Pathology

Necrosis Irreversible cell and tissue injury • mechanisms • morphology • patterns • inflammation • clinical recognition • emergency implications Necrosis is the morphologic pattern of cell or tissue death that follows severe injury and is characterised by loss of membrane integrity, enzymatic digestion of cellular components and an inflammatory response. It is most often caused by ischaemia, toxins, infection or physical injury. In modern cell-death biology, necrosis may be the final appearance of accidental destruction or of a regulated lytic pathway such as necroptosis or pyroptosis. For emergency medicine learners, necrosis is not merely a histology term. It represents tissue that has crossed the point of no return, may release dangerous intracellular contents, and can create complications such as shock, sepsis, hyperkalaemia, compartment syndromes, organ failure and permanent loss of function. Learning outcomes Define necrosis and distinguish it from reversible cell injury and apoptosis. Explain ATP depletion, calcium overload, ROS, mitochondrial failure, lysosomal digestion and membrane rupture in necrotic death. Recognise nuclear changes (pyknosis, karyorrhexis and karyolysis) and cytoplasmic changes. Identify coagulative, liquefactive, caseous, fat, fibrinoid and gangrenous patterns and their usual settings. Explain the inflammatory consequences of DAMP release and how necrosis enlarges tissue injury. Interpret clinical, laboratory, imaging and histopathology findings in common necrotic lesions. Apply emergency management principles to infarction, sepsis, burns, trauma, pancreatitis, limb ischaemia and toxic injury. 1. Definition and essential features Necrosis is cell death with structural disintegration and usually uncontrolled release of intracellular contents into the extracellular space. Four linked features are central: Irreversible energy failure: mitochondria cannot restore oxidative phosphorylation or ATP. Membrane damage: plasma, mitochondrial and lysosomal membranes lose selective permeability and rupture. Enzymatic digestion: endogenous lysosomal enzymes and sometimes leukocyte enzymes digest the dead tissue. Inflammation: DAMPs activate innate immunity, causing vascular leakage and leukocyte recruitment. Feature Necrosis Reversible injury Apoptosis Cell size Usually swelling (oncosis). Swelling may reverse. Shrinkage. Membrane Severe damage and rupture. Blebs/permeability changes may recover. Intact until apoptotic-body clearance. Nucleus Pyknosis, karyorrhexis, karyolysis and disappearance. Chromatin clumping may recover. Condensation and orderly fragmentation. Contents Leak into tissue and blood. Remain mostly contained. Remain in membrane-bound fragments. Inflammation Usually prominent. Minimal from the injured cell. Usually minimal. Energy dependence Overwhelming injury and ATP failure. Energy reserve remains. Requires organised signalling and ATP. 2. Pathogenesis: from injury to necrotic tissue 2.1 Initiating insults Ischaemia, hypoxia, toxins, infection, immune injury, heat, cold, radiation and mechanical trauma can start the cascade. The final morphology depends on dose, duration, organ, tissue architecture, presence of microbes and the speed of inflammation. 2.2 ATP depletion and failure of ion pumps Loss of oxidative phosphorylation lowers ATP. The Na+/K+-ATPase fails, sodium and water enter, potassium leaves and the cell swells. The Ca2+ pump fails, so cytosolic calcium rises. Ribosomes detach from rough ER, protein synthesis falls and the cytoskeleton loses repair capacity. 2.3 Calcium-dependent destruction High cytosolic Ca2+ activates phospholipases, proteases, endonucleases and ATPases. Phospholipases destroy membrane phospholipids; proteases degrade cytoskeletal proteins; endonucleases fragment DNA; ATPases consume remaining energy. Calcium also promotes mitochondrial permeability transition. 2.4 Reactive oxygen species (ROS) Damaged mitochondria, xanthine oxidase, activated neutrophils and reperfusion generate superoxide, hydrogen peroxide and hydroxyl radicals. ROS cause lipid peroxidation, protein oxidation and DNA damage. Antioxidants such as glutathione and catalase may be overwhelmed. 2.5 Mitochondrial permeability transition Persistent pore opening collapses the proton gradient, stops ATP production and causes mitochondrial swelling. Severe ATP failure prevents orderly apoptosis and favours necrosis. Damaged mitochondria release their own DAMPs, including mitochondrial DNA and ATP, further activating inflammation. 2.6 Lysosomal leakage and autodigestion Lysosomal membranes become permeable. Acid hydrolases, proteases, lipases and nucleases digest cytoplasmic and nuclear structures. Inflammatory leukocytes add neutrophil proteases and ROS. Liquefaction is especially prominent when enzymatic digestion is extensive, as in brain infarction and abscesses. 2.7 Membrane rupture and DAMP release Membrane phospholipid loss, cytoskeletal breakdown and osmotic swelling produce blebs and discontinuities. Once the membrane ruptures, intracellular enzymes and DAMPs enter the interstitium and circulation. These signals activate pattern-recognition receptors on macrophages, neutrophils and endothelial cells. 3. Morphology of necrosis 3.1 Cytoplasmic changes Increased eosinophilia: RNA loss and protein denaturation make the cytoplasm intensely pink. Glassy or homogeneous cytoplasm: loss of glycogen and ribosomes. Vacuolation: swollen mitochondria and fragmented organelles. Myelin figures: whorled phospholipid remnants of damaged membranes. Cell outlines: may persist as pale “ghost cells” in coagulative necrosis or disappear completely in liquefactive necrosis. 3.2 Nuclear changes Term Description Mechanism Pyknosis Shrunken, intensely basophilic nucleus with dense chromatin. Chromatin condensation and DNA compaction. Karyorrhexis Fragmentation of the pyknotic nucleus. Endonuclease-mediated DNA cleavage and mechanical fragmentation. Karyolysis Fading and dissolution of nuclear staining. DNase digestion of chromatin. All three changes may be seen during necrosis, often progressing from pyknosis to karyorrhexis and then karyolysis. The timing varies by organ and type of injury. 4. Patterns of necrosis 4.1 Coagulative necrosis Coagulative necrosis is caused mainly by ischaemia in solid organs. Protein denaturation predominates over enzymatic digestion, so the basic tissue architecture remains visible for several days. Cells are eosinophilic, anucleate and appear as “ghosts.” The affected area is often firm, pale and wedge-shaped. Typical organs: heart, kidney and spleen; it can occur in liver and other solid organs. Classic cause: arterial infarction, except in the brain. Inflammation: neutrophils enter early, followed by macrophages; later fibrosis replaces the infarct. Clinical consequence: permanent loss of the infarcted functional tissue and scar formation. 4.2 Liquefactive necrosis Liquefactive necrosis occurs when enzymatic digestion dominates. The tissue becomes soft, liquid or cystic. Neutrophils and microbial enzymes contribute in abscesses; resident microglia and lysosomes digest the dead tissue after a brain infarct. Brain infarction: dead neural tissue is removed, leaving a fluid-filled cavity and gliosis. Abscess: pus contains neutrophils, bacteria, necrotic debris and protein-rich fluid. Clinical risk: raised intracranial pressure, mass effect, rupture into ventricles or bloodstream and sepsis. 4.3 Caseous necrosis Caseous necrosis has a soft, friable, granular, cheese-like appearance. It is classically found in tuberculosis and some deep fungal infections within granulomas. Microscopically, the necrotic centre lacks cellular detail and is surrounded by epithelioid

Causes and Types of Cell Death Why cells die • physiologic and pathologic death • accidental and regulated pathways • morphology • inflammation • clinical interpretation Cell death is the permanent loss of cellular function and structural integrity. It is essential during development and tissue maintenance, but it becomes disease when too many cells die, when the wrong cells die, or when death occurs at the wrong time. Emergency clinicians need to recognise both the cause of death and the mechanism because the causes are often reversible during the tissue-at-risk window even though an established dead cell is not. This lesson provides the classification framework used before studying necrosis and apoptosis in depth. It deliberately separates what started the injury, how the cell died and what the dead cell looks like in tissue. Learning outcomes Define cell death and distinguish it from reversible injury and loss of function. Classify cell death by purpose (physiologic or pathologic), predictability (accidental or regulated) and morphology. Explain major causes: hypoxia/ischaemia, physical agents, chemicals, infections, immune injury, nutritional-metabolic disease, genetic disorders and ageing. Compare apoptosis, necrosis, necroptosis, pyroptosis, ferroptosis, parthanatos, mitochondrial permeability transition-driven death and other regulated pathways. Explain why necrosis is a morphological pattern whereas many modern death pathways are molecular programmes. Link death pathways to DAMP release, inflammation, organ failure and emergency treatment. Use history, examination, laboratory tests, imaging and pathology to infer the most likely cause and type of death. 