79 HISTOPATHOLOGY

# 80 HISTOPATHOLOGY

Histopathology is the diagnostic backbone of surgical oncology. Every specimen you send from theatre passes through the same pipeline: fixation, processing, sectioning, staining, microscopy and, increasingly, immunohistochemistry. MRCS Part A questions rarely ask you to be a pathologist — they ask whether you understand the pipeline well enough to choose the right test, interpret the report, and recognise classic histological patterns. This lesson distils the high-yield essentials.

Tissue processing and fixation

Fresh tissue autolyses within minutes. Fixation halts this and preserves cellular architecture.

- Formalin (10% buffered formaldehyde) is the standard fixative. It cross-links amino groups in proteins, locking structure in place. Specimens are then dehydrated, embedded in paraffin wax, cut at 4 µm and mounted on slides — the FFPE block (Formalin-Fixed Paraffin-Embedded).

- Glutaraldehyde is reserved for electron microscopy.

- Frozen section bypasses fixation entirely. The specimen is snap-frozen in liquid nitrogen or isopentane, sectioned on a cryostat and stained within 15–20 minutes. Resolution is inferior to FFPE because ice crystals distort cells, but the speed is invaluable intraoperatively.

👩‍⚕️ Classic frozen-section indications: assessing resection margins (e.g. breast lumpectomy, head-and-neck), evaluating a sentinel lymph node, and distinguishing lymphoma from reactive lymphadenopathy before committing to definitive surgery. Definitive diagnosis still requires the paraffin section the next day.

Staining

➡ H&E (Haematoxylin and Eosin) is the universal screening stain. Haematoxylin is basic and stains acidic structures — DNA, RNA, ribosomes — so nuclei appear blue/purple. Eosin is acidic and stains basic proteins, so cytoplasm and collagen appear pink.

Special stains target specific molecules:

Stain	Target	Classic use
Congo red	Amyloid	Apple-green birefringence under polarised light
Prussian blue (Perls')	Ferric iron	Haemochromatosis, haemosiderin
PAS	Glycogen, basement membrane	Whipple's, fungi, glycogen storage
Ziehl-Neelsen	Mycobacteria	Acid-fast bacilli in TB
Masson's trichrome	Collagen	Fibrosis
Oil Red O	Fat	Requires frozen section (alcohol dissolves fat)
Silver (Grocott/Warthin-Starry)	Fungi, spirochaetes	Pneumocystis, H. pylori

Immunohistochemistry (IHC)

IHC uses antibodies tagged with a chromogen to detect specific antigens — invaluable when H&E morphology alone cannot distinguish tumour origin (e.g. "carcinoma of unknown primary", small round blue cell tumours).

Lineage	Marker(s)
Epithelial	Cytokeratin (CK), EMA
Mesenchymal	Vimentin (broad), desmin & SMA (muscle)
Lymphoid	CD45 (LCA) pan-lymphoid; CD20 B-cell (target of rituximab); CD3 T-cell; CD15 + CD30 Reed-Sternberg cells
Neuroendocrine	Chromogranin, synaptophysin, CD56
Melanoma	S100, HMB-45, Melan-A
GIST	c-KIT (CD117), DOG-1
Breast	ER, PR, HER2 (drives endocrine and trastuzumab therapy)
Prostate	PSA, PSAP
Thyroid	Thyroglobulin (follicular/papillary); calcitonin + chromogranin (medullary)
Mesothelioma	Calretinin, WT-1

👩‍⚕️ A high-yield rule: if you can't tell whether a tumour is carcinoma, sarcoma, lymphoma or melanoma, run a four-stain panel — cytokeratin, vimentin, CD45, S100. One will usually light up.

Cellular adaptations

Adaptation	Change	Example
Hyperplasia	↑ cell number	Endometrium under unopposed oestrogen; BPH
Hypertrophy	↑ cell size	LV in hypertension; skeletal muscle in athletes
Atrophy	↓ cell size/number	Disuse muscle wasting; post-menopausal endometrium
Metaplasia	One mature cell type → another	Barrett's (squamous → columnar); bronchial squamous metaplasia in smokers
Dysplasia	Disordered, atypical growth	Cervical CIN; pre-malignant
Anaplasia	Complete loss of differentiation	Hallmark of malignancy

Metaplasia is reversible if the stimulus is removed; dysplasia is the bridge to neoplasia.

