Pathogenesis of Diseases: Molecular, Cellular, Tissue and Organ Mechanisms

Learning objectives

• Define pathogenesis and distinguish it from etiology, morphology, clinical manifestations and natural history.
• Trace disease mechanisms across molecular, cellular, tissue, organ and whole-person levels.
• Explain the central mechanisms of cellular injury: ATP depletion, mitochondrial damage, calcium dysregulation, oxidative stress, membrane damage, protein misfolding and DNA injury.
• Describe how inflammation, immune responses, vascular changes, fibrosis, repair and abnormal cell growth contribute to disease.
• Apply mechanism-based reasoning to infection, myocardial infarction, sepsis, diabetes, cancer and shock.
• Recognise reversible versus irreversible stages and identify points where emergency intervention can change the pathway.

---

1. Definition and scope

Pathogenesis is the stepwise biological progression of a disease from its initiating cause to its structural changes, functional disturbances, clinical manifestations and outcome. It includes events at several levels:

Level	Question	Example in myocardial infarction
Molecular	Which genes, proteins, receptors, enzymes or mediators change?	Ischaemia alters ATP production, ion pumps and cell-death signalling.
Cellular	How do individual cells adapt, become injured, die or proliferate?	Cardiomyocytes lose contractility and undergo necrosis.
Tissue	What is the structural and inflammatory response of the tissue?	Necrotic myocardium triggers neutrophilic inflammation and later scar formation.
Organ	How does tissue injury impair organ physiology?	Reduced contractile myocardium lowers cardiac output.
Systemic	How does the body respond and what secondary effects appear?	Sympathetic activation, pulmonary oedema, arrhythmia or cardiogenic shock.
Clinical	What symptoms, signs, tests and complications result?	Chest pain, ECG changes, troponin rise and possible heart failure.

Etiology tells us what started the process; pathogenesis tells us how it progressed. One etiology can produce different diseases in different hosts, and different causes can converge on the same final pathway.

2. The general disease pathway

1. Susceptible host: genetics, age, immune status, nutrition and organ reserve influence the response.
2. Initiating event: infection, toxin, trauma, ischaemia, mutation, immune attack, nutritional deficiency or abnormal load.
3. Recognition: cellular sensors detect pathogen-associated or damage-associated molecular patterns, altered metabolites, mechanical stress or receptor stimulation.
4. Signal transduction: intracellular pathways alter transcription, metabolism, ion flux, cytoskeleton and cell–cell communication.
5. Cellular decision: adaptation, reversible injury, senescence, apoptosis, necrosis, autophagy, proliferation or malignant transformation.
6. Tissue response: oedema, inflammation, thrombosis, haemorrhage, regeneration, fibrosis, remodelling or tumour formation.
7. Organ dysfunction: impaired perfusion, oxygenation, filtration, secretion, contraction, conduction or neural function.
8. Clinical expression: symptoms, signs, laboratory abnormalities, imaging findings and syndromes.
9. Outcome: resolution, repair, chronic disease, relapse, complication, disability or death.

3. Host response determines the final disease

The same stimulus can cause a minor transient change in one person and fatal disease in another. Outcome depends on:

• Dose and duration: a brief low-dose exposure may be reversible; repeated or intense exposure may exceed repair capacity.
• Route and site: the same organism or toxin behaves differently in skin, lung, gut, blood or brain.
• Genetics: receptors, enzymes, haemoglobin, immune genes and detoxification pathways vary.
• Age and developmental stage: infants and older adults have different reserves and drug handling.
• Comorbidities: diabetes, renal failure, pregnancy, malnutrition and immunosuppression modify injury.
• Repair capacity: stem-cell reserve, blood supply, extracellular matrix and inflammation affect recovery.
• Time to treatment: early oxygenation, reperfusion, antibiotics, antidotes or haemorrhage control can stop progression.

4. Cellular adaptation before injury

Cells respond to increased demand or stress by changing size, number, phenotype or metabolism. Adaptation is often reversible; persistent stress can progress to injury.

