Fiche de révision : Cell Injury and Death Mechanisms

📋 Course Outline

1. Cell responses to stress and injury
2. Causes of cell injury
3. Reversible injury and cell death
4. Necrosis patterns and biomarkers
5. Apoptosis pathways and morphology
6. Autophagy and necroptosis
7. Oxidative stress and membrane damage
8. Hypoxia and reperfusion injury
9. Chemical, infectious, and immune injury
10. Cellular adaptations to stress
11. Intracellular accumulations and pigments
12. Calcification and cellular aging

📖 1. Cell responses to stress and injury

🔑 Key Concepts & Definitions

• Homeostasis : Homeostasis is the steady functional state that a normal cell maintains despite physiologic demands and constraints from its surroundings.
• Adaptation : Adaptation is a reversible change in function and structure that lets a cell survive a stress when limits are not exceeded.
• Cell injury : Cell injury is the progression of cellular dysfunction that occurs when stress exceeds adaptive limits, critical nutrients are lacking, or essential functions are impaired.
• Reversible cell injury : Reversible cell injury is early or mild damage where structural and functional alterations can be corrected if the injurious stimulus stops.
• Irreversible injury : Irreversible injury is persistent or severe damage that passes beyond a repair limit and leads to cell death.

📝 Essential Points

• Removal of the stress allows the cell to return to its original state without harmful consequences.
• If injury persists or becomes severe, cells progress from reversible injury to irreversible injury and ultimately cell death.
• Noxious stimuli act first at molecular or biochemical levels, with morphologic changes developing after a time lag.
• Depending on severity, cells may adapt, undergo injury, or die after oxygen deprivation.
• Cell death proceeds mainly via necrosis or apoptosis, with necrosis typically associated with inflammation and apoptosis occurring with defined regulated pathways.

💡 Memory Hook

A→R→I→D: Adaptation, Reversible injury, Irreversible injury, Death (necrosis/apoptosis).

📖 2. Causes of cell injury

🔑 Key Concepts & Definitions

• Oxygen deprivation : Oxygen deprivation is a condition in which low oxygen reduces aerobic oxidative respiration, triggering cell injury and possibly cell death.
• Physical agents : Physical agents are nonchemical insults such as trauma, extreme temperature, pressure changes, radiation, and electric shock that damage cells.
• Chemical agents and drugs : Chemical agents and drugs are harmful substances that injure cells directly or by disrupting electrolytes, fluids, or metabolic processes.
• Infectious agents : Infectious agents are biological pathogens ranging from viruses to parasites that cause cell injury through diverse mechanisms.
• Immunologic reactions : Immunologic reactions are immune processes that can injure cells, including autoimmune reactions to self antigens and reactions to external agents.

📝 Essential Points

• Hypoxia (oxygen deficiency) causes cell injury by reducing aerobic oxidative respiration, and it can result from ischemia, cardiorespiratory failure, or impaired oxygen carriage such as anemia or carbon monoxide poisoning.
• Etiologic factors can be grouped into genetic causes and environmental causes, and many common diseases are multifactorial rather than single-agent.
• Nutritional imbalances cause cell injury via both deficiency and excess, including protein-calorie deficiency, vitamin deficiencies, and diets that contribute to disorders such as atherosclerosis.
• Genetic abnormalities can injure cells by deficient protein function or by triggering cell death when damaged DNA or misfolded proteins accumulate beyond repair.
• Cell injury can be caused by electrolyte/fluid derangements from hypertonic solutes, and even oxygen can be toxic at high concentrations.

💡 Memory Hook

Think of major causes as O-P-C-I-I: Oxygen deprivation, Physical agents, Chemical agents, Infectious agents, Immunologic reactions.

📖 3. Reversible injury and cell death

🔑 Key Concepts & Definitions

• Point of no return : The point of no return is the poorly defined moment when ongoing injury becomes irreparable and cells shift from reversible injury to irreversible cell death.
• Hydropic change : Hydropic change is a form of reversible injury marked by water-driven cellular swelling and clear cytoplasmic vacuoles from ER distention.
• Necrosis : Necrosis is pathologic cell death caused by severe injury that destroys membranes, leaks contents, and triggers local inflammation with enzymatic digestion.
• Damage-associated molecular patterns : Damage-associated molecular patterns are molecules released by severely injured cells that signal danger and promote phagocytosis and inflammatory cytokine production.