Cell death

Apoptosis is programmed, energy-dependent and tidy. Single cells shrink, chromatin condenses, the cell fragments into apoptotic bodies that are phagocytosed — no inflammation. Two pathways converge on caspase activation:

- Intrinsic (mitochondrial) — triggered by DNA damage or stress. Bax punches mitochondrial membranes, cytochrome c leaks into cytoplasm, activating caspase-9. Inhibited by Bcl-2.

- Extrinsic (death receptor) — Fas-FasL or TNF receptor ligation activates caspase-8.

Necrosis is pathological, unprogrammed and messy. Cells swell, membranes rupture, contents spill out and provoke acute inflammation.

Necrosis type	Setting	Histology
Coagulative	Ischaemia (MI, splenic infarct) — most common	Cell outlines preserved, "ghost" architecture
Liquefactive	Brain infarct, bacterial abscess	Tissue digested to fluid
Caseous	TB, some fungal infections	"Cheesy" amorphous debris, granuloma
Fat	Acute pancreatitis, breast trauma	Chalky saponification with calcium
Fibrinoid	Immune-complex vasculitis, malignant hypertension	Pink amorphous protein in vessel walls

Inflammation

Acute inflammation is the first line — onset in minutes to hours, dominated by neutrophils, driven by TNF, IL-1, histamine, bradykinin and complement. Cardinal signs: rubor, calor, tumor, dolor, functio laesa.

Chronic inflammation takes over after days to weeks; lymphocytes, plasma cells and macrophages dominate. Hallmark lesion is the granuloma — a tight aggregate of epithelioid macrophages, often with multinucleated giant cells.

- Caseating granulomas → tuberculosis

- Non-caseating granulomas → sarcoidosis, Crohn's, foreign body reaction, GCA, leprosy

Healing

Tissue capacity to regenerate depends on cell type:

- Labile (continuously dividing): skin epithelium, gut epithelium, haematopoietic — full regeneration.

- Stable (quiescent but can re-enter cycle): hepatocytes, renal tubule, fibroblasts — regenerate if stroma intact.

- Permanent (cannot divide): neurones, cardiac muscle, skeletal muscle — heal by scarring only.

Wound healing progresses through haemostasis → inflammation → proliferation (granulation tissue, angiogenesis, fibroblast collagen) → remodelling. Excessive collagen produces a hypertrophic scar (within wound boundary) or keloid (beyond boundary).

Tumour staging and grading

- Grading = how the tumour looks — degree of differentiation (G1 well → G4 anaplastic).

- Staging = how far the tumour has spread — the prognostic heavyweight.

TNM: Tumour size/local invasion, regional Nodes, distant Metastasis.

Cytology

- Fine-needle aspiration (FNA) — cells aspirated through a 21–25G needle; preserves no architecture but rapid and cheap. Used for thyroid, breast, lymph node, salivary gland.

- Exfoliative cytology — naturally shed cells: cervical smear, sputum, urine, pleural fluid.

Cytology yields categorical reports, not tissue diagnoses:

System	Site
B1–B5	Breast core biopsy
C1–C5	Breast FNA
Thy1–Thy5	Thyroid FNA
P1–P5	Pancreas
U1–U5	Urine
M1–M5	Salivary gland
R1–R5	Respiratory

Category 1 = inadequate; 2 = benign; 3 = atypia/uncertain; 4 = suspicious; 5 = malignant. A C5/B5 breast result virtually confirms cancer; a C3 mandates core biopsy.

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Test yourself

A 65-year-old presents with fatigue, bone pain, recurrent infections, anaemia and hypercalcaemia. Bone marrow biopsy shows which cell type?

• ((Plasma cells::☑️ Multiple myeloma — clonal plasma cells produce monoclonal Ig, causing lytic bone, anaemia, renal failure.))
• ((Lymphoblasts::Acute lymphoblastic leukaemia — children, not this CRAB picture.))
• ((Reed-Sternberg cells::Hodgkin lymphoma — lymphadenopathy, not lytic bone marrow.))
• ((Megakaryocytes::Myeloproliferative disorders — no monoclonal protein.))

👩‍⚕️ CRAB criteria: hyperCalcaemia, Renal failure, Anaemia, Bone lesions.

A patient with back pain has raised ESR, abnormal serum electrophoresis and sacral lytic lesions. Sacral biopsy is most likely to show?