Adaptation	Mechanism	Example
Hypertrophy	Increased cell size due to mechanical load or trophic signals; new proteins and organelles are produced.	Left-ventricular hypertrophy in hypertension; skeletal muscle after exercise.
Hyperplasia	Increased cell number through growth-factor-driven proliferation.	Endometrial proliferation, liver regeneration, benign prostatic hyperplasia.
Atrophy	Reduced cell size and function from decreased workload, blood supply, nutrition, innervation or hormonal stimulation.	Disuse muscle wasting or brain atrophy.
Metaplasia	Reversible replacement of one mature cell type by another better able to tolerate stress.	Bronchial squamous metaplasia in smokers.
Autophagy	Cells recycle organelles and proteins during nutrient deprivation or stress.	Protective early response that may become insufficient in prolonged injury.
Senescence	Stable growth arrest with altered secretory activity after stress or replicative exhaustion.	Ageing tissues and chronic inflammatory microenvironments.

Adaptation becomes pathological when it impairs function, predisposes to injury or persists after the stimulus is removed.

5. Central mechanisms of cellular injury

5.1 ATP depletion and energy failure

ATP is required for ion pumps, protein synthesis, membrane integrity, contraction and repair. Hypoxia, ischaemia, mitochondrial toxins and severe metabolic stress reduce oxidative phosphorylation.

• Na⁺/K⁺-ATPase failure causes sodium and water influx, cell swelling and potassium loss.
• Ca²⁺ pumps fail, allowing cytosolic calcium to rise.
• Anaerobic glycolysis increases, glycogen is consumed and lactic acid accumulates, lowering intracellular pH.
• Ribosomes detach from rough endoplasmic reticulum, reducing protein synthesis.
• Persistent energy failure leads to membrane damage, mitochondrial permeability and cell death.

5.2 Mitochondrial damage

Mitochondria produce ATP and regulate apoptosis. Injury may result from hypoxia, calcium overload, toxins, oxidative stress or inherited mitochondrial disorders.

• Loss of mitochondrial membrane potential reduces ATP production.
• Opening of permeability-transition pores causes swelling and failure of oxidative phosphorylation.
• Cytochrome c release activates caspases and apoptosis.
• Reactive oxygen species increase and damage lipids, proteins and DNA.

5.3 Calcium dysregulation

Normally, cytosolic calcium is kept low. Injury increases calcium entry from outside the cell and release from the endoplasmic reticulum. Calcium activates phospholipases, proteases, endonucleases and ATPases, damaging membranes, cytoskeleton, proteins and DNA. Calcium also worsens mitochondrial injury and activates both necrotic and apoptotic pathways.

5.4 Oxidative and nitrosative stress

Reactive oxygen species (ROS) and reactive nitrogen species are produced by mitochondria, phagocytes, oxidases, radiation and toxins. At controlled levels they signal adaptation and host defence; excessive levels produce oxidative stress.

Target	Damage	Clinical example
Lipids	Membrane lipid peroxidation increases permeability and disrupts organelles.	Reperfusion injury, toxin-induced liver injury.
Proteins	Oxidation, cross-linking or fragmentation alters enzymes, receptors and structural proteins.	Protein misfolding and neurodegeneration.
DNA	Base modification, strand breaks and mutations activate repair or cell death.	Radiation carcinogenesis, chronic inflammation-associated cancer.
Vascular mediators	NO inactivation and endothelial dysfunction impair vasodilation and perfusion.	Sepsis, atherosclerosis and pulmonary hypertension.

Antioxidant systems include superoxide dismutase, catalase, glutathione and dietary antioxidant pathways. These systems can be overwhelmed by severe injury.

5.5 Membrane and barrier damage

Plasma, mitochondrial, lysosomal and endoplasmic-reticulum membranes maintain compartments. Damage may result from lipid breakdown, cytoskeletal disruption, ROS, calcium-activated enzymes, toxins or mechanical injury.

• Plasma-membrane damage causes loss of contents, influx of ions and inability to maintain gradients.
• Mitochondrial membrane damage causes energy failure and apoptosis.
• Lysosomal membrane leakage releases hydrolases and digests cellular components.
• Endothelial barrier damage causes oedema, haemorrhage, thrombosis and impaired tissue perfusion.