📝 Essential Points

• Early biochemical changes after injury may appear within minutes to hours, while light microscopy or gross changes may take hours to days to become visible.
• In ischemic myocardium, cell swelling can occur within minutes and may progress to irreversibility within 1 to 2 hours.
• In the same ischemia example, unmistakable light-microscopic evidence of cell death may not be seen until 4 to 12 hours after onset.
• With reversible injury, cell swelling is the earliest manifestation, and plasma membrane blebbing plus swollen organelles reflect failed ATP-dependent ion pumping due to ATP depletion or mitochondrial damage.
• Irreversible progression is consistently associated with inability to restore mitochondrial dysfunction and severe damage to lysosomal membranes.

💡 Memory Hook

Ischemia timeline: swelling in minutes → irreversibility in 1–2 h → light-microscopic death at 4–12 h.

📖 4. Necrosis patterns and biomarkers

🔑 Key Concepts & Definitions

• Coagulative necrosis : Coagulative necrosis is a necrosis pattern where the dead tissue’s architecture is preserved for at least some days, leaving a firm texture.
• Liquefactive necrosis : Liquefactive necrosis is a necrosis pattern in which dead cells are digested, turning tissue into a viscous liquid.
• Caseous necrosis : Caseous necrosis is a necrosis pattern that forms friable, cheeselike yellow-white debris with a distinctive inflammatory border.
• Fat necrosis : Fat necrosis is focal destruction of fat that commonly produces chalky-white calcium soap deposits after lipid breakdown.
• Fibrinoid necrosis : Fibrinoid necrosis is a special vascular necrosis pattern seen in immune reactions, producing bright pink amorphous vessel-wall staining on H&E.

📝 Essential Points

• Coagulative necrosis preserves cellular outlines while nuclei are lost, because injury denatures structural proteins and enzymes and blocks proteolysis for days or weeks.
• Liquefactive necrosis is seen in focal bacterial infections and occasionally fungal infections, where leukocyte enzymes digest tissue into a creamy yellow pus-like material.
• Caseous necrosis consists of fragmented or lysed cells and amorphous granular debris enclosed by an inflammatory border forming a granuloma-like appearance.
• Fat necrosis results when activated pancreatic lipases leak and liquefy fat cell membranes, generating fatty acids that combine with calcium to form visible chalky-white saponification deposits.
• Fibrinoid necrosis occurs when antigen–antibody immune complexes deposit in vessel walls with leaked plasma proteins, creating bright pink amorphous H&E staining.
• Necrotic cells show increased eosinophilia on H&E due to loss of cytoplasmic RNA and accumulation of denatured proteins that bind eosin.

💡 Memory Hook

Coagulative = “coagulum” keeps outlines; Liquefactive = “liquefy” into mush/pus; Caseous = “cheesy”; Fat = “chalky soap”; Fibrinoid = immune-complex “pink” vessel wall.

📖 5. Apoptosis pathways and morphology

🔑 Key Concepts & Definitions

• Cell shrinkage : Apoptosis shows reduced cell size with dense, eosinophilic cytoplasm and relatively normal organelles packed more tightly than before.
• Chromatin condensation : Apoptosis is characterized by peripheral aggregation of chromatin beneath the nuclear membrane into dense masses of varied shapes and sizes.
• Apoptotic bodies : Apoptosis involves membrane blebbing followed by fragmentation into membrane-bound apoptotic bodies containing cytoplasm and tightly packed organelles.
• Caspase activation marker : Apoptosis depends on proteolytic caspase activation, and active caspases serve as a marker of cells undergoing apoptosis.
• Mitochondrial intrinsic pathway : The intrinsic pathway of apoptosis is initiated by mitochondrial outer membrane permeability changes that release pro-apoptotic proteins and trigger caspase cascades.