• ((Sheets of plasma cells::☑️ Multiple myeloma; monoclonal band on electrophoresis seals the diagnosis.))
• ((Osteoblastic rimming::Suggests metastatic prostate or osteoblastoma, not lytic myeloma.))
• ((Granulomas::TB or sarcoid — wouldn't give a monoclonal paraprotein.))
• ((Spindle cells::Sarcoma — different clinical picture, no paraprotein.))

What stain identifies amyloid?

• ((Congo red::☑️ Apple-green birefringence under polarised light is pathognomonic.))
• ((Prussian blue::Stains ferric iron (haemochromatosis, haemosiderin).))
• ((PAS::Stains glycogen, basement membranes, fungi.))
• ((Ziehl-Neelsen::Acid-fast stain for mycobacteria.))

Under polarised light, Congo red-stained amyloid shows?

• ((Apple-green birefringence::☑️ Classic and pathognomonic feature of amyloid.))
• ((Negative birefringence::Describes monosodium urate (gout), not amyloid.))
• ((Positive birefringence::Calcium pyrophosphate (pseudogout) — rhomboid crystals.))
• ((No birefringence::Would exclude amyloid and crystal arthropathy.))

A resected liver turns deep blue on staining. The underlying pathology is?

• ((Haemochromatosis::☑️ Prussian blue (Perls') reacts with ferric iron to form an insoluble blue pigment.))
• ((Wilson's disease::Copper accumulation — stained with rhodanine, not Prussian blue.))
• ((Alpha-1-antitrypsin deficiency::PAS-positive diastase-resistant globules in hepatocytes.))
• ((Hepatocellular carcinoma::Diagnosed on H&E architecture, not a colour reaction.))

Joint aspirate from a gouty toe shows?

• ((Needle-shaped, negatively birefringent crystals::☑️ Monosodium urate — yellow when parallel to the slow axis.))
• ((Needle-shaped, positively birefringent crystals::Pseudogout crystals are rhomboid and positive — opposite pattern.))
• ((Rhomboid, positively birefringent crystals::Calcium pyrophosphate — pseudogout, not gout.))
• ((Cholesterol crystals::Seen in chronic bursitis or atheroma, not acute gout.))

Which stain is used for routine frozen sections?

• ((H&E::☑️ Same stain as paraffin sections — quick to perform on the cryostat slide.))
• ((Oil Red O::Used for fat — requires frozen tissue but is not the routine screening stain.))
• ((PAS::Reserved for glycogen, basement membranes, fungi.))
• ((Immunohistochemistry::Too slow for the 15–20 minute intraoperative window.))

Hassall's corpuscles in a histology specimen indicate which organ?

• ((Thymus::☑️ Concentric whorls of keratinised epithelial cells in the medulla — pathognomonic.))
• ((Thyroid::Contains colloid-filled follicles, no Hassall's corpuscles.))
• ((Parathyroid::Chief and oxyphil cells; no Hassall's corpuscles.))
• ((Spleen::White and red pulp; lacks Hassall's corpuscles.))

A young adult has an anterior mediastinal mass. Biopsy shows epithelial cells with admixed lymphocytes. Diagnosis?

• ((Thymoma::☑️ Epithelial tumour of the thymus; classically associated with myasthenia gravis.))
• ((Squamous cell carcinoma::Would show keratin pearls and intercellular bridges.))
• ((Adenocarcinoma::Glandular pattern with mucin — wrong location for this morphology.))
• ((Basal cell carcinoma::Cutaneous tumour, never primary mediastinal.))

👩‍⚕️ Mediastinal mass mnemonic: 4 Ts — Thymoma, Teratoma, "Terrible" lymphoma, Thyroid retrosternal.

A 30-year-old man has a mediastinal mass. FNAC shows glandular cells, no neuroendocrine markers, ill-defined clumps. Diagnosis?

• ((Thymic carcinoma::☑️ Malignant thymic epithelial tumour without neuroendocrine differentiation.))
• ((Small cell carcinoma::Would be chromogranin/synaptophysin positive.))
• ((Hodgkin lymphoma::CD15/CD30 positive Reed-Sternberg cells — not glandular.))
• ((Teratoma::Mixed tissues (fat, cartilage, hair) on histology.))

Histology of Morton's neuroma shows?

