I now have comprehensive information from Robbins Pathology (both Basic and Pathologic Basis of Disease) to compile thorough cell injury notes with mnemonics. Let me write the complete answer.
Cell Injury - Notes & Mnemonics
Sources: Robbins & Kumar Basic Pathology; Robbins, Cotran & Kumar Pathologic Basis of Disease
1. Overview
Cells maintain homeostasis through constant adaptation. When stress exceeds adaptive capacity, cell injury occurs. Injury is reversible up to a point; if the stimulus is severe or persistent, the cell crosses the "point of no return" and dies.
Progression: Normal → Adaptation → Reversible Injury → Irreversible Injury → Cell Death
2. Causes of Cell Injury
Mnemonic: "HIT PING"
| Letter | Cause | Key Details |
|---|
| H | Hypoxia / Ischemia | Most common cause; ↓O₂ delivery → ↓ATP → ion pump failure |
| I | Immunologic reactions | Autoimmune, allergy, chronic inflammation |
| T | Toxins / Chemical agents | CO, arsenic, cyanide, alcohol, drugs |
| P | Physical agents | Trauma, radiation, burns, electric shock, pressure changes |
| I | Infectious agents | Viruses, bacteria, fungi, parasites |
| N | Nutritional imbalances | Protein-calorie deficiency; vitamin deficiencies; obesity |
| G | Genetic abnormalities | Down syndrome, sickle cell, enzyme defects, misfolded proteins |
3. Reversible vs. Irreversible Cell Injury
Reversible Cell Injury
The cell can recover if the injurious stimulus is removed.
Morphologic features - mnemonic: "FEMALE"
- F - Fatty change (lipid vacuoles in cytoplasm - especially liver)
- E - Eosinophilia (↑eosinophilic staining, ↓cytoplasmic RNA)
- M - Membrane blebbing (plasma membrane blebs + loss of microvilli)
- A - ATP-dependent Na⁺-K⁺ pump failure → cell swelling
- L - Loose ribosomes (detach from ER) + ER dilation
- E - Electron microscopy shows mitochondrial swelling, myelin figures, chromatin clumping
Key change: Cellular swelling (hydropic change / vacuolar degeneration) - the hallmark. Cells swell due to failure of the Na⁺/K⁺-ATPase pump.
Irreversible Cell Injury ("Point of No Return")
Two key ultrastructural markers of irreversibility:
- Densely flocculent amorphous densities in mitochondria (calcium phospholipid deposits)
- Gross membrane disruption (plasma membrane + lysosomal rupture)
4. Mechanisms of Cell Injury
Mnemonic: "MR. ODE"
| Letter | Mechanism |
|---|
| M | Mitochondrial damage → ↓ATP, cytochrome c release → apoptosis |
| R | Reactive oxygen species (ROS) / Oxidative stress → lipid peroxidation, DNA + protein damage |
| O | Oxygen deprivation (hypoxia/ischemia) → ATP depletion → failure of energy-dependent functions |
| D | DNA damage → caspase activation → apoptosis |
| E | ER stress → unfolded protein response (UPR) → if unresolved, apoptosis |
ATP Depletion Cascade (ischemia):
↓O₂ → ↓Oxidative phosphorylation → ↓ATP
↓
Na⁺/K⁺-ATPase fails → Na⁺ enters cell, K⁺ exits
↓
Water influx → Cell swelling + ER swelling
↓
Ca²⁺ influx → activates phospholipases, proteases, ATPases
↓
Mitochondrial permeability ↑ → cytochrome c → apoptosis
Ischemia-Reperfusion Injury:
Paradoxically, restoring blood flow worsens injury by:
- Burst of ROS generation
- Increased inflammation (neutrophil influx)
5. Cell Death Pathways
Necrosis vs. Apoptosis - Mnemonic: "NECROSIS = No Energy, Cell Rupture, Outpour, Swelling, Inflammation, Scary"
| Feature | Necrosis | Apoptosis |
|---|
| Cause | Pathologic (ischemia, toxins) | Physiologic OR pathologic |
| Cell size | Swells | Shrinks |
| Nucleus | Karyolysis, karyorrhexis, pyknosis | Fragmentation (ladder pattern) |