1. What counts as cell death? A cell is considered dead when it has irreversibly lost the capacity to maintain homeostasis, restore energy production and preserve the plasma membrane. A transient loss of function, ATP depletion or membrane blebbing may still be reversible. In practice, death is inferred from a combination of biochemical, structural and functional evidence. Concept Definition Clinical example Reversible injury Stress impairs function but the cell can recover if the cause is removed. Transient renal tubular hypoxia during dehydration. Irreversible injury Mitochondrial failure and membrane/nuclear damage make recovery impossible. Prolonged coronary occlusion causing infarct-core necrosis. Cell death Permanent loss of cellular function and structural integrity, by accidental destruction or an organised death programme. Apoptosis of autoreactive lymphocytes; necrosis in a brain infarct. Organ failure Loss of enough cells or tissue architecture that an organ can no longer meet physiologic demands. Acute kidney injury after shock. Three questions to ask for every dead-cell pattern What was the initiating cause? (for example, ischaemia, toxin, infection or immune attack) Which molecular pathway operated? (for example, caspase-dependent apoptosis or iron-dependent ferroptosis) What is the tissue morphology and inflammatory consequence? (for example, coagulative necrosis with neutrophils) 2. Classification by purpose and setting 2.1 Physiologic cell death Physiologic death is controlled, proportionate and beneficial. It shapes organs, removes cells that have completed their task and maintains a stable cell population. It is most often apoptosis, although other regulated pathways may contribute in selected tissues. Physiologic process Why cells die Example Embryologic remodelling Removes transient structures and sculpts organs. Separation of fingers and toes; neural development. Hormone-dependent involution Loss of trophic stimulation activates an orderly death programme. Endometrial shedding; post-lactational breast involution. Immune-cell contraction Removes excess lymphocytes after an immune response. Death of activated T cells after clearance of infection. Cell turnover Replaces old cells while preserving barriers and tissue function. Intestinal epithelial cells migrating to the villus tip. Removal of damaged cells Prevents propagation of cells carrying dangerous mutations. Apoptosis after irreparable DNA damage. Resolution of inflammation Removes spent neutrophils and limits collateral injury. Macrophage clearance of apoptotic neutrophils. 2.2 Pathologic cell death Pathologic death follows an injurious stimulus or an inappropriate activation of a death pathway. It can be focal, as in an infarct, or systemic, as in multi-organ injury during severe shock. The same trigger may cause more than one pathway depending on severity, cell type, oxygen availability and the timing of treatment. 3. Classification by predictability: accidental versus regulated 3.1 Accidental cell death Accidental cell death (ACD) is an immediate or near-immediate consequence of overwhelming physical, chemical or mechanical injury. The cell is destroyed before a coordinated signalling programme can be mounted. Severe burns, crushing, membrane-solubilising toxins and extreme osmotic injury are examples. ACD usually has necrotic morphology, membrane rupture and inflammation. 3.2 Regulated cell death Regulated cell death (RCD) depends on genetically encoded signalling and can be modified by inhibitors, gene defects or changes in the cellular environment. RCD is not necessarily “peaceful”: necroptosis, pyroptosis and ferroptosis are regulated yet often lytic and inflammatory. Regulation describes the mechanism, not the clinical severity. Classification axis Examples What it predicts Physiologic vs pathologic Developmental apoptosis vs infarction. Whether death is part of normal maintenance or disease. Accidental vs regulated Crush injury vs apoptosis/necroptosis. Whether a molecular programme can be modulated. Lytic vs non-lytic morphology Necrosis/pyroptosis vs apoptosis. Likelihood of DAMP release and inflammation. Inflammatory vs immunologically quiet Pyroptosis vs rapid efferocytosis. Extent of local and systemic inflammatory response. 4. Causes of cell death 4.1 Hypoxia, ischaemia and loss of oxygen delivery Hypoxia is insufficient oxygen; ischaemia is reduced blood flow and therefore also limits glucose delivery and waste removal. Ischaemia is usually more damaging than hypoxia alone. Oxidative phosphorylation falls, ATP is depleted, ion pumps fail and cells swell. If the insult persists, mitochondrial permeability transition and membrane rupture produce irreversible death. Reduced arterial flow: thrombosis, embolus, atherosclerotic occlusion or vasospasm. Reduced venous drainage: congestion, torsion and strangulation increase tissue pressure and reduce the effective perfusion gradient. Systemic oxygen-delivery failure: shock, severe anaemia, respiratory failure or carbon-monoxide poisoning. Microvascular failure: sepsis, disseminated intravascular coagulation and endothelial swelling create patchy tissue hypoxia. 4.2 Physical agents Agent Cellular damage Examples Heat Protein denaturation, membrane disruption, dehydration and vascular injury. Thermal burns, hyperthermia and heat stroke. Cold Ice crystals, vasoconstriction, ischaemia and reperfusion injury. Frostbite and accidental hypothermia. Mechanical force Direct membrane and cytoskeletal rupture; crush-related ischaemia. Trauma, compartment syndrome and blast injury. Radiation DNA strand breaks and ROS; rapidly dividing cells are vulnerable. Radiotherapy, nuclear exposure and excessive ultraviolet light. Electricity Thermal burns, arrhythmia,

Irreversible Cell Death Point of no return • mitochondrial failure • membrane destruction • nuclear breakdown • necrosis • apoptosis • emergency recognition Cell injury becomes irreversible when the cell can no longer restore mitochondrial energy production and membrane integrity even after the original stress is removed. The final result is cell death. This is not a single instant that can be measured at the bedside; it is a biologic transition inferred from persistent biochemical failure, structural destruction, leakage of intracellular contents and characteristic patterns of tissue injury. This lesson is written for emergency medicine and pathology learners. It links molecular mechanisms to the signs, laboratory results, histology and urgent decisions encountered in shock, myocardial infarction, stroke, sepsis, poisoning, severe infection, pancreatitis and major trauma. Learning outcomes Define irreversible cell injury and explain the point of no return. Describe the two canonical biochemical events: failure to reverse mitochondrial dysfunction and severe membrane damage. Explain the roles of ATP depletion, calcium overload, reactive oxygen species (ROS), permeability transition, lysosomal enzymes, ER stress and DNA damage. Recognise cytoplasmic, nuclear and ultrastructural changes that distinguish reversible injury from cell death. Compare necrosis, apoptosis, necroptosis, pyroptosis and ferroptosis without confusing morphology with mechanism. Interpret common biomarkers of tissue necrosis while accounting for timing, organ specificity and false positives. Apply the concepts to emergency presentations and decide what can still be salvaged. 1. Orientation: injury, adaptation and death State What is happening? Can the cell recover? Typical example Homeostasis Energy production, ion gradients, protein turnover and membrane trafficking are balanced. Yes; this is the normal state. Well-perfused skeletal muscle at rest. Adaptation Reversible changes in size, number, phenotype or metabolism help the cell tolerate a new demand. Usually yes if the stress is removed. Hypertrophy in pressure overload; atrophy during immobilisation. Reversible injury ATP falls, ion pumps fail and metabolism shifts, but the membranes and mitochondria retain enough reserve to recover. Yes, if treatment is early and adequate. Transient hypoxia causing cellular swelling. Irreversible injury / cell death Persistent mitochondrial failure and severe membrane or nuclear damage lead to self-amplifying destruction. No; removal of the original stress cannot restore the dead cell. Prolonged infarction with coagulative necrosis. Core exam statement Irreversibility is defined by functional failure, not simply by the duration of exposure. The same period of ischaemia may be reversible in one tissue and fatal in another. Neurons tolerate severe hypoxia for only minutes, while some connective tissues remain viable longer. Temperature, collateral blood supply, metabolic demand, previous disease and the speed of reperfusion all alter the threshold. 2. The point of no return The point of no return is the stage at which the molecular damage becomes self-sustaining. Even if oxygen and nutrients are restored, the cell cannot rebuild ATP, reseal its membranes or control destructive enzymes. There is no single universal switch; rather, several linked failures reinforce one another. 2.1 The two canonical irreversible events Canonical event Mechanism Visible consequences Inability to reverse mitochondrial dysfunction Persistent electron-transport failure, loss of membrane potential, mitochondrial permeability transition and severe oxidative damage prevent oxidative phosphorylation from restarting. ATP remains critically low; Na+/K+-ATPase and Ca2+ pumps fail; biosynthesis stops; the cell cannot maintain volume or membrane gradients. Severe loss of membrane integrity Plasma, mitochondrial, lysosomal and ER membranes are damaged by lipid peroxidation, phospholipase activation, calcium-dependent proteases, cytoskeletal breakdown and physical rupture. Ion and water influx, enzyme leakage, lysosomal digestion, organelle swelling, release of DAMPs and an inflammatory response. 2.2 Why the failure becomes self-amplifying ATP depletion prevents active transport and repair. Calcium accumulation activates phospholipases, proteases, endonucleases and ATPases, which consume the remaining energy and dismantle membranes, cytoskeleton and DNA. ROS generation damages lipids, proteins and nucleic acids; injured mitochondria generate still more ROS. Membrane rupture releases enzymes and DAMPs, amplifying inflammation and secondary tissue injury. Lysosomal leakage allows acid hydrolases to digest cytoplasmic and nuclear components. 