5.6 Protein, endoplasmic-reticulum and DNA injury

Misfolded proteins activate the unfolded-protein response. If stress is corrected, translation slows and chaperones restore function; if stress persists, apoptosis is activated. DNA damage triggers cell-cycle arrest and repair. Unrepairable damage leads to apoptosis; inaccurate repair can produce mutation and neoplasia.

6. Reversible and irreversible injury

Feature	Reversible injury	Irreversible injury
Energy	ATP falls but can recover when the cause is removed.	Persistent mitochondrial dysfunction and inability to restore ATP.
Cell appearance	Cell swelling, fatty change, membrane blebs and organelle dilation.	Severe swelling, membrane rupture, lysosomal leakage and nuclear breakdown.
Membranes	Ion gradients are disturbed but membrane integrity remains sufficient.	Plasma and organelle membranes lose integrity.
Nucleus	Chromatin clumping may occur.	Pyknosis, karyorrhexis and karyolysis.
Outcome	Recovery if oxygen, nutrients and homeostasis return in time.	Cell death by necrosis, apoptosis or other regulated death pathways.

There is no single universal clock. Reversibility depends on cell type, injury intensity, temperature, oxygen reserve and treatment. Brain tissue is particularly vulnerable to prolonged hypoxia.

7. Cell death pathways

7.1 Necrosis

Necrosis follows severe injury with loss of membrane integrity, enzyme leakage and inflammation. Nuclear changes progress from pyknosis (shrinkage) to karyorrhexis (fragmentation) and karyolysis (dissolution).

Pattern	Typical setting	Mechanism/appearance
Coagulative	Ischaemia in solid organs except brain.	Cell outlines persist temporarily; tissue is firm and pale.
Liquefactive	Brain infarction and bacterial abscess.	Enzymatic digestion creates liquid debris.
Caseous	Tuberculosis and some fungal infections.	Cheese-like granular necrotic material within granulomas.
Fat necrosis	Acute pancreatitis or traumatic adipose injury.	Lipase releases fatty acids that bind calcium; chalky deposits may form.
Fibrinoid	Immune-mediated vascular injury.	Protein-rich material in vessel walls.
Gangrenous	Clinical term for extensive ischaemic necrosis, often limb or bowel.	Dry, wet or gas gangrene according to infection and tissue conditions.

7.2 Apoptosis

Apoptosis is regulated cell death with cell shrinkage, chromatin condensation, apoptotic bodies and minimal surrounding inflammation. It removes damaged, infected, senescent or unnecessary cells.

• Intrinsic pathway: mitochondrial stress changes BCL-2-family balance, releases cytochrome c and activates caspase-9.
• Extrinsic pathway: death receptors such as Fas or TNF receptors activate caspase-8.
• Execution phase: caspases cleave proteins and DNA; phagocytes remove apoptotic bodies.

Excess apoptosis contributes to neurodegeneration, immune deficiency and tissue atrophy. Insufficient apoptosis contributes to cancer, autoimmune disease and persistence of infected cells.

7.3 Other regulated death mechanisms

Pyroptosis is an inflammatory programmed death associated with inflammasomes and gasdermin pore formation. Necroptosis is a regulated necrotic pathway activated when death-receptor signalling is diverted from apoptosis. Ferroptosis is iron-dependent lipid peroxidation. These mechanisms are important research areas and may overlap in severe disease.

8. Inflammation as a pathogenic mechanism

Inflammation is a protective response to infection or tissue injury that removes the cause, clears damaged tissue and initiates repair. It becomes pathogenic when excessive, prolonged, misdirected or unable to resolve.

Acute inflammation

1. Recognition of pathogen- or damage-associated signals by resident macrophages, mast cells and epithelial cells.
2. Release of histamine, prostaglandins, leukotrienes, cytokines and chemokines.
3. Vasodilation and increased vascular permeability, producing redness, heat, swelling and exudate.
4. Endothelial activation, leukocyte rolling, adhesion and transmigration.
5. Neutrophil recruitment, phagocytosis, oxidative killing and extracellular-trap formation.
6. Resolution, abscess formation, fibrosis or progression to chronic inflammation.