| Membrane | Disrupted - contents leak | Intact - apoptotic bodies form |
| Inflammation | YES (inflammatory response) | NO (phagocytes clean up silently) |
| Mechanism | Passive, uncontrolled | Active, caspase-mediated, ATP-dependent |
Nuclear Changes in Necrosis - Mnemonic: "PKL" (Pickle)
- Pyknosis - nuclear shrinkage + condensation
- Karyorrhexis - nuclear fragmentation
- Karyolysis - nuclear dissolution (DNA digested by DNases)
6. Types of Necrosis
Mnemonic: "CLFC Gas" (Coag, Liq, Fat, Caseous, Gangrenous, Fibrinoid)
| Type | Location | Gross Appearance | Key Cause |
|---|
| Coagulative | All solid organs (except brain) | Firm, preserved architecture | Ischemia (MI, renal infarct) |
| Liquefactive | Brain; bacterial abscesses | Pus, liquid mass | Brain ischemia; bacterial infections |
| Fat necrosis | Pancreas, breast | Chalky-white (saponification) | Pancreatitis; trauma |
| Caseous | Lymph nodes, lung (TB) | Cheese-like, crumbly | TB (M. tuberculosis) |
| Gangrenous | Limbs | Dry (coagulative) or wet (+ liquefactive) | Ischemia ± infection |
| Fibrinoid | Blood vessel walls | Bright pink, smudged | Immune complex deposition, malignant HTN |
7. Apoptosis Pathways
Intrinsic (Mitochondrial) Pathway
- Triggered by: DNA damage, growth factor withdrawal, misfolded proteins (ER stress)
- Key players: BCL-2 family (anti-apoptotic: BCL-2, BCL-XL; pro-apoptotic: BAX, BAK)
- BAX/BAK form pores → cytochrome c leaks from mitochondria → binds APAF-1 → apoptosome → activates caspase-9 → executes caspase cascade
Extrinsic (Death Receptor) Pathway
- Triggered by: FasL binding Fas (CD95); TNF binding TNFR1
- Activates caspase-8 → executes caspase cascade
- Role: elimination of self-reactive lymphocytes; CTL-mediated killing
Mnemonic for pathways: "In-Mit, Ex-Death"
- Intrinsic = Mitochondrial
- Extrinsic = Death receptor
8. Cellular Adaptations to Stress
Mnemonic: "HAMM" (Hypertrophy, Atrophy, Metaplasia, hyperplasia (extra M))
| Adaptation | Definition | Example |
|---|
| Hypertrophy | ↑ cell size, ↑ organ size | Cardiac hypertrophy in hypertension |
| Hyperplasia | ↑ cell number | Endometrial hyperplasia with estrogen |
| Atrophy | ↓ cell size + metabolic activity | Disuse atrophy, denervation atrophy |
| Metaplasia | Change in cell phenotype | Barrett esophagus (squamous→columnar) |
9. Autophagy
- Triggered by: nutrient deprivation
- Process: cellular organelles sequestered into autophagosomes → fuse with lysosomes → digested
- Function: energy source during starvation; quality control (removes damaged organelles)
- If stress unrelieved: autophagy triggers apoptosis
- Dysregulated in: cancer, IBD, neurodegenerative diseases
10. Other Cell Death Types
| Type | Key Feature |
|---|
| Necroptosis | Regulated necrosis - features of both necrosis + apoptosis; mediated by RIPK3/MLKL |
| Pyroptosis | Death with release of pro-inflammatory cytokines (IL-1β, IL-18); key in inflammasome activation |
| Ferroptosis | Iron-dependent, ROS-driven lipid peroxidation |
Quick Summary Mnemonic: "CRIME AVAN"
For a rapid rundown of all major cell injury concepts:
- C - Causes (HIT PING)
- R - Reversible injury (cellular swelling, fatty change)
- I - Irreversible = flocculent mitochondrial densities + membrane rupture
- M - Mechanisms (MR. ODE)
- E - End-point = Necrosis or Apoptosis
- A - Adaptations (HAMM)
- V - Various necrosis types (CLFC Gas)
- A - Apoptosis pathways (In-Mit, Ex-Death)
- N - Nuclear changes in necrosis (PKL = Pyknosis, Karyorrhexis, Karyolysis)
Robbins & Kumar Basic Pathology, pp. 17-20 | Robbins, Cotran & Kumar Pathologic Basis of Disease, pp. 47-65