3. Molecular mechanisms of irreversible injury 3.1 Mitochondrial injury and permeability transition Mitochondria are both the energy source and a death-signalling organelle. Hypoxia, toxins, calcium overload and ROS open the mitochondrial permeability transition pore (MPTP) in the inner mitochondrial membrane. The proton gradient collapses, oxidative phosphorylation stops and the organelle swells. ATP synthase may reverse and consume residual ATP rather than produce it. Energy failure: persistent ATP depletion prevents ion pumping, protein synthesis, membrane repair and orderly autophagy. Outer-membrane permeabilisation: release of cytochrome c activates the intrinsic apoptotic pathway when the cell still has enough organisation for programmed death. Necrotic mitochondrial failure: when ATP is profoundly depleted, swelling and rupture dominate, producing necrotic death and inflammation. ROS amplification: damaged complexes I and III leak electrons to oxygen, producing superoxide, hydrogen peroxide and hydroxyl radicals. 3.2 Calcium overload Cytosolic calcium is normally kept extremely low. Failure of the plasma-membrane Ca2+ pump, release from the ER and influx through injured membranes raise intracellular calcium. Calcium activates: Target Result Phospholipases Breakdown of membrane phospholipids, loss of membrane barriers and generation of inflammatory lipid mediators. Proteases (calpains) Degradation of cytoskeletal proteins and membrane anchors; cell becomes mechanically fragile. Endonucleases DNA and chromatin fragmentation. ATPases Further ATP consumption during a period when production is failing. Mitochondrial permeability pathways Loss of membrane potential, cytochrome c release and ROS production. 3.3 Oxidative stress and lipid peroxidation ROS are generated by injured mitochondria, activated leukocytes, xanthine oxidase, cytochrome P450 metabolism, radiation and reperfusion. Antioxidant systems (glutathione, catalase, superoxide dismutase and vitamins) may be overwhelmed. Lipid peroxidation makes membranes leaky and disrupts receptors, channels and mitochondrial cristae. Protein oxidation alters enzymes, transporters, contractile proteins and structural scaffolds. DNA oxidation causes strand breaks, mutations and activation of p53-dependent death pathways. Reperfusion injury can produce a burst of ROS when oxygen returns to a cell whose antioxidant capacity is depleted. 3.4 Plasma-membrane and cytoskeletal destruction Phospholipase activation removes membrane phospholipids; proteases degrade spectrin and other cytoskeletal elements; ATP depletion prevents replacement of damaged lipids. The membrane loses selective permeability, so sodium, calcium and water enter while potassium, metabolites and enzymes leave. Eventually it blebs and ruptures. Loss of

Reversible Cell Injury: ATP Depletion, Cellular Swelling, Fatty Change, Causes, Diagnosis and Recovery Reversible cell injury is an early, potentially recoverable disturbance in cell function and structure caused by a stress that has not yet destroyed essential membranes, mitochondrial recovery or genomic integrity. The two classic morphologic patterns are cellular swelling (hydropic change) and fatty change, especially in the liver and heart. The clinical priority is to identify and remove the cause before the cell crosses into irreversible injury and death. Rescue principle: restore oxygen delivery, perfusion, glucose and electrolytes; stop the toxin or offending drug; treat infection or obstruction; and reassess rapidly. Reversibility is time- and cause-dependent. Learning outcomes Define reversible cell injury and distinguish it from adaptation and irreversible injury. Explain ATP depletion, ion-pump failure, anaerobic glycolysis, acidosis and reduced protein synthesis. Describe cellular swelling, fatty change, ER/ribosome changes, cytoskeletal injury and autophagy. Recognise common causes in emergency medicine and organ-specific presentations. Interpret early laboratory, imaging and microscopic clues. Apply practical steps that restore viability and prevent progression to cell death. 1. Where reversible injury fits State Cellular condition Can it recover? Adaptation Cell changes size, number, phenotype or metabolism while maintaining viability. Usually, if stimulus is removed. Reversible injury ATP and function fall; swelling/fatty change and organelle disturbance occur, but basic membrane integrity and repair capacity remain. Yes, if the cause is removed promptly. Irreversible injury Mitochondria cannot recover; severe membrane/DNA damage and calcium/ROS amplification occur. No; progresses to death. Cell death Necrosis, apoptosis or another regulated pathway removes the cell. The individual cell cannot recover; tissue may repair or scar. 2. Common causes Cause Typical reversible mechanism Clinical example Hypoxia/ischaemia Reduced oxidative phosphorylation and ATP. Early shock, transient coronary or renal hypoperfusion. Physical injury Membrane/cytoskeletal stress, heat/cold or pressure changes. Early burn, frostbite, electrical injury or trauma. Chemicals/drugs Enzyme inhibition, reactive metabolites, mitochondrial toxicity or osmotic imbalance. Early paracetamol, alcohol, carbon-monoxide or medication toxicity. Infection Microbial competition, toxins, inflammation and altered metabolism. Early viral hepatitis, sepsis-related organ dysfunction. Nutritional/metabolic imbalance Fat accumulation, osmotic shifts, glucose/energy failure. Fatty liver, diabetic metabolic stress, electrolyte disorders. Immune/inflammatory injury Cytokine, complement, ROS and protease effects. Early hypersensitivity or inflammatory tissue injury. 3. ATP depletion: the initiating lesion 3.1 Why ATP falls Reduced oxygen delivery limits mitochondrial oxidative phosphorylation. Direct mitochondrial toxins inhibit electron transport or ATP synthase. Substrate deficiency, severe hypoglycaemia or metabolic block limits fuel. Excessive ATP consumption occurs during seizures, hyperthermia, muscle activity or ion pumping against injury. 3.2 The energy-failure cascade ATP-dependent process Early consequence Morphologic/clinical clue Na+/K+-ATPase Sodium and water enter; potassium leaves. Cellular swelling, weight gain, tissue oedema. Ca2+ pumps Cytosolic calcium rises. Enzyme activation, contractile dysfunction and arrhythmia risk. Protein synthesis Ribosomes detach from rough ER. Reduced enzymes, receptors and repair proteins. Membrane/lipid repair Phospholipids are not replaced adequately. Blebs, increased permeability and leakage if prolonged. Cytoskeletal maintenance Microvilli and junctions lose structure. Reduced absorption/barrier function and epithelial sloughing. 4. Anaerobic glycolysis and acidosis As oxidative phosphorylation declines, cells increase anaerobic glycolysis to generate limited ATP. Glycogen stores are consumed; lactate and inorganic phosphate accumulate; intracellular pH falls. Chromatin may clump, enzyme activity changes and contractile function falls. In myocardium, acidosis reduces contractility and can contribute to arrhythmia. In skeletal muscle, lactate and ATP depletion cause weakness and pain. In brain, energy failure disrupts ion gradients and synaptic function, producing confusion or seizures. In kidney, tubular transport and concentrating ability fall before structural necrosis is obvious. 5. Cellular swelling and hydropic change Cellular swelling is the most common early manifestation of reversible injury. Water enters because ATP-dependent ion gradients fail, intracellular proteins/osmoles accumulate and membrane permeability changes. Hydropic change is marked by clear cytoplasmic vacuoles representing distended endoplasmic reticulum and other organelles. Level Finding Gross Organ may be pale, heavier, tense and swollen; tissue function is impaired. Light microscopy Cell enlargement, pale granular cytoplasm, small vacuoles and sometimes membrane blebs. Ultrastructure Plasma-membrane blebs, loss of microvilli, ER dilation, mitochondrial swelling and ribosome detachment. Function Reduced transport, secretion, contractility, conduction or filtration depending on organ. Swelling is not specific for one cause. It can result from hypoxia, toxins, infection, metabolic disturbance or membrane injury; the clinical context is essential. 6. Fatty change Fatty change (steatosis) is abnormal accumulation of triglyceride within parenchymal cells. It is common in hepatocytes because the liver handles lipid uptake, synthesis, oxidation and export, but it can also occur in myocardium, skeletal muscle and kidney. 6.1 Mechanisms Increased delivery of free fatty acids to the cell. Increased triglyceride synthesis or esterification. Reduced beta-oxidation from hypoxia, mitochondrial injury or toxins. Reduced apoprotein synthesis or impaired lipoprotein export. Excessive alcohol metabolism altering redox state and lipid handling. Pattern Appearance Clinical note Microvesicular steatosis Many small droplets, nucleus remains central. Seen in selected toxins, pregnancy-related liver disease and metabolic disorders; may be severe despite small droplets. Macrovesicular steatosis One or few large droplets displace the nucleus. Common in metabolic dysfunction, alcohol-related disease and obesity; may regress if the cause is corrected. Cardiac fatty change Fine droplets in myocytes, sometimes producing a “tigered” appearance grossly. Hypoxia, toxins and metabolic disease can reduce contractile reserve. Steatosis can be reversible, but persistent lipotoxicity, inflammation and fibrosis can progress to steatohepatitis and organ failure. 7. Endoplasmic reticulum and protein synthesis Ribosome detachment reduces synthesis of enzymes, membrane proteins and structural components. Rough ER dilates; smooth ER may proliferate in response to drug detoxification. The unfolded-protein response temporarily reduces translation and increases chaperones. Secretory cells are particularly vulnerable because they have high protein-folding demands. Persistent ER stress activates CHOP and apoptosis pathways. Smooth-ER proliferation can increase cytochrome P450 metabolism and drug interactions. Impaired protein synthesis slows membrane repair and makes the cell less able to recover. 8. Mitochondrial and lysosomal changes 8.1 Mitochondria Early mitochondrial swelling and reduced cristae reflect energy stress. If the inner membrane remains recoverable, reoxygenation can restore ATP. Persistent calcium/ROS causes permeability transition, loss of membrane potential and irreversible death. 8.2 Lysosomes and autophagy Autophagic vacuoles remove damaged organelles and recycle substrates. This is

