01Core principlesThe concepts and mechanisms needed to understand the subject.
Pathology connects cause, cellular response, structural change and clinical consequence. Homeostasis is active: cells adjust size, number, phenotype and metabolic activity to demand. Hypertrophy enlarges cells; hyperplasia increases cell number in capable tissues; atrophy reduces cell substance and function; metaplasia replaces one differentiated programme with another better able to tolerate stress. Adaptation can be protective yet create vulnerability, as persistent metaplasia may lose specialised function and provide a background for dysplasia.
Injury occurs when stress exceeds adaptation. ATP depletion impairs ion pumps, causing swelling and calcium disturbance; oxidative stress damages lipids, proteins and DNA; mitochondrial permeability and membrane failure amplify damage. Reversible changes can resolve after the insult ends. Irreversible injury produces cell death. Necrosis describes morphological death with membrane disruption, enzyme leakage and inflammatory response; patterns such as coagulative, liquefactive, caseous and fat necrosis reflect tissue and mechanism. Apoptosis uses regulated protease pathways to remove cells in development, homeostasis or damage with membrane-contained fragments.
Inflammation delivers defence and repair but can also injure. Vascular dilation and permeability bring proteins and leukocytes; adhesion and chemotaxis direct cells to the focus; mediators coordinate pain, fever, killing and resolution. Neutrophils often dominate early bacterial or sterile acute injury, while macrophages orchestrate clearance and repair. Chronic inflammation persists when the stimulus remains or regulation fails, producing lymphocytes, macrophages, tissue destruction and fibrosis. Granulomas are organised macrophage-rich responses that narrow mechanisms but are not organism-specific.
Neoplasia is clonal growth driven by accumulated genomic and epigenetic changes plus selection in a tissue environment. Benign and malignant describe behaviour, not merely size. Dysplasia is disordered epithelial maturation; carcinoma in situ remains above the basement membrane; invasion crosses anatomical boundaries and enables lymphatic, blood or surface spread. Grade estimates biological aggressiveness from differentiation and proliferation; stage measures anatomical extent and usually carries stronger prognostic weight. Modern classification often requires morphology, immunophenotype and molecular data, so fixation, sampling and tumour content determine what can be concluded.
Key points
- Cells adapt to stress through hypertrophy, hyperplasia, atrophy and metaplasia; persistence or excess can progress to dysfunction, injury or neoplastic risk.
- Reversible injury disrupts energy, ion balance and organelles; irreversible injury crosses a point at which membrane and mitochondrial failure lead to cell death.
- Necrosis usually releases intracellular contents and provokes inflammation, while apoptosis packages regulated cell death with less surrounding reaction.
- Acute inflammation recruits fluid and leukocytes; chronic inflammation combines persistent injury, mononuclear cells, repair, angiogenesis and fibrosis.
- Healing depends on tissue regenerative capacity, extracellular matrix, perfusion, infection, mechanics and systemic factors; fibrosis restores integrity at a functional cost.
- Virchow's triad links thrombosis to endothelial injury, abnormal flow and hypercoagulability; embolisation and infarction depend on vascular anatomy and tissue reserve.
- Tumour classification integrates architecture, cytology, invasion, grade, stage and validated molecular features; a representative specimen and clinical context are essential.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Increased demand, hormonal signals, reduced use, pressure, nutrition and chronic irritation alter cell size, number or differentiation before overt cell death occurs.
Swelling, fatty change and organelle dysfunction may resolve if the insult ends before membrane integrity and mitochondrial recovery are lost.
Membrane disruption releases intracellular molecules, activates inflammation and produces tissue-specific morphological patterns that suggest but do not alone prove cause.
Cell type, distribution, necrosis, organisms, immune deposits and fibrosis together constrain timing and mechanism more than the label acute or chronic alone.
Regeneration restores specialised cells where stem capacity and matrix permit; scar substitutes collagen where damage is extensive or tissue cannot regenerate.
Invasion, destructive growth, metastasis and molecular lineage define malignancy more reliably than size, circumscription or cytological atypia in isolation.
03Interpreting evidenceInformation, measurements and their limitations.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Representative tissue sampling - Why
- Capture the lesion, interface and relevant normal tissue while preserving material for diagnosis.
- Interpretation and limitations
- A small or superficial sample may miss invasion, heterogeneity or necrosis; biopsy route and target should be planned with the treating and pathology teams.
- 02
Fixation and processing review - Why
- Preserve morphology and biomolecules sufficiently for sectioning, staining and ancillary studies.
- Interpretation and limitations
- Delay, underfixation, crush, cautery and decalcification can distort morphology or molecular assays; artefact should be reported as a limitation.
- 03
Histological pattern assessment - Why
- Evaluate architecture, cell morphology, stroma, inflammation, necrosis and invasion.
- Interpretation and limitations
- Patterns generate and narrow differentials but must be integrated with site, age, imaging and specimen type; mimicry is common.
- 04
Immunohistochemistry - Why
- Demonstrate proteins that support lineage, differentiation, infection or predictive classification.
- Interpretation and limitations
- Use a hypothesis-led panel with internal controls; staining is rarely perfectly specific and technical loss can create false negativity.