Chronic inflammation

Persistent infection, autoimmune disease, foreign material, toxins and unresolved acute inflammation recruit macrophages, lymphocytes and plasma cells. Cytokines and growth factors cause continuing injury, angiogenesis, tissue remodelling and fibrosis. The process can produce both organ failure and cancer-promoting microenvironments.

Protective role	Pathogenic consequence
Containment of infection	Abscess, granuloma, fibrosis or sepsis.
Removal of damaged tissue	Excessive protease and ROS injury to healthy cells.
Repair and regeneration	Scarring, strictures, adhesions and organ remodelling.
Immune surveillance	Autoimmunity, hypersensitivity or transplant rejection.

9. Immune and hypersensitivity mechanisms

• Antibody-mediated injury: antibodies block receptors, stimulate receptors, opsonise cells or activate complement.
• Immune-complex injury: antigen–antibody deposits activate complement and inflammation in vessels, kidneys or joints.
• T-cell-mediated injury: cytotoxic cells kill target cells; helper T cells recruit macrophages and other inflammatory cells.
• IgE-mediated hypersensitivity: mast-cell degranulation produces bronchospasm, oedema, urticaria and shock.
• Delayed hypersensitivity: persistent T-cell activation produces macrophage inflammation and tissue damage.

The immune system can be a cause, a mechanism and a modifier. For example, a virus may initiate hepatitis, but immune-mediated cytotoxicity contributes substantially to hepatocyte injury.

10. Vascular, perfusion and reperfusion mechanisms

Ischaemia and infarction

Reduced perfusion decreases oxygen and nutrient delivery and prevents removal of waste. ATP falls, pumps fail, cells swell and irreversible injury develops if perfusion is not restored. The pattern depends on collateral circulation, tissue sensitivity, rate of obstruction and duration.

Reperfusion injury

Restoring blood flow is essential, but sudden reperfusion can generate ROS, calcium overload, endothelial activation, neutrophil recruitment and mitochondrial permeability. This may enlarge the injury despite successful reopening of the vessel.

Shock

Shock pathogenesis is inadequate tissue perfusion and oxygen use. Hypovolaemic shock reduces preload; cardiogenic shock reduces pump output; obstructive shock prevents filling or ejection; distributive shock produces vasodilation, maldistribution and capillary leak. Cellular hypoxia causes mitochondrial failure, acidosis, inflammation and progressive organ dysfunction.

11. Infection: mechanisms of microbial injury

Mechanism	How injury occurs	Example
Direct cytolysis	Replication or invasion destroys host cells.	Viral cytopathic effect.
Toxin-mediated	Exotoxin, endotoxin or secreted enzyme alters cell function or kills cells.	Botulinum toxin, cholera toxin, bacterial sepsis mediators.
Immune-mediated	Host inflammation or adaptive immunity damages infected and neighbouring tissue.	Viral hepatitis, post-streptococcal disease.
Resource competition	Organisms consume nutrients or alter the microbiome.	Malabsorption or anaemia from helminth infection.
Obstruction/mass effect	Organisms, cysts or granulomas obstruct ducts or vessels.	Schistosomal fibrosis, hydatid cyst.
Persistence/latency	Organism evades immunity and reactivates later.	Tuberculosis, herpesviruses and HIV.

12. Fibrosis, repair and remodelling

Repair restores tissue after injury by regeneration or scar formation. Growth factors stimulate fibroblast migration, extracellular-matrix deposition, angiogenesis and remodelling. Repair is essential, but excessive fibrosis replaces functional tissue with stiff scar.

• Regeneration: remaining cells proliferate and restore architecture when the scaffold is preserved.
• Scar formation: collagen replaces destroyed tissue when injury is severe or the matrix is damaged.
• Remodelling: matrix metalloproteinases and inhibitors reshape the scar over time.
• Pathological fibrosis: persistent inflammation or abnormal repair causes cirrhosis, pulmonary fibrosis, kidney scarring or strictures.