Cell Injury and Cell Death: Causes, Mechanisms, Reversible Injury, Irreversible Death, Necrosis and Apoptosis Cell injury occurs when a stress exceeds the cell’s ability to adapt and maintain homeostasis. The initial changes may be reversible if the cause is removed. Persistent or severe injury damages membranes, mitochondria, DNA and essential proteins so that the cell crosses a point of no return and dies. Cell death may occur by necrosis, apoptosis or other regulated pathways such as necroptosis, pyroptosis and ferroptosis. Clinical sequence: stimulus → adaptation (if possible) → reversible injury → irreversible injury → cell death → inflammation or silent clearance → repair, scarring or organ failure. Learning outcomes Define cell injury, reversible injury, irreversible injury and cell death. Classify major causes of cell injury and explain how intensity, duration and cell type alter the outcome. Describe ATP depletion, mitochondrial dysfunction, calcium influx, oxidative stress, membrane damage, protein misfolding and DNA injury. Distinguish reversible injury from irreversible injury by function, biochemistry and morphology. Compare necrosis, apoptosis and other regulated cell-death pathways. Relate cellular mechanisms to emergency presentations such as shock, hypoxia, poisoning, sepsis, myocardial infarction and organ failure. 1. Homeostasis, adaptation and injury State Cellular condition Potential outcome Homeostasis Energy production, ion gradients, protein quality, membranes and organelles remain within functional limits. Normal structure and function. Adaptation Reversible change in size, number, phenotype or metabolism accommodates a stress. New steady state; may regress when the stimulus stops. Reversible injury Functional/metabolic disturbance with preserved basic membrane integrity and recoverable damage. Recovery if oxygen, nutrients or cause are restored promptly. Irreversible injury Severe mitochondrial dysfunction and membrane/DNA damage prevent recovery. Necrosis, apoptosis or another cell-death pathway. Cell death Loss of essential cellular function and structural integrity, or controlled dismantling. Inflammation, silent clearance, regeneration or scar. 2. Major causes of cell injury Cause Mechanisms Examples Oxygen deprivation Hypoxia, ischaemia, anaemia or respiratory failure reduce oxidative phosphorylation and ATP. Myocardial infarction, shock, stroke, strangulated bowel. Physical agents Trauma, temperature extremes, radiation, electricity, pressure and noise damage membranes/DNA. Burns, frostbite, fractures, radiation dermatitis, electrical injury. Chemicals/drugs Direct toxicity, reactive metabolites, enzyme inhibition, membrane dissolution or osmotic injury. Paracetamol overdose, cyanide, carbon monoxide, alcohol, heavy metals. Infectious agents Direct cytotoxicity, toxins, invasion and dysregulated immune response. Viral cytolysis, bacterial sepsis, fungal invasion. Immune reactions Antibodies, complement, immune complexes, T cells and cytokines damage host tissue. Autoimmunity, hypersensitivity, transplant rejection. Genetic defects Abnormal proteins, enzymes, membranes, DNA repair or organelles increase vulnerability. Mitochondrial disease, storage disease, channelopathy. Nutritional imbalance Deficiency, excess, malabsorption and metabolic overload disturb energy and structure. Protein-energy malnutrition, obesity-related lipotoxicity, vitamin deficiency. Ageing Reduced repair, mitochondrial dysfunction, senescence and accumulated oxidative/genomic injury. Frailty, neurodegeneration, reduced organ reserve. 3. What determines severity? Type, duration and intensity: brief mild hypoxia may be reversible; prolonged severe ischaemia causes infarction. Cell type: neurons have limited glycolytic reserve; hepatocytes tolerate some hypoxia; skeletal muscle can adapt to intermittent stress. Metabolic state: energy stores, nutrient status, mitochondrial reserve and prior disease alter tolerance. Blood supply and collateral flow: organs with collateral circulation may survive an arterial obstruction longer. Temperature and pH: hypothermia may slow metabolism, while fever/acidosis can increase injury. Genetics and age: DNA repair, antioxidant capacity and regenerative reserve vary. Competing stresses: infection, shock, drug exposure and inflammation amplify each other. 4. Central mechanisms of cell injury 4.1 ATP depletion and mitochondrial failure Reduced oxygen or mitochondrial toxins decrease oxidative phosphorylation and ATP. Ion pumps fail, sodium and water enter, the endoplasmic reticulum dilates, protein synthesis falls and anaerobic glycolysis raises lactate. Persistent mitochondrial permeability transition collapses the membrane potential, stops ATP production and may release cytochrome c to trigger apoptosis. ATP-dependent process Effect of failure Na+/K+-ATPase Cell swelling, membrane blebs and potassium loss. Ca2+ pumps Cytosolic calcium rises and activates destructive enzymes. Protein synthesis and folding Ribosome detachment, ER stress and reduced repair. Actin/cytoskeletal maintenance Loss of microvilli, membrane blebs and impaired trafficking. Membrane/lipid maintenance Phospholipid loss, increased permeability and eventual rupture. 4.2 Mitochondrial damage Mitochondria are both energy factories and injury sensors. Damage causes ATP failure, increased reactive oxygen species, calcium dysregulation and release of pro-apoptotic proteins. Mild stress can be contained by mitophagy; severe damage causes irreversible injury. 4.3 Calcium influx and enzyme activation Raised cytosolic calcium activates phospholipases, proteases, endonucleases and ATPases. These break membrane phospholipids, cytoskeletal proteins and DNA while consuming remaining ATP. Mitochondrial calcium overload amplifies oxidative stress and permeability transition. 4.4 Oxidative stress Reactive oxygen species (ROS) arise from mitochondria, phagocytes, radiation, toxins and reperfusion. Antioxidant systems include glutathione, catalase, superoxide dismutase, vitamins and Nrf2-driven gene expression. Excess ROS cause lipid peroxidation, protein oxidation, DNA strand breaks and mitochondrial damage. 4.5 Membrane damage Plasma membrane damage causes leakage of enzymes/proteins and loss of ion gradients. Mitochondrial membrane damage reduces ATP and releases apoptotic proteins. Lysosomal membrane damage releases hydrolases that digest cytoplasm. Endoplasmic-reticulum membrane disruption impairs protein folding and calcium storage. 4.6 Protein misfolding and DNA damage Heat, toxins, oxidative stress and mutations produce misfolded proteins. Chaperones, unfolded-protein response and proteasomes initially restore proteostasis. Persistent misfolding activates apoptosis. DNA damage activates repair and p53-mediated arrest; if repair fails, apoptosis or senescence limits propagation of mutations. 5. Reversible cell injury Reversible injury is a functional and structural disturbance that can return toward normal after removal of the cause. The earliest biochemical changes often precede visible morphology. Mechanism Typical change Microscopic/gross correlate Moderate ATP fall Ion-pump failure and increased anaerobic glycolysis. Cellular swelling, hydropic change and organelle dilation. Ribosome detachment Reduced protein synthesis. ER dilation and loss of basophilia. Fatty change Triglyceride accumulation from impaired oxidation/export. Vacuoles in hepatocytes or myocytes; may be reversible. Cytoskeletal disruption Loss of microvilli and surface specialisation. Blebs, irregular shape and reduced transport. Autophagy/proteostasis response Recycling and chaperone induction. Autophagic vacuoles or stress proteins. Symptoms and biomarkers depend on the organ. Mild reversible myocardial injury may produce transient enzyme elevation; reversible tubular injury may reduce concentrating ability before creatinine rises. 6. Irreversible injury and the point of no return The exact molecular point varies, but two consistent events