• ((Perineurial fibrosis (not a true neuroma)::☑️ Reactive fibrotic thickening of the interdigital plantar nerve.))
• ((Schwann cell proliferation::Describes a true schwannoma, not Morton's.))
• ((Caseating granuloma::Tuberculosis — not a compressive interdigital lesion.))
• ((Malignant nerve sheath cells::Would be MPNST, a sarcoma.))

The classic histological feature of well-differentiated squamous cell carcinoma is?

• ((Keratin pearls::☑️ Concentric whorls of keratinised cells — pathognomonic of SCC.))
• ((Rosette formation::Neuroendocrine tumours (e.g. neuroblastoma).))
• ((Signet ring cells::Diffuse gastric carcinoma — nuclei pushed to side by mucin.))
• ((Reed-Sternberg cells::Hodgkin lymphoma.))

A neck node biopsy confirms sarcoidosis. The predominant lesion is?

• ((Non-caseating epithelioid granulomas::☑️ Classic feature; may contain asteroid or Schaumann bodies.))
• ((Caseating granulomas::Tuberculosis — central necrosis is absent in sarcoid.))
• ((Sheets of neutrophils::Acute bacterial lymphadenitis.))
• ((Reed-Sternberg cells::Hodgkin lymphoma — not granulomatous.))

A 50-year-old with chronic cough, weight loss, evening fevers and upper-lobe cavities has acid-fast bacilli in sputum. The granulomas centre on which cell?

• ((Macrophages (epithelioid + Langhans giant cells)::☑️ Form caseating granulomas in TB.))
• ((Neutrophils::Drive acute pyogenic abscesses, not granulomas.))
• ((Plasma cells::Chronic inflammation generally, not granuloma formation.))
• ((Eosinophils::Parasitic infection and allergy — not TB.))

A 45-year-old has cavitating upper-lobe lung disease. Biopsy shows central necrosis ringed by epithelioid macrophages and giant cells. Diagnosis?

• ((Tuberculosis::☑️ Caseating granulomas with Langhans giant cells — classic.))
• ((Sarcoidosis::Non-caseating granulomas; rarely cavitates.))
• ((Actinomycosis::Sulphur granules and sinus tracts, not caseating granulomas.))
• ((Squamous cell carcinoma::Can cavitate but shows keratinising malignant cells.))

A 68-year-old with headache, jaw claudication, scalp tenderness and blurred vision. Temporal artery biopsy shows?

• ((Granulomatous transmural inflammation with giant cells::☑️ Giant cell arteritis — disrupts internal elastic lamina.))
• ((Caseating granulomas::TB — wrong vessel, wrong clinical picture.))
• ((Fibrinoid necrosis with neutrophils::Polyarteritis nodosa — small to medium muscular arteries, no giant cells.))
• ((Lymphocytic infiltrate only::Non-specific — would not satisfy GCA criteria.))

👩‍⚕️ Skip lesions in GCA mean a negative biopsy doesn't exclude it — start steroids on clinical suspicion.

A young man has a testicular mass. Histology shows sheets of clear cells separated by fibrous septa infiltrated by lymphocytes. Diagnosis?

• ((Classical seminoma::☑️ Uniform cells with glycogen-rich clear cytoplasm; PLAP positive.))
• ((Spermatocytic seminoma::Older men, three cell sizes, no lymphoid stroma.))
• ((Embryonal carcinoma::Pleomorphic, glandular/papillary, raised AFP/β-hCG.))
• ((Yolk sac tumour::Schiller-Duval bodies, raised AFP.))

A young woman with a thyroid lump shows oncocytic (Hürthle) epithelial cells with dense lymphocytic infiltrate. Diagnosis?

• ((Hashimoto's thyroiditis::☑️ Autoimmune destruction with Hürthle cell metaplasia and germinal centres.))
• ((Papillary thyroid carcinoma::Orphan-Annie nuclei, psammoma bodies, nuclear grooves.))
• ((Follicular carcinoma::Capsular/vascular invasion — no lymphoid stroma.))
• ((Medullary carcinoma::Parafollicular C cells with amyloid stroma; calcitonin positive.))

A patient with painful thyroid swelling has defective epithelial cells and siderophages on histology. Diagnosis?

• ((Haemorrhagic thyroid cyst::☑️ Cyst rupture or bleed causes pain; siderophages = haemosiderin-laden macrophages.))
• ((Medullary carcinoma::Painless; amyloid stroma; calcitonin positive.))
• ((Papillary carcinoma::Painless; Orphan-Annie nuclei and psammoma bodies.))
• ((Follicular carcinoma::Painless, encapsulated; no siderophages.))