13. Abnormal cell growth and neoplastic pathogenesis

Cancer develops when genetic and epigenetic alterations confer growth advantage, resistance to death, replicative immortality, angiogenesis, invasion or immune evasion. Mutations may affect:

• Oncogenes: activated growth-promoting genes.
• Tumour-suppressor genes: lost restraints on proliferation or survival.
• DNA-repair genes: failure increases mutation accumulation.
• Apoptosis regulators: abnormal survival of damaged cells.
• Epigenetic control: altered gene expression without changing DNA sequence.

The tumour microenvironment—blood vessels, fibroblasts, immune cells and extracellular matrix—also contributes to invasion, metastasis and treatment resistance.

14. Mechanism-based examples

14.1 Sepsis

Initiating cause: infection or microbial products. Recognition: pattern-recognition receptors activate NF-κB and inflammatory pathways. Systemic mechanism: cytokines, endothelial activation, vasodilation, capillary leak, coagulation activation, mitochondrial dysfunction and impaired oxygen use. Clinical result: hypotension, altered mental state, oliguria, elevated lactate and organ dysfunction. The same inflammatory response that contains microbes can become the cause of shock and organ injury.

14.2 Acute myocardial infarction

Plaque rupture or erosion exposes thrombogenic material. Platelets activate, coagulation generates thrombin and a coronary thrombus obstructs flow. Ischaemia causes ATP depletion, ion-pump failure, calcium overload and myocyte necrosis. Inflammation clears necrotic tissue, while fibroblasts form a scar. Reperfusion can rescue myocardium but may add oxidative and inflammatory injury.

14.3 Type 1 diabetes

Genetic susceptibility and environmental triggers lead to immune-mediated destruction of pancreatic beta cells. Insulin deficiency causes hyperglycaemia, lipolysis, ketone production, osmotic diuresis and dehydration. Diabetic ketoacidosis is therefore a metabolic consequence of the underlying autoimmune pathogenesis, not a separate disease cause.

14.4 Chronic asthma

Airway hyper-responsiveness, epithelial injury, type-2 inflammation, bronchoconstriction, mucus production and airway remodelling narrow airways. Viral infection, allergens, smoke, exercise or medication exposure can trigger acute worsening. Repeated inflammation produces structural remodelling and a decline in function.

14.5 Cancer

Carcinogen exposure or inherited susceptibility produces driver alterations. Clonal selection favours cells that proliferate, evade apoptosis, recruit blood supply and invade. The patient’s symptoms may arise from mass effect, tissue destruction, obstruction, bleeding, hormone production or systemic cytokines.

15. Reversible checkpoints and emergency intervention

Pathogenic checkpoint	Potential intervention	Emergency relevance
Airway/oxygen failure	Airway opening, oxygen, ventilation and treatment of the cause.	Prevents hypoxic mitochondrial injury.
Low circulating volume	Control bleeding, give appropriate fluids/blood and vasopressors when indicated.	Stops ischaemia and shock progression.
Vascular obstruction	Reperfusion, anticoagulation, thrombolysis or surgery according to diagnosis.	Limits infarction and organ loss.
Toxin exposure	Remove exposure, decontaminate selectively and administer a specific antidote.	May reverse receptor or enzyme dysfunction before cell death.
Infection	Source control, timely antimicrobials and supportive organ care.	Reduces microbial load and dysregulated inflammation.
Severe inflammation	Cause-specific immunomodulation, haemodynamic support and organ protection.	Prevents cytokine-driven capillary leak and failure.
Metabolic derangement	Correct glucose, electrolytes, acid–base disorder and endocrine deficiency carefully.	Restores cellular gradients and prevents arrhythmia/coma.

16. Clinical reasoning checklist

1. Identify the likely initiating cause and the dangerous alternatives.
2. Ask which organ or cell type is most vulnerable and why.
3. Determine whether the process is reversible or has crossed an irreversible threshold.
4. Look for compensatory responses that may conceal deterioration.
5. Choose tests that identify the mechanism, severity or treatable cause—not just abnormal results.
6. Start time-critical treatment while confirmation is pending when the risk of delay is greater than the risk of treatment.
7. Reassess after intervention; a changing trajectory is evidence about the mechanism.
8. Document cause, mechanism, severity, uncertainty, response and next review.