Mechanisms of Infection: Colonisation, Invasion, Virulence, Immune Evasion, Tissue Injury and Sepsis Mechanisms of infection describe how an infectious agent reaches a host, attaches to a suitable site, survives local defences, multiplies, invades or disseminates, damages tissue and interacts with the immune system. Infection is therefore a dynamic host–pathogen process—not merely the presence of a microorganism in a specimen. Clinical sequence: exposure → adhesion/colonisation → multiplication → invasion or toxin effect → local inflammation → dissemination or containment → resolution, persistence, chronic disease or sepsis. Learning outcomes Describe the stages from exposure to clinically significant infection. Explain adhesion, colonisation, invasion, replication, tissue tropism, biofilm formation and dissemination. Compare toxin-mediated, immune-mediated and direct cytopathic tissue injury. Explain innate and adaptive immune evasion, latency, intracellular survival and persistence. Recognise local, systemic, chronic, latent and opportunistic patterns. Connect mechanisms to specimen choice, diagnosis, treatment, source control and sepsis recognition. 1. Infection is an interaction, not a culture result Term What it means Clinical example Exposure Contact with an infectious agent through a portal of entry. Inhalation of respiratory particles, ingestion of contaminated food, needlestick or mosquito bite. Colonisation Adhesion and multiplication without tissue injury or symptoms. Nasal Staphylococcus carriage or intestinal microbiota. Infection Agent survives, multiplies or interacts with host tissues, with or without symptoms. Asymptomatic malaria parasitaemia or early viral infection. Disease Clinical dysfunction or tissue injury due to pathogen and/or host response. Pneumonia, meningitis, cellulitis or diarrhoeal disease. Invasion Crossing epithelial barriers, entering tissue, lymphatics or blood. Cellulitis progressing to bacteraemia; intestinal pathogen entering mucosa. Dissemination Spread from a primary focus to remote sites through blood, lymph, nerves, airways or tissue planes. Miliary TB, meningococcaemia, disseminated herpes or metastatic abscesses. 2. Stage 1—entry and overcoming barriers 2.1 Physical and chemical barriers Intact skin, mucus, cilia, cough, tears and urine physically remove agents. Acid in the stomach, bile, lysozyme, defensins and antimicrobial peptides inhibit organisms. Normal microbiota compete for nutrients and attachment sites. Macrophages, dendritic cells and epithelial pattern-recognition receptors detect danger rapidly. 2.2 How pathogens breach barriers Strategy Examples Mechanical/traumatic entry Needle, bite, wound, surgery, burns, catheter or insect proboscis. Adhesion to mucosa Respiratory viruses, uropathogenic E. coli, Vibrio, Neisseria and enteric pathogens. Enzyme-mediated penetration Proteases, hyaluronidase, collagenase, neuraminidase or phospholipases disrupt tissue/secretions. Vector or vehicle delivery Mosquito inoculates Plasmodium; contaminated water delivers enteric agents. Intracellular entry Viruses use receptors/endocytosis; bacteria may induce phagocytosis or invade epithelial cells. 3. Stage 2—adhesion and colonisation To establish a focus, the agent must remain at a site long enough to resist flow, mucus, peristalsis, urine, cilia and immune clearance. Adhesins bind host receptors, often in a tissue- and species-specific manner. This determines tropism. Mechanism Pathogen advantage Clinical consequence Fimbriae/pili/adhesins Specific attachment to epithelial receptors. Urinary tract infection, pharyngeal colonisation or enteric disease. Viral receptor binding Determines susceptible cell type and organ. Respiratory, hepatic, neural or immunologic tropism. Capsule/slime layer Reduces phagocytosis and retains water/nutrients. Persistent colonisation and invasive disease. Biofilm matrix Anchors communities to tissue or devices and slows antimicrobial penetration. Chronic wound, catheter, prosthetic-joint or dental infection. Antigenic variation Changes exposed proteins and delays adaptive clearance. Recurrent or chronic infection. Colonisation may be protective or harmful depending on site. A commensal organism can cause infection after barrier breach, aspiration, immunosuppression or movement to a normally sterile compartment. 4. Stage 3—multiplication and nutrient acquisition Successful pathogens obtain carbon, nitrogen, iron and other nutrients while coping with temperature, pH, oxygen tension, osmotic stress and immune attack. Bacteria may secrete siderophores, viruses redirect host metabolism, fungi use hyphae and parasites alter host nutrient pathways. 4.1 Microbial growth and inoculum A high inoculum may overwhelm local defences before adaptive immunity develops. Growth rate and generation time influence incubation and acute severity. Slow-growing or dormant organisms may produce chronic disease and require prolonged treatment. Quorum sensing coordinates biofilm, toxin or virulence-gene expression in some bacteria. 4.2 Host factors that alter multiplication Diabetes, malnutrition, neutropenia, HIV, steroid therapy, pregnancy and extremes of age. Acid suppression, broad-spectrum antibiotics, surgery, burns and devices. Obstruction/stasis in urinary, biliary, respiratory or gastrointestinal systems. Prior immunity, vaccination and protective microbiota. 5. Stage 4—local invasion and tissue spread Invasion occurs when organisms or their products cross an epithelial barrier and enter tissue. Spread may follow planes, lymphatics, blood, nerves, airways or contiguous extension. Spread pattern Mechanism Example Contiguous extension Enzymes, pressure, necrosis or tissue planes carry infection locally. Cellulitis, necrotising fasciitis, sinus infection spreading to orbit. Lymphatic spread Organisms/cells enter lymphatics and regional nodes. Lymphangitis and tender regional lymphadenopathy. Bloodstream spread Transient or sustained bacteraemia/viraemia/fungaemia. Sepsis, endocarditis, metastatic abscesses or organ seeding. Neural spread Some viruses move along peripheral nerves. Herpesviruses and rabies-related neuroinvasion. Airway/aspiration spread Secretions reach lower airways or adjacent structures. Aspiration pneumonia and endobronchial dissemination. Transplacental or perinatal Agent crosses placenta or contacts infant during birth/feeding. Congenital infection or neonatal sepsis. 6. Stage 5—virulence factors Virulence factor Mechanism Pathology Adhesins Bind host receptors and resist clearance. Stable colonisation of mucosa or devices. Capsules Block phagocytosis/complement and promote persistence. Invasive meningitis, pneumonia or sepsis. Enzymes Break extracellular matrix, clots or antimicrobial molecules. Deep invasion, abscess extension and tissue destruction. Secretion systems Inject or release effector proteins into host cells. Cytoskeletal manipulation, intracellular survival and inflammation. Biofilms Organised matrix protects a community and creates slow-growing subpopulations. Device-associated, chronic and relapsing infection. Iron acquisition Siderophores and receptor systems overcome host iron sequestration. Growth in blood and inflamed tissue. Antigenic variation Changes surface antigens or switches expression. Persistent/recurrent infection and vaccine escape in selected settings. Intracellular residence Survives in epithelial cells, macrophages or other protected niches. Chronic infection, poor antibiotic access and relapse. 7. Toxin-mediated injury 7.1 Exotoxins Exotoxins are microbial proteins released or injected into host cells. Some inhibit protein synthesis, disrupt membranes, alter neurotransmission, activate signalling pathways or cause intense cytokine release. Toxin effect Pathophysiologic result Clinical pattern Neurotoxicity Blocks neurotransmitter release or causes neuronal dysfunction. Flaccid/spastic paralysis or altered autonomic function. Enterotoxicity Alters ion transport and secretion in intestinal epithelium. Watery diarrhoea, dehydration and electrolyte loss. Cytotoxicity Damages membranes, ribosomes or intracellular

Transmission Mechanisms of Infectious Diseases: The Chain of Infection, Routes and Prevention Transmission is the movement of an infectious agent from a reservoir or source to a portal of entry in a susceptible host. The same organism may use more than one route, and a single patient may acquire infection through community, household, occupational, healthcare-associated, sexual, foodborne or vector-borne exposures. Understanding the route allows clinicians to choose isolation, PPE, vaccination, environmental controls, post-exposure prophylaxis and public-health action. Emergency-medicine principle: apply standard precautions to every patient, add route-specific precautions as soon as the syndrome suggests them, and avoid delaying life-saving care while arranging appropriate isolation and notification. Learning outcomes Describe the six links in the chain of infection. Distinguish direct contact, indirect contact, droplet, airborne, vehicle-borne, vector-borne, bloodborne, sexual and vertical transmission. Explain portals of exit and entry, reservoirs, fomites, vectors, inoculum and host susceptibility. Apply transmission-based precautions in emergency and healthcare settings. Recognise healthcare-associated, iatrogenic, zoonotic and food/water routes. Select practical measures that break each link and respond to exposures or outbreaks. 1. The chain of infection Transmission requires a connected sequence. Infection-prevention measures work by breaking one or more links. Link Question Examples Ways to break it 1. Infectious agent What organism or agent is present, and how hardy/virulent is it? Bacterium, virus, fungus, parasite or prion; resistance to drying/disinfectant. Diagnosis, treatment, vaccination, sterilisation, disinfection and antimicrobial stewardship. 2. Reservoir Where does the organism live and multiply? Human, animal, soil, water, food, equipment, skin, respiratory or gastrointestinal tract. Source control, environmental cleaning, animal/vector control, isolation and safe waste management. 3. Portal of exit How does it leave the reservoir? Respiratory secretions, stool, urine, blood, wound drainage, genital fluids, skin scales, placenta. Respiratory etiquette, masks, wound covering, safe sharps, continence care and PPE. 4. Mode of transmission How does it travel? Contact, droplet, airborne, vehicle, vector, blood/needle, sexual or vertical routes. Hand hygiene, PPE, ventilation, cleaning, safe food/water, vector control and safe injection. 