrophages - follow (24-48 hours onward); slower but longer-lasting

EVENT 3: Phagocytosis and Destruction

Step 1 - Recognition and Opsonization

• Phagocytes recognize microbes via:
  • Pattern recognition receptors (e.g., Toll-like receptors, mannose receptors)
  • Opsonins - coat microbes and enhance phagocytosis: IgG antibodies (Fc receptor), C3b (complement receptor), plasma lectins
• Opsonization dramatically increases efficiency of phagocytosis

Step 2 - Engulfment

• Pseudopods extend around the particle → form a phagosome
• Phagosome fuses with lysosomes → phagolysosome
• Lysosomal contents discharged into phagolysosome

Step 3 - Killing and Degradation

• NADPH oxidase assembled on phagosome membrane → converts O2 to superoxide (O2•-)
• O2•- → H2O2 → then myeloperoxidase (MPO) converts H2O2 + Cl- → hypochlorous acid (HOCl) - the most potent microbicidal agent
• Defect in NADPH oxidase → Chronic Granulomatous Disease (CGD) - recurrent infections

• Lysosomal enzymes: elastase, cathepsins, defensins
• Lysozyme: degrades bacterial cell walls
• Lactoferrin: chelates iron (needed for bacterial growth)
• Major basic protein (eosinophils): kills parasites

Leukocyte-mediated Tissue Damage

• ROS, proteases, elastase → damage host connective tissue
• This is the basis of tissue injury in many inflammatory diseases (e.g., glomerulonephritis, ARDS)

Summary Flow Chart

INJURY/INFECTION
      ↓
Mast cells/macrophages release histamine, TNF, IL-1
      ↓
VASODILATION + ↑ VASCULAR PERMEABILITY
      ↓
Blood flow slows → Stasis → Margination of WBCs
      ↓
Rolling (Selectins) → Activation (Chemokines) → Firm Adhesion (Integrins)
      ↓
Transmigration (PECAM-1) → Chemotaxis toward injury
      ↓
PHAGOCYTOSIS → Opsonization → Engulfment → Killing (ROS + enzymes)
      ↓
Clearance of agent → Resolution OR → Chronic inflammation