17. Common misconceptions

Misconception	Correction
Pathogenesis is the same as etiology.	Etiology is what starts disease; pathogenesis is the sequence that follows.
All cell death is necrosis.	Apoptosis and other regulated pathways can remove cells without the same inflammatory rupture.
Inflammation is always harmful.	It protects and repairs, but excessive or unresolved inflammation causes disease.
Reperfusion always ends injury.	Reperfusion saves tissue but can produce oxidative and inflammatory injury.
Structural disease must be visible on imaging.	Molecular, cellular and functional abnormalities may precede visible morphology.
One cause produces one outcome.	Host factors, dose, route, time and treatment modify the final disease.
Mechanism is only academic.	Mechanism predicts complications, selects tests and identifies points for treatment.

18. Examination-ready definitions

• Pathogenesis: the stepwise biological progression from an initiating cause to disease manifestations and outcome.
• Cellular injury: functional and structural damage occurring when stress exceeds a cell’s adaptive capacity.
• Reversible injury: cellular dysfunction that can recover when the injurious stimulus is removed.
• Irreversible injury: damage beyond recovery, characterised by inability to restore mitochondrial function and severe membrane/nuclear injury.
• Necrosis: cell death with membrane breakdown, enzyme leakage and inflammation.
• Apoptosis: regulated cell deletion with cell shrinkage, caspase activation and usually minimal inflammation.
• Oxidative stress: injury caused when reactive oxygen/nitrogen species exceed antioxidant defences.
• Inflammation: a vascular and cellular response that removes injury and initiates repair, but can itself cause tissue damage.
• Fibrosis: excessive deposition of extracellular matrix that replaces or stiffens functional tissue.
• Remodelling: structural and functional reorganisation after injury, disease or altered load.

19. Quick self-test

1. Separate the etiology, pathogenesis, morphology and clinical manifestations of myocardial infarction.
2. Why does ATP depletion cause cell swelling and lactic acidosis?
3. Compare necrosis and apoptosis in membrane integrity and inflammation.
4. How can inflammation protect tissue and also destroy it?
5. Why can reperfusion produce additional injury after an artery is reopened?
6. What factors explain why the same infectious agent causes mild disease in one patient and fatal disease in another?
7. List three points in the pathogenesis of sepsis at which emergency treatment can change outcome.

20. Key take-home points

• Pathogenesis explains the “how” of disease and links etiology to clinical outcome.
• Disease unfolds across molecular, cellular, tissue, organ and systemic levels.
• ATP depletion, mitochondrial injury, calcium overload, ROS, membrane damage, protein misfolding and DNA damage are interdependent injury mechanisms.
• Cell adaptation may be protective but can become pathological if persistent.
• Necrosis is inflammatory membrane rupture; apoptosis is regulated cell deletion; regulated inflammatory death pathways also exist.
• Inflammation, thrombosis, vascular leak, fibrosis and remodelling are common final pathways of many diseases.
• Host factors determine severity and reversibility.
• Emergency treatment works by interrupting pathogenesis before irreversible injury: oxygenation, perfusion, antidotes, antimicrobials, reperfusion and metabolic correction.
• Mechanism-based reasoning is more transferable than memorising isolated disease names.
• The next curriculum post, Pathology Specimens, will explain how tissue, cells and body fluids are collected, preserved, transported and interpreted to reveal these mechanisms.

Sources and further reading

• Pathology: The Clinical Description of Human Disease
• Mechanisms and Morphology of Cellular Injury, Adaptation, and Death
• NCBI Bookshelf: Necrosis Pathology
• NCBI Bookshelf: Histology, Cell Death
• NCBI Bookshelf: Acute Inflammatory Response
• NCBI Bookshelf: Chronic Inflammation
• NCBI Bookshelf: The front line of host defense