5. Portal of entry How does it enter the next host? Respiratory mucosa, conjunctiva, mouth, broken skin, genitourinary tract, placenta, invasive device. Barrier precautions, aseptic technique, vaccination, wound care, condoms and device bundles. 6. Susceptible host Who lacks adequate defence? Infants, older adults, pregnancy, diabetes, HIV, neutropenia, malnutrition, unvaccinated people. Vaccination, prophylaxis, nutrition, chronic-disease control, early treatment and protective isolation. 2. Direct and indirect transmission Pattern Definition Examples Key prevention Direct contact Immediate transfer by skin-to-skin or mucosal contact without an intermediate object. Scabies, herpes lesions, sexual contact, touching draining wounds. Hand hygiene, gloves, covering lesions, contact precautions and treating cases/contacts. Indirect contact Transfer through a contaminated inanimate object or surface (fomite). Shared equipment, bed rails, phones, instruments, contaminated medication vials. Cleaning/disinfection, dedicated equipment, safe injection and hand hygiene. Droplet spread Respiratory particles from coughing, sneezing, talking or procedures reach mucosa at close range before settling or dispersing. Influenza, pertussis and some meningococcal respiratory exposures. Source masking, distance, eye/face protection, surgical mask and ventilation. Airborne spread Small suspended particles or droplet nuclei remain airborne and travel beyond immediate close contact under suitable conditions. Measles, pulmonary tuberculosis and varicella are classic examples; organism-specific rules apply. Airborne isolation/respirator, negative-pressure room where available, ventilation and limiting movement. The familiar “droplet ≤5 micrometres” threshold is a teaching aid rather than a universal biologic boundary. Particle behaviour depends on humidity, airflow, procedure, pathogen and environment. Follow current local and organism-specific guidance. 3. Respiratory transmission in detail 3.1 Source and particle generation Respiratory agents leave through the nose or mouth in breathing, talking, coughing, sneezing, suctioning, intubation, bronchoscopy, nebulisation and other procedures. Particle size, viral/bacterial load, ventilation, humidity, distance and duration determine exposure. Situation Risk amplifier Control Close conversation/coughing High source load, face-to-face proximity and prolonged time. Source mask, distance, ventilation, staff mask/eye protection where indicated. Aerosol-generating procedure High particle generation and exposure to airway secretions. Plan procedure, minimise staff, use appropriate respirator/eye protection, ventilation and decontamination. Poorly ventilated room Accumulation of suspended particles. Fresh-air exchange, appropriate filtration/air cleaning and reduced occupancy. Transport of patient Shared spaces expose other patients and staff. Mask the patient if safe, notify receiving area, use the shortest route and clean equipment. 3.2 Droplet versus airborne precautions Droplet: place patient in a single room/cohort where possible, use a medical mask for close care, eye protection for splash risk and mask the patient during transport. Airborne: use an airborne infection isolation room or equivalent ventilation when available, fit-tested respirator, door closed, limit transport and mask the patient. Uncertainty: if a high-consequence airborne infection is possible, use the highest practical precaution while seeking infection-prevention and public-health advice. 4. Contact transmission 4.1 Direct contact Transmission occurs when infected skin, mucosa, lesions, secretions or body fluids contact a susceptible host. The dose and integrity of the recipient’s skin/mucosa matter. Sexual transmission is a specialised direct-contact route involving genital, anal or oral mucosa and body fluids. 4.2 Indirect contact and fomites Organisms survive on equipment, surfaces, fabrics, hands, phones, keyboards or medication containers. The risk depends on environmental survival, inoculum, cleaning, hand contact and portals of entry. A fomite is not the same as a reservoir: an object may transfer an organism without supporting multiplication. High-risk practice Why it transmits Safer practice Using one stethoscope between patients without cleaning Moves skin/respiratory organisms between hosts. Clean/disinfect between patients or dedicate equipment. Touching a sterile catheter or wound after contact with a surface Transfers organisms to a portal of entry. Hand hygiene and aseptic non-touch technique. Reusing multidose vials or needles unsafely Contaminates medication and inoculates patients directly. Safe injection policy, single-use equipment and correct vial handling. Moving contaminated linen without containment Disperses skin, stool or respiratory organisms. Bag at the point of use, avoid shaking and follow laundry policy. 5. Vehicle-borne transmission A vehicle is a non-living medium that carries an agent to multiple hosts. Common vehicles include food, water, blood, medications, biological products, inadequately sterilised instruments and contaminated solutions. Vehicle Examples Prevention Food/water Cholera, typhoid, norovirus, hepatitis A, Giardia and many foodborne toxins. Safe water, cooking, hand hygiene, sanitation, food storage and

Infectious Organisms: Bacteria, Viruses, Fungi, Parasites and Prions Infectious organisms are biological agents capable of entering a host, surviving or replicating, interacting with tissues and causing infection. An infection may be asymptomatic, colonising, local, invasive, disseminated or fatal. Not every microbe is pathogenic: the normal microbiota can protect against invasion, while disease depends on the agent, dose, route, virulence, host susceptibility and immune response. Emergency-medicine principle: identify the likely organism class early, obtain appropriate specimens before antimicrobials when this is safe, begin time-critical treatment when indicated, apply infection-prevention precautions and reassess when laboratory or clinical data change the probability. Learning outcomes Define infectious organism, pathogen, colonisation, infection, disease, virulence and opportunistic infection. Compare bacteria, viruses, fungi, protozoa, helminths, ectoparasites and prions by structure, replication and clinical behaviour. Recognise important bacterial, viral, fungal and parasitic patterns relevant to emergency presentations. Explain how toxins, adhesins, capsules, biofilms, intracellular survival, antigenic variation and immune evasion cause disease. Select specimen types and first-line diagnostic approaches without confusing colonisation with invasive infection. Use antimicrobial-stewardship principles and identify situations that require isolation, public-health notification or urgent referral. 1. Core definitions Term Meaning Example/clinical implication Microorganism/microbe A microscopic living organism or infectious entity; includes bacteria, fungi, protozoa and some parasites. Many are harmless or beneficial; classification alone does not prove disease. Pathogen An organism or agent able to cause disease under particular host and environmental conditions. Pathogenicity is the capacity to cause disease; it differs from severity. Colonisation Presence and multiplication on/in a host without tissue invasion or disease. Nasopharyngeal carriage of bacteria may not require antibiotics. Infection Entry and multiplication/interaction of an infectious agent in a host, with or without symptoms. Laboratory detection must be interpreted with signs, site and host factors. Infectious disease Clinical dysfunction or tissue injury caused by infection and host response. Pneumonia, meningitis, sepsis and malaria are diseases, not merely positive cultures. Virulence Degree of damage or severity produced by a pathogen in a particular host. Toxin production, invasion and immune evasion can increase virulence. Opportunistic pathogen An organism that causes disease when barriers or immunity are impaired. Candida, Pseudomonas or Pneumocystis may be dangerous in selected hosts. Pathogenicity island/virulence factor Gene or product that supports adhesion, invasion, toxin production, immune evasion or nutrient acquisition. Virulence may be gained through mutation, plasmids, phages or horizontal gene transfer. 2. The major classes at a glance Class Cellular status How it multiplies Typical treatment class Examples Bacteria Prokaryotic cells with DNA, ribosomes and a membrane/cell wall. Binary fission; some form spores or survive intracellularly. Antibacterials selected by syndrome, susceptibility and local guidance. Staphylococcus, Streptococcus, Enterobacterales, Mycobacterium. Viruses Acellular particles containing DNA or RNA in a capsid, sometimes an envelope. Obligate intracellular replication using host and viral machinery. Vaccines/prevention, supportive care and selected antivirals. Influenza, HIV, hepatitis viruses, herpesviruses, SARS-CoV-2. Fungi Eukaryotic yeasts, moulds or dimorphic organisms. Budding, fragmentation or spores; hyphae may invade tissue. Antifungals; source control and immune restoration are often critical. Candida, Cryptococcus, Aspergillus, dermatophytes. Protozoa Unicellular eukaryotic parasites. Asexual and/or sexual life-cycle stages, often in vectors or hosts. Antiprotozoal drugs and supportive/complication management. Plasmodium, Entamoeba, Giardia, Trypanosoma. Helminths Multicellular parasitic worms. Egg, larval and adult stages; some require intermediate hosts. Anthelmintics, surgery or treatment of inflammatory complications. Schistosoma, Taenia, Ascaris, hookworm. Ectoparasites Arthropods living on the skin or hair. Egg/nymph/adult or other arthropod life cycles. Topical/oral antiparasitic therapy and environmental control. Scabies mites, lice, ticks, fleas. Prions Abnormally folded infectious proteins without DNA or RNA. Template-directed misfolding of host prion protein. No proven curative antimicrobial; strict infection-control and specialist care. Creutzfeldt–Jakob disease and related transmissible spongiform encephalopathies. 