📝 Essential Points

• Apoptosis typically lacks an inflammatory response, so light microscopy may miss it even when tissue damage is extensive.
• Early necrosis shows cell swelling, whereas apoptosis shows cell shrinkage plus later nuclear fragmentation and apoptotic body formation.
• Apoptotic cells expose phosphatidylserine on the outer plasma membrane leaflet, which is recognized by macrophage receptors to promote clearance.
• In the mitochondrial pathway, pro-apoptotic BAX and BAK oligomerize in the outer mitochondrial membrane after BH3-only activation, enabling leakage of cytochrome c.
• In the Fas-mediated extrinsic pathway, Fas ligand trimerization recruits FADD and activates caspase-8, which then drives the same executioner caspase sequence as the intrinsic pathway.

💡 Memory Hook

Morphology: Shrink, Condense, Blebb → Bodies; Pathways: Mito (BAX/BAK→cytochrome c→caspase-9) or Fas (FADD→caspase-8→caspase-3/6).

📖 6. Autophagy and necroptosis

🔑 Key Concepts & Definitions

• Autophagy : Autophagy is a conserved process that delivers cytoplasmic material to lysosomes for degradation, helping cells survive stress such as nutrient deprivation.
• Autophagosome : An autophagosome is a double-membrane vesicle that sequesters cytoplasmic cargo before fusing with lysosomes for degradation.
• LC3 lipidation : LC3 lipidation is the conjugation of LC3 to PE during autophagy, making LC3 a marker for identifying cells where autophagy is occurring.
• Necroptosis : Necroptosis is a genetically controlled programmed necrosis that resembles necrosis morphologically while proceeding without caspase activation.
• RIPK1-RIPK3-MLKL pathway : The RIPK1-RIPK3-MLKL signaling axis is the necroptosis mechanism in which RIPK1/RIPK3 activation leads to MLKL-dependent plasma membrane disruption.

📝 Essential Points

• Autophagy is triggered by cues such as nutrient deprivation or growth factor depletion that activate an initiation complex to recruit Atg proteins to form isolation membranes.
• Autophagosome maturation occurs via fusion with lysosomes, and the inner membrane and enclosed cargo are digested by lysosomal hydrolases for metabolite recycling.
• Necroptosis is initiated by receptor-ligand signaling, with TNFR1 ligation being a widely studied trigger.
• Necroptosis requires RIPK1 and RIPK3, leading to MLKL phosphorylation, MLKL oligomer formation, MLKL membrane translocation, and plasma membrane disruption.
• Necroptosis evokes inflammatory cell death because release of cellular contents accompanies membrane rupture, even though caspases are not activated.

💡 Memory Hook

Autophagy: LC3 PE-lipidation tags the autophagosome; Necroptosis: RIPK1→RIPK3→MLKL makes membrane pores.

📖 7. Oxidative stress and membrane damage

🔑 Key Concepts & Definitions

• Reactive oxygen species : Reactive oxygen species are oxygen-derived free radicals that can exceed antioxidant defenses and damage key cellular components.
• Oxidative stress : Oxidative stress is the buildup of damaging free radicals when production increases or antioxidant removal decreases.
• Lipid peroxidation : Lipid peroxidation is oxidative damage to membrane phospholipids in which free radicals attack unsaturated fatty-acid double bonds.
• Lipid free radical propagation : Lipid free radical propagation is an autocatalytic chain reaction where early lipid damage generates new reactive intermediates and amplifies membrane injury.
• SOD catalase scavenging : SOD and catalase are antioxidant enzymes that neutralize ROS by converting superoxide to hydrogen peroxide and decomposing hydrogen peroxide to water and oxygen.

📝 Essential Points

• ROS form during normal oxidative phosphorylation when oxygen is incompletely reduced, producing superoxide, hydrogen peroxide, and hydroxyl radicals.
• Oxidative stress can also follow reperfusion because restored oxygen with weakened antioxidant defenses promotes new free radical generation.
• Lipid peroxidation damages membranes by OH-driven attack on unsaturated fatty-acid double bonds, yielding unstable peroxides that trigger propagation.
• Iron and copper can catalyze ROS formation via the Fenton reaction, which converts hydrogen peroxide into hydroxyl radicals.
• Cells limit ROS via antioxidants and enzymes including SOD, catalase in peroxisomes, and glutathione peroxidase to detoxify hydrogen peroxide.