- 05
Molecular or cytogenetic assay - Why
- Identify a validated alteration for classification, prognosis, inherited-risk assessment or treatment selection.
- Interpretation and limitations
- Check tumour fraction, assay scope and whether the alteration is defining, supportive or merely associated; failed testing is not a negative result.
- 06
Integrated pathology report - Why
- Combine specimen, morphology, ancillary evidence, margins, grade, stage elements and limitations into an actionable diagnosis.
- Interpretation and limitations
- Read qualifiers and comments as part of the result; classification version and multidisciplinary correlation matter when features conflict.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked caseReason from a small biopsy to a safe conclusionInputs: a superficial gastric biopsy shows marked atypia and ulceration. The fragments lack an intact epithelial–stromal interface and adequate underlying lamina propria, so stromal invasion cannot be assessed reliably; imaging suggests a deeper mass.+
- 1Separate observed pathology from the clinical question: severe epithelial atypia is present, but fragmentation and the absent assessable epithelial–stromal interface and underlying stroma prevent reliable assessment of invasion in this specimen. Gastric carcinoma can invade within the mucosa; missing muscularis mucosae alone would not establish this limitation.
- 2Identify distortion mechanisms: ulceration and repair can produce reactive atypia, while fragmentation and missing tissue planes limit architecture.
- 3Integrate discordant imaging without letting it invent histology: a deeper mass raises concern and makes the superficial negative-for-invasion statement non-exclusionary.
- 4Report the highest supported epithelial abnormality with explicit sampling limitation rather than calling either benign repair or invasive carcinoma beyond evidence.
- 5Recommend multidisciplinary clinicoradiological correlation and suitably targeted repeat or definitive sampling when it will change management.
- 6Verify the next specimen by confirming lesion targeting, adequate depth and preservation, then reconcile morphology and imaging before final integrated classification.
02Mechanism pathwayTrace ischaemia to infarctionAn arterial occlusion abruptly reduces oxygen delivery to a solid organ.+
- 1Relate flow interruption to ATP depletion, ion-pump failure, cellular swelling, acidosis and calcium-mediated injury.
- 2Assess duration, collateral supply, reperfusion, tissue metabolic demand and pre-existing oxygenation to estimate reversibility.
- 3Once membrane and mitochondrial failure become irreversible, expect necrosis with leakage and an acute inflammatory response.
- 4Predict later macrophage clearance and repair by regeneration or scar according to the organ's proliferative capacity and matrix integrity.
03Tumour integrationBuild a classification from layered evidenceA resection contains a heterogeneous malignant tumour and treatment depends on subtype and stage.+
- 1Sample viable tumour, invasive front, interfaces, margins and relevant nodes according to the current organ dataset.
- 2Define morphology and grade, then select immunohistochemistry from a focused lineage differential.
- 3Use validated molecular testing when it is defining or treatment-relevant, preserving tumour-rich material and recording assay limitations.
- 4Issue an integrated report with diagnosis, grade, stage elements, margins, biomarkers and uncertainties for multidisciplinary review.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
- Correlate pathology with imaging, operative findings and clinical course; unexplained discordance warrants review of identity, sampling and interpretation.
- Track pending immunohistochemical or molecular addenda so provisional reports do not become unintended final decisions.
- Preserve and document tissue allocation for ancillary studies, especially when the sample is small and future treatment biomarkers may be needed.
- Use current organ-specific datasets and tumour-classification editions, recording any change that alters diagnosis or staging.
- When treatment response or recurrence differs from predicted biology, consider heterogeneity, transformation, sampling error and a new process before repeating old assumptions.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Morphology records time
Neutrophils, macrophages, granulation tissue, collagen and remodelling often reflect phases of injury, but timing varies with tissue, perfusion, infection and host state.
Reperfusion can add injury
Restored oxygen may generate reactive species, calcium influx and inflammation even while salvaging reversibly injured tissue; benefit and injury coexist.
Granuloma is a pattern
Granulomatous inflammation directs investigation toward infections, foreign material and immune diseases but cannot identify one cause without additional evidence.
Grade and stage answer different questions
Grade estimates cellular aggressiveness, whereas stage describes extent; a low-grade tumour can still be advanced and clinically dangerous.
The block is finite
Repeated broad ancillary panels can exhaust a small specimen, so tests should be sequenced by diagnostic and treatment value with tissue stewardship.
07Common pitfallsFrequent interpretation and management errors.
- 01
Equating adaptation with harmlessness or treating metaplasia, dysplasia and invasive malignancy as interchangeable stages.
- 02
Using apoptosis and necrosis as synonyms despite their different mechanisms, morphology and inflammatory consequences.
- 03
Calling granulomatous inflammation diagnostic of one infection without organisms, molecular evidence or compatible clinical context.
- 04
Assuming a superficial negative biopsy excludes a deeper lesion that was not representatively sampled.
- 05
Ordering an indiscriminate immunohistochemical panel without a morphological differential, controls or a tissue-preservation plan.
- 06
Reading only the bold diagnosis while missing specimen limitations, provisional status, margins, stage elements or pending addenda.