3. Bacteria 3.1 Structure and physiology Bacteria are prokaryotic cells with a nucleoid, 70S ribosomes, cytoplasmic membrane and a cell wall of variable composition. They lack a membrane-bound nucleus and mitochondria. Their small size, rapid generation and ability to exchange genes allow rapid adaptation. Feature Clinical significance Cell wall Peptidoglycan provides shape and is a target for beta-lactams and glycopeptides. Gram staining reflects wall/envelope structure. Gram-positive envelope Thick peptidoglycan with teichoic acids; examples include Staphylococcus, Streptococcus and Enterococcus. Gram-negative envelope Thin peptidoglycan plus outer membrane containing lipopolysaccharide; endotoxin can drive systemic inflammation. Capsule/slime layer Reduces phagocytosis and supports adherence; important in pneumococcus, meningococcus, Klebsiella and others. Flagella/pili/fimbriae Motility, adhesion and conjugative gene transfer; urinary and intestinal colonisation often depends on adhesins. Biofilm Organised community in extracellular matrix on devices or tissues; reduces antimicrobial penetration and immune clearance. Spores Highly resistant survival forms in Bacillus and Clostridium; require rigorous decontamination and environmental control. Intracellular survival Some organisms survive in macrophages or epithelial cells, changing specimen collection and drug penetration requirements. 3.2 Shapes and arrangements Cocci: Staphylococci form clusters; streptococci and enterococci form chains/pairs; Neisseria are diplococci. Bacilli: Rods include Enterobacterales, Pseudomonas, Bacillus, Clostridium and mycobacteria. Curved/spiral organisms: Vibrio, Campylobacter, Helicobacter, Treponema and Borrelia require specific culture or molecular methods. Intracellular or wall-deficient organisms: Chlamydia, Rickettsia, Mycoplasma and Coxiella have distinctive diagnostic and treatment considerations. 3.3 Bacterial disease mechanisms Mechanism Example effect Adhesion Fimbriae/adhesins attach to respiratory, urinary or intestinal epithelium. Invasion Enzymes, secretion systems and motility breach barriers and enter tissue/blood. Exotoxins Secreted proteins produce specific effects such as neurotoxicity, enteric secretion or cytolysis. Endotoxin/LPS Activates innate immunity, cytokines, vasodilation, capillary leak and potentially septic shock. Immune evasion Capsules, antigenic variation, intracellular residence, IgA protease and complement resistance prolong infection. Tissue destruction and inflammation Direct cytotoxicity plus host neutrophil/immune response cause abscess, necrosis or organ dysfunction. 3.4 Clinically important bacterial patterns Pyogenic infection: fever, neutrophilia, pus, abscess, cellulitis, pneumonia or meningitis. Sepsis: infection-associated organ dysfunction due to dysregulated host response; the organism may be in blood or only at a focus. Intracellular/slow-growing infection: tuberculosis, brucellosis, rickettsial disease and atypical pneumonia may require special specimens, prolonged culture or molecular tests. Toxin-mediated illness: clinical disease can persist after the organism is absent because toxin effects continue. Device-associated infection: biofilms produce persistent, relapsing infection and may require device removal/source control. 4. Viruses 4.1 Structure Viruses contain a DNA or RNA genome enclosed in a protein capsid; some

Other Cellular Adaptations: Atrophy, Metaplasia, Autophagy, Senescence, Dysplasia and Stress Responses Not every adaptive response is an increase in cell size or number. Cells may shrink, change phenotype, recycle their components, enter a durable growth arrest, increase protective proteins or alter metabolism. These responses can preserve viability, but they may also reduce specialised function, produce fibrosis, create tissue vulnerability or mark a field at risk of neoplasia. Classification note: atrophy, metaplasia and autophagy are commonly taught as cellular adaptations. Senescence and dysplasia are related stress responses but are not identical: senescence is a durable cell-cycle arrest, while dysplasia is disordered atypical growth with premalignant potential. Learning outcomes Define atrophy, metaplasia, autophagy, senescence, dysplasia and intracellular stress responses. Explain the mechanisms and causes of physiologic and pathologic atrophy. Describe metaplasia as progenitor-cell reprogramming and recognise its major clinical examples. Explain how autophagy, proteostasis, antioxidant responses and the unfolded-protein response preserve cell survival. Distinguish senescence and dysplasia from reversible adaptation and from neoplasia. Apply the concepts to disuse, denervation, chronic irritation, reflux, ageing, cancer risk and clinical investigation. 1. Atrophy Atrophy is a reduction in cell size and often in tissue mass and function. Cells reduce protein synthesis, increase degradation and may lose organelles. If the stimulus is corrected before irreversible injury, atrophy can be partially or fully reversible. 1.1 Physiologic atrophy Example Mechanism Clinical meaning Embryologic involution Programmed loss of transient structures during development. Normal morphogenesis, not disease. Thymic involution Age- and hormone-related reduction of lymphoid tissue with fatty replacement. Normal ageing, although immune reserve changes. Postpartum uterine involution Loss of pregnancy-related hypertrophic/hyperplastic tissue through apoptosis and autophagy. Normal return toward pre-pregnancy size. Ovarian/endometrial involution Loss of trophic hormone stimulation with menopause. Reduced tissue thickness and reproductive function. 1.2 Pathologic atrophy Cause Mechanism Examples Disuse Reduced mechanical signalling, reduced protein synthesis and increased proteasomal/autophagic breakdown. Immobilised limb, prolonged bed rest, microgravity. Denervation Loss of neural trophic input and muscle activation. Peripheral nerve injury, motor-neuron disease. Reduced blood supply Chronic hypoperfusion limits oxygen and nutrient delivery. Renal atrophy distal to renal-artery stenosis; brain atrophy after chronic vascular disease. Inadequate nutrition/cachexia Catabolism supplies substrates; inflammatory cytokines worsen muscle and fat loss. Advanced cancer, chronic infection, severe malnutrition or heart failure. Loss of endocrine stimulation Target cells lose growth/survival signals. Endometrial or breast atrophy after menopause; prostate atrophy after androgen deprivation. Pressure Compression reduces blood flow and increases resorption. Bone erosion or renal parenchymal thinning from obstruction. Ageing Reduced anabolic signalling, mitochondrial dysfunction, senescence and cumulative injury. Sarcopenia, brain atrophy and gonadal involution. 1.3 Mechanisms of atrophy Reduced synthesis: low insulin/IGF-1, low mTOR activity and reduced transcription decrease production of contractile and structural proteins. Ubiquitin–proteasome degradation: proteins are tagged with ubiquitin and degraded into amino acids; muscle-specific ligases rise during disuse and illness. Autophagy: lysosomes digest organelles and cytoplasmic material, recycling substrates but potentially reducing cell mass when prolonged. Apoptosis: loss of trophic signals may remove individual cells, decreasing total tissue mass. Lipofuscin: residual non-degradable material can accumulate in long-standing atrophic cells, especially in ageing myocardium and liver. 2. Metaplasia Metaplasia is a reversible replacement of one mature cell type by another mature cell type that better tolerates a persistent stress. It usually reflects reprogramming of local stem/progenitor cells by cytokines, growth factors and extracellular-matrix signals—not direct conversion of one mature cell into another. Site/stimulus Change Adaptive benefit Cost/risk Bronchi in smokers or chronic irritant exposure Ciliated columnar epithelium → stratified squamous epithelium More resistant to smoke and abrasion. Loss of cilia/mucus clearance, infection and dysplasia/carcinoma risk. Endocervical transformation zone Columnar → squamous epithelium Better resists the vaginal environment. HPV-related dysplasia commonly begins in this vulnerable zone. Barrett oesophagus from reflux Squamous epithelium → intestinal-type columnar epithelium with goblet cells Improved resistance to acid and bile. Increased risk of dysplasia and oesophageal adenocarcinoma. Gastric intestinal metaplasia Gastric mucosa acquires intestinal phenotype after chronic injury/inflammation. Altered barrier and repair response. Marker of a field at risk for gastric neoplasia in selected settings. Vitamin A deficiency Mucosal columnar epithelium → squamous phenotype More robust surface barrier. Loss of mucus, ciliary and secretory function. Osseous/cartilaginous metaplasia Mesenchymal progenitors form bone/cartilage in injured soft tissue. May stabilise a chronic lesion. Can impair movement or complicate imaging/surgery. 