💡 Memory Hook

Oxidative stress = More ROS or Less Scavenging; ROS then “peroxidize lipids” by OH → unstable peroxides → membrane-propagation chain.

📖 8. Hypoxia and reperfusion injury

🔑 Key Concepts & Definitions

• Ischemia : Ischemia is hypoxia caused by reduced blood flow, most often from mechanical arterial obstruction, that compromises delivery of substrates for energy production.
• Hypoxia : Hypoxia is reduced oxygen availability while blood flow is maintained enough for glycolysis to continue for a time via anaerobic energy generation.
• Ischemia-reperfusion injury : Ischemia-reperfusion injury is paradoxical cell damage and death that occurs when blood flow is restored to previously ischemic tissue.
• Oxidative stress in reperfusion : Oxidative stress in reperfusion is increased reactive oxygen and nitrogen species generation when oxygen is reintroduced after ischemia.

📝 Essential Points

• In ischemia, both aerobic metabolism fails and anaerobic glycolysis eventually stops after glycolytic substrates are exhausted or glycolysis is inhibited by metabolite buildup.
• Reperfusion injury can kill cells that might have recovered from reversible ischemia by initiating new damaging processes during reoxygenation.
• ROS and reactive nitrogen species in reperfused tissue can be driven by incomplete oxygen reduction in leukocytes, damaged endothelium, and parenchymal cells.
• Reperfusion exacerbates intracellular Ca2+ overload via influx due to membrane damage and ROS-mediated sarcoplasmic reticulum injury, promoting mitochondrial permeability transition and ATP depletion.
• Neutrophil recruitment amplifies reperfusion injury, and blocking cytokines or adhesion molecules can reduce neutrophil extravasation and tissue damage.
• Complement activation may worsen injury when IgM is deposited in ischemic tissue and complement binds and activates upon blood flow restoration.

💡 Memory Hook

Ischemia is oxygen-starved; reperfusion is oxygen-sparked—ROS + Ca2+ + neutrophils + complement = fresh damage.

📖 9. Chemical, infectious, and immune injury

🔑 Key Concepts & Definitions

• Direct toxicity : Direct toxicity is a chemical injury mechanism where a chemical binds critical cellular components and disrupts essential function.
• Toxic metabolite conversion : Toxic metabolite conversion is a chemical injury mechanism in which an initially inactive compound is metabolized into a reactive product that damages cells.
• Inflammation in reperfusion : Inflammation in reperfusion is an immune-driven injury process that recruits neutrophils to reperfused tissue and worsens damage.
• Complement activation : Complement activation is an immune injury mechanism where complement proteins bind activated targets and amplify injury and inflammation.
• IgM antibody deposition : IgM antibody deposition is the immune event where some IgM antibodies preferentially lodge in ischemic tissues and serve as triggers for complement binding.

📝 Essential Points

• Mercuric chloride injury is caused by mercury binding to sulfhydryl groups on membrane proteins, increasing membrane permeability and inhibiting ion transport.
• In direct toxic injury, the most affected cells are those that initially absorb, use, excrete, or concentrate the chemical, starting with reversible swelling and later progressing to necrosis.
• Many toxic chemicals are bioactivated when cytochrome P-450 mixed-function oxidases convert them into reactive metabolites that injure cells via free radical formation and lipid peroxidation.
• Reperfusion-related inflammation involves danger signals from dead cells, cytokines from resident immune cells, and increased adhesion molecule expression that recruits circulating neutrophils.
• Some IgM antibodies deposit in ischemic tissues and, after blood flow returns, complement binds and activates locally to exacerbate cell injury and inflammation.

💡 Memory Hook

Think “P-450 makes peroxides, IgM calls complement, neutrophils finish the damage.”

📖 10. Cellular adaptations to stress

🔑 Key Concepts & Definitions

• Hypertrophy : Hypertrophy is an increase in cell and organ size, typically driven by increased workload and growth-factor signaling in tissues that cannot divide.
• Hyperplasia : Hyperplasia is an increase in cell numbers produced by hormones or growth factors, occurring in tissues whose cells can divide or that contain stem cells.
• Atrophy : Atrophy is a decrease in cell and organ size caused by reduced nutrient supply or disuse, reflecting less synthesis and more protein breakdown.
• Metaplasia : Metaplasia is a reversible switch in differentiated cell phenotype, often induced by chronic irritation to make cells more stress-tolerant.