2.1 Reversal and prevention Remove the stimulus: stop smoking, treat reflux, eradicate chronic infection where appropriate, correct nutritional deficiency and control inflammation. Reversal is possible but not guaranteed after long-standing injury. Metaplastic tissue requires site-specific surveillance when guidelines identify a meaningful dysplasia/cancer risk. 3. Autophagy and organelle quality control Autophagy is a lysosome-dependent recycling system. It is active at baseline and increases during starvation, hypoxia, infection, toxin exposure and organelle damage. It removes defective mitochondria, aggregates and portions of cytoplasm, then returns amino acids, fatty acids and nucleotides to the metabolic pool. Stage What happens Why it matters Stress sensing Low energy activates AMPK and suppresses mTOR; nutrient withdrawal permits autophagy initiation. Shifts the cell from growth to conservation and recycling. Phagophore formation A membrane grows around selected cytoplasmic cargo. Allows selective or bulk capture of damaged material. Autophagosome maturation The double-membrane vesicle closes and traffics toward lysosomes. Contains potentially toxic cargo. Fusion and digestion Autophagosome fuses with lysosome; hydrolases degrade cargo. Recycles substrates and removes defective organelles. Resolution or failure Autophagy falls when stress resolves; impaired clearance causes accumulation and injury. Relevant to neurodegeneration, infection, cancer and ischaemia/reperfusion. 3.1 Selective autophagy Mitophagy: removes damaged mitochondria and limits reactive oxygen species. ER-phagy: removes excess or damaged endoplasmic reticulum. Selective aggrephagy: clears protein aggregates. Xenophagy: targets some intracellular pathogens, although organisms may manipulate autophagy to survive. Autophagy is usually a survival response, not a synonym for cell death. Excessive or unsuccessful autophagy may accompany cell death, but morphology and molecular context are required. 4. Proteostasis and the unfolded-protein response Proteostasis maintains the correct synthesis, folding, trafficking and removal of proteins. Heat, oxidative stress, toxins, hypoxia and gene mutations can produce misfolded proteins. Response Mechanism Example Heat-shock response Heat-shock proteins act as chaperones to refold or direct damaged proteins to degradation. Fever, burns,

Cellular Hyperplasia: Regulated Cell Proliferation, Physiologic and Pathologic Forms, Organ Examples and Cancer Risk Hyperplasia is an increase in the number of cells in a tissue or organ caused by regulated proliferation of mature cells, progenitor cells or tissue stem cells. The organ may enlarge and function may increase, but the defining event is more cells, not simply larger cells. Hyperplasia can be physiologic, compensatory or pathologic. It usually regresses when the stimulus disappears, yet persistent stimulation, genomic instability or atypical architecture can create a field in which neoplasia is more likely. Core distinction: hyperplasia requires a cell population capable of entering the cell cycle. It is therefore common in epithelia, glands, liver and bone marrow, but it is not the principal response of mature cardiac myocytes or neurons. Learning outcomes Define hyperplasia and differentiate it from hypertrophy, regeneration, metaplasia, dysplasia and neoplasia. Explain cell-cycle entry, growth-factor signalling, stem/progenitor activation and termination of hyperplasia. Compare hormonal physiologic, compensatory physiologic and pathologic hyperplasia. Describe hyperplasia in endometrium, breast, prostate, thyroid, liver, bone marrow, skin, lymphoid tissue and other organs. Recognise the difference between benign regulated hyperplasia and atypical or premalignant proliferation. Apply the concepts to clinical cases, investigations, treatment and prevention of complications. 1. Hyperplasia and related concepts Concept Definition Relationship to hyperplasia Hyperplasia Increase in cell number through regulated cell proliferation. Primary topic; needs cells that can divide or a progenitor reserve. Hypertrophy Increase in individual cell size from increased protein/organelle synthesis. May accompany hyperplasia, especially in uterus and glands, but is a different mechanism. Regeneration Replacement of lost cells with the same mature cell type and restoration of tissue structure/function. Often uses compensatory hyperplasia, as in liver regeneration, but regeneration also requires appropriate architecture and differentiation. Metaplasia Replacement of one mature cell phenotype by another better adapted to chronic stress. May involve progenitor-cell reprogramming; it is not simply an increase in cell number. Dysplasia Disordered epithelial growth with atypia, abnormal maturation and architectural disturbance. May arise in a hyperplastic/metaplastic field and signals increased cancer risk. Neoplasia Clonal autonomous proliferation that persists despite removal of the initiating stimulus. Hyperplasia remains regulated; neoplasia escapes normal growth controls. 2. The cell-cycle basis of hyperplasia 2.1 From quiescence to proliferation Many adult cells rest in G0 until a growth signal appears. Mitogens activate surface receptors and intracellular pathways; cyclin D–CDK4/6 phosphorylates RB, releases E2F and permits the G1-to-S transition. DNA is replicated in S phase, chromosomes segregate in G2/M, and cytokinesis creates two daughter cells. Checkpoints, DNA repair and apoptosis prevent propagation of major errors. Step Key event Pathology implication Signal reception Hormone, growth factor, cytokine, antigen or mechanical signal binds a receptor. Excess ligand or receptor activation can produce excessive proliferation. Early response RAS–RAF–MEK–ERK, PI3K–AKT–mTOR, JAK–STAT and MYC increase growth and biosynthesis. Mutations in these pathways can convert regulated hyperplasia to neoplasia. Cell-cycle commitment Cyclins/CDKs phosphorylate RB; E2F activates DNA-synthesis genes. CDK inhibitors such as p21 and p27 restrain entry; loss of checkpoints increases atypia. DNA replication and mitosis Cell duplicates its genome, checks damage and divides. High mitotic rate may be physiologic, reactive, atypical or malignant depending on morphology. Termination Withdrawal of stimulus, contact inhibition, differentiation, apoptosis or senescence limits expansion. Failure of termination is a defining feature of neoplastic growth. 2.2 Stem and progenitor cells Hyperplasia often originates from tissue stem/progenitor cells rather than mature cells directly. The niche supplies Wnt, Notch, Hedgehog, BMP, cytokine and extracellular-matrix signals that balance self-renewal, differentiation and cell death. The liver, intestinal epithelium, skin, endometrium and bone marrow rely on such reserves. 3. Major categories of hyperplasia Category Trigger Examples Reversibility Hormonal physiologic hyperplasia Normal hormone or growth signal during a life stage or cycle. Endometrial proliferation, breast glandular growth in pregnancy. Usually regresses when the hormonal stimulus falls. Compensatory physiologic hyperplasia Partial tissue loss or increased demand. Liver regrowth after partial hepatectomy, bone-marrow response to blood loss. Usually stops when mass/function is restored. Pathologic hormonal hyperplasia Excessive, prolonged or unbalanced hormonal stimulation. Endometrial hyperplasia from unopposed oestrogen, benign prostatic hyperplasia. May regress after correcting the stimulus; atypical forms require surveillance/treatment. Reactive/inflammatory hyperplasia Antigen, cytokine, infection, injury or chronic irritation. Reactive lymphoid hyperplasia, epidermal hyperplasia in chronic dermatitis. Often regresses when inflammation/infection is controlled. Viral-associated hyperplasia Viral proteins alter cell-cycle and differentiation signals. Some HPV-related squamous proliferations and viral warts. Variable; persistent infection can cause dysplasia and cancer risk. 4. Hormonal physiologic hyperplasia 4.1 Endometrium Oestrogen stimulates proliferative-phase glands and stroma after menstruation. Progesterone then converts the endometrium to a secretory state. When hormonal cycling is coordinated, proliferation is controlled and shedding restores baseline. Persistent unopposed oestrogen—whether from anovulation, obesity, exogenous therapy or an oestrogen-producing lesion—can produce excessive glandular proliferation. 4.2 Breast During pregnancy, oestrogen, progesterone, prolactin and other signals expand lobular and ductal epithelium, combining hyperplasia and hypertrophy to prepare for lactation. After lactation, withdrawal of hormones and apoptosis return much of the tissue toward baseline. 4.3 Puberty and growth Endocrine signals stimulate hyperplasia of glandular, reproductive and supportive tissues during development. Normal growth differs from an isolated pathologic mass because it is coordinated with maturation, vascular supply and function. 5. Compensatory hyperplasia and regeneration 5.1 Liver The liver can restore mass after partial resection or cell loss. Cytokines such as IL-6 prime hepatocytes; growth factors including HGF and EGF then promote cell-cycle entry. Kupffer cells, stellate cells, endothelial cells and the extracellular matrix coordinate the response. Regeneration may restore mass without reproducing the exact original lobular arrangement if chronic inflammation or fibrosis is present. Healthy regenerative context Impaired or abnormal context Preserved architecture and adequate nutrition. Cirrhosis, fibrosis, ongoing viral injury, alcohol or malnutrition. Growth stops when functional mass is restored. Repeated injury drives nodules, altered vascular pathways and cancer risk. Proliferating hepatocytes retain differentiation. Dysplastic nodules or malignant clones may arise in a chronically injured field. 5.2 Bone marrow Hypoxia, tissue oxygen demand, erythropoietin, thrombopoietin, colony-stimulating factors and inflammatory cytokines expand selected marrow lineages. Acute blood loss or haemolysis can increase erythroid hyperplasia; infection can increase myeloid production. The pattern,