📝 Essential Points

• Cardiac hypertrophy can switch myosin heavy chain isoforms from an α isoform to a β isoform, producing slower and more energy-economical contraction.
• Hypertrophy signaling activates transcription factors such as GATA4, NFAT, and MEF2, increasing muscle-protein gene expression including fetal/embryonic genes like ANF.
• Hyperplasia requires cells capable of division, and it can be physiologic (e.g., compensatory liver regeneration) or pathologic when hormone/growth-factor stimulation is excessive or inappropriate.
• Atrophy mechanisms include reduced protein synthesis with less trophic signaling and increased protein degradation via the ubiquitin-proteasome pathway plus often increased autophagy.
• Metaplasia commonly changes columnar to squamous epithelium in chronic irritation, and the altered lining may lose protective functions (mucus/cilia) and, if persistent, predispose to malignant transformation.

💡 Memory Hook

HAT-M: Hypertrophy = bigger cells, Hyperplasia = more cells, Atrophy = less tissue, Metaplasia = swapped cell type.

📖 11. Intracellular accumulations and pigments

🔑 Key Concepts & Definitions

• Intracellular accumulations : Intracellular accumulations are abnormal build-ups of substances in cytoplasm, organelles, or nucleus that may be harmless or cause further cell injury.
• Steatosis (fatty change) : Steatosis is the abnormal accumulation of triglycerides in parenchymal cells, classically seen in the liver and often linked to reversible injury.
• Lysosomal storage diseases : Lysosomal storage diseases are inherited disorders where defective lysosomal enzymes prevent degradation of metabolites, driving progressive intracellular accumulation.
• Lipofuscin : Lipofuscin is an insoluble wear-and-tear pigment made of lipid-phospholipid polymers plus protein that marks oxidative injury but is not itself harmful to cells.
• Hemosiderin : Hemosiderin is a golden yellow-brown iron-storage pigment formed from ferritin micelles when there is local or systemic iron excess.

📝 Essential Points

• Intracellular accumulations arise from inadequate removal, folding/transport/secretion defects, lysosomal enzyme failure, or deposition of indigestible exogenous materials.
• If the triggering overload is controlled or stopped, intracellular accumulation can be reversible, but inherited storage diseases usually progress and may injure tissue.
• Fatty change involves abnormal triglycerides in cells, and phospholipids can accumulate as components of myelin figures in necrotic cells.
• Glycogen appears as clear cytoplasmic vacuoles on routine histology because it dissolves in aqueous fixatives, and PAS or Best carmine stains glycogen rose-to-violet.
• Carbon pigment causes anthracosis by being taken up by alveolar macrophages and transported to tracheobronchial lymph nodes, blackening lungs and those nodes.

💡 Memory Hook

Reversible overload vs progressive lysosomal failure: stop the insult to reverse, but enzyme-less lysosomes keep accumulating.

📖 12. Calcification and cellular aging

🔑 Key Concepts & Definitions

• Pathologic calcification : Pathologic calcification is abnormal deposition of calcium salts in tissues along with smaller amounts of other mineral salts.
• Dystrophic calcification : Dystrophic calcification is calcium deposition in sites of cell injury or necrosis despite normal serum calcium and without calcium-metabolism derangements.
• Metastatic calcification : Metastatic calcification is calcium deposition in otherwise normal tissues, usually driven by hypercalcemia from disturbed calcium metabolism.
• Replicative senescence : Replicative senescence is a permanent nondividing growth arrest reached after a limited number of cell divisions.
• Telomere attrition : Telomere attrition is progressive shortening of chromosomal ends during replication that ultimately triggers cell cycle arrest.

📝 Essential Points

• Dystrophic calcification occurs in necrosis areas (coagulative, caseous, or liquefactive) and in foci of enzymatic fat necrosis.
• Dystrophic calcification is usually associated with advanced atherosclerotic atheromas and commonly develops in aging or damaged heart valves.
• Serum calcium remains normal in dystrophic calcification, whereas metastatic calcification arises with hypercalcemia and can affect gastric mucosa, kidneys, lungs, systemic arteries, and pulmonary veins.
• On H&E staining, calcium salts appear as basophilic amorphous granular deposits that can be intracellular, extracellular, or both.
• Cells from children can undergo more replication rounds than cells from older people due to replicative senescence.
• Caloric restriction increases longevity and is linked to reduced IGF-1 signaling and increased sirtuins, which influence DNA repair and other stress responses.

💡 Memory Hook

Think DYSTROPHIC = Damage with Normal calcium; METASTATIC = Normal tissue with high calcium.

📅 Key Dates

📊 Synthesis Tables

Necrosis vs apoptosis (key features)

⚠️ Common Pitfalls & Confusions

1. Mistaking apoptosis for “unregulated” cell death; apoptosis is regulated by caspases and distinct intrinsic/extrinsic pathways.
2. Thinking the point of no return is precisely defined; it is still largely nebulous and is linked to irreversible mitochondrial/lysosomal dysfunction.
3. Confusing the ischemia timeline: swelling occurs within minutes, irreversibility within 1–2 h, but light-microscopic death may appear only at 4–12 h.
4. Mixing necrosis patterns and their causes (e.g., caseous is granuloma-like in tuberculosis; liquefactive is pus-like in focal bacterial infections; fat necrosis needs pancreatic lipases and chalky calcium soaps).
5. Forgetting that necrosis is associated with inflammatory response due to membrane rupture and DAMP release, while apoptosis typically lacks an inflammatory response.
6. Assuming autophagy always kills cells; it is primarily a survival mechanism under nutrient deprivation and only contributes to death when inadequate to cope.
7. Interchanging oxidative stress and reperfusion injury mechanisms; reperfusion adds ROS and Ca2+ overload and also involves neutrophils and complement in some settings.

✅ Exam Checklist

1. Define homeostasis and adaptation, and state the stage sequence from normal cell to adaptation to reversible injury to irreversible injury to cell death.
2. List major categories of injurious stimuli, including oxygen deprivation, physical agents, chemical agents/drugs, infectious agents, immunologic reactions, and genetic and nutritional causes.
3. Explain the progression principle that early biochemical changes can occur within minutes to hours before light-gross changes (hours to days).
4. Identify the earliest morphology of reversible injury (cell swelling; hydropic/vacuolar change) and key ultrastructural features (blebbing, mitochondrial swelling, ER dilation, loss of microvilli/eosinophilia).
5. State what characterizes irreversibility and link it to inability to restore mitochondrial dysfunction and severe damage to lysosomal membranes.
6. Differentiate necrosis and apoptosis by at least five morphologic/functional features (cell size, nucleus changes, plasma membrane integrity, inflammation, and clearance/contents leakage).
7. Match necrosis patterns to characteristic morphology and example triggers: coagulative (architecture preserved, infarcts), liquefactive (pus; brain infarcts), caseous (tuberculosis granuloma-like border), fat (chalky calcium soaps), fibrinoid (immune complexes with bright pink vessel walls).
8. Describe apoptosis morphology (cell shrinkage, chromatin condensation, blebbing/apoptotic bodies) and state why apoptosis is hard to detect by light microscopy.
9. Give the two main apoptosis pathway initiations and convergence: mitochondrial (BAX/BAK, cytochrome c, caspase-9) and death receptor (Fas/FADD, caspase-8, shared executioner caspases).
10. Explain how autophagy works stepwise (isolation membrane/autophagosome formation, fusion with lysosomes, LC3 lipidation as marker) and why it is generally protective under stress.
11. For regulated necrosis mechanisms, contrast necroptosis (TNFR1, RIPK1/RIPK3, MLKL pores; inflammatory contents release) with pyroptosis (inflammasome → caspase-1 → IL-1 activation) and ferroptosis (iron-dependent lipid peroxidation; caspase-independent).
12. From hypoxia/ischemia to reperfusion, summarize key mechanisms: ATP depletion in ischemia, then in reperfusion oxidative stress, Ca2+ overload, neutrophil inflammation, and complement activation with IgM deposition in some cases.