A thyroid biopsy is strongly CD20 positive. Diagnosis?

• ((Primary thyroid lymphoma::☑️ Typically B-cell — DLBCL or MALT lymphoma arising in Hashimoto's.))
• ((Papillary carcinoma::Epithelial — cytokeratin and thyroglobulin positive, CD20 negative.))
• ((Medullary carcinoma::Chromogranin and calcitonin positive, not lymphoid.))
• ((Anaplastic carcinoma::Pleomorphic epithelial malignancy; lacks lymphoid markers.))

A 45-year-old woman has a rapidly enlarging thyroid swelling, normal TFTs, atypical CD20-positive lymphoid cells with pale empty nuclei. Diagnosis?

• ((Thyroid lymphoma::☑️ B-cell marker positivity plus atypical lymphocytes — classic in long-standing Hashimoto's.))
• ((Papillary carcinoma::Epithelial; CD20 negative; psammoma bodies and grooved nuclei.))
• ((Hashimoto's thyroiditis::Polyclonal lymphocytes, no atypia.))
• ((Follicular carcinoma::Follicular epithelial pattern; CD20 negative.))

Which histological feature is classic for medullary thyroid carcinoma?

• ((Amyloid stroma::☑️ Derived from procalcitonin; Congo red positive; chromogranin and calcitonin positive.))
• ((Hürthle cells::Hashimoto's or follicular variant.))
• ((Psammoma bodies::Papillary thyroid carcinoma (also meningioma, serous ovarian).))
• ((Keratin pearls::Squamous cell carcinoma.))

Mesothelioma is best described as?

• ((Tumour of pleural (or peritoneal) serosal cells::☑️ Arises from mesothelium; calretinin and WT-1 positive.))
• ((Primary tumour of lung parenchyma::That would be bronchogenic carcinoma.))
• ((Vascular tumour::Describes angiosarcoma or Kaposi sarcoma.))
• ((Tumour of tubular epithelium::Describes adenocarcinoma — not mesothelial.))

A lymph node biopsy in suspected Hodgkin lymphoma shows?

• ((Reed-Sternberg cells::☑️ Large bilobed "owl-eye" nuclei; CD15 and CD30 positive.))
• ((Small round blue cells::Paediatric Ewing's, neuroblastoma, rhabdomyosarcoma.))
• ((Granulomas::TB, sarcoid — not Hodgkin.))
• ((Plasma cells::Myeloma or chronic inflammation.))

A patient with bilateral hilar lymphadenopathy has FNAC showing glandular cells with pleomorphic nuclei, negative for neuroendocrine markers. Diagnosis?

• ((Adenocarcinoma::☑️ Glandular malignant cells, chromogranin/synaptophysin negative.))
• ((Small cell carcinoma::Chromogranin and synaptophysin positive with nuclear moulding.))
• ((Squamous cell carcinoma::Keratinisation and intercellular bridges, not glandular.))
• ((Mesothelioma::Calretinin and WT-1 positive; arises from pleura.))

Histology of rheumatoid synovium shows?

• ((Necrobiotic (palisading) granuloma with synovial hyperplasia and pannus::☑️ Fibrinoid necrosis ringed by palisading histiocytes.))
• ((Caseating granulomas::Tuberculosis.))
• ((Langhans giant cells::TB and sarcoid, not RA.))
• ((Psammoma bodies::Papillary thyroid carcinoma, meningioma.))

A 20-year-old woman has an ovarian cyst. Histology shows keratinising squamous epithelium with fat, muscle, thyroid and neural tissue in the wall. Diagnosis?

• ((Mature cystic teratoma (dermoid)::☑️ Tissues from all three germ layers; benign in young women.))
• ((Serous cystadenoma::Single layer of ciliated columnar epithelium, no germ layer tissues.))
• ((Dysgerminoma::Sheets of primitive germ cells with lymphocytic stroma; malignant.))
• ((Squamous cell carcinoma::Invasive squamous with atypia — here epithelium is mature and benign.))

Revision summary

- Formalin = standard fixative (protein cross-linking). Frozen section = 15–20 min, lower resolution; used for margins, sentinel nodes, lymphoma vs reactive.

- H&E: haematoxylin → nuclei blue; eosin → cytoplasm pink.

- Key special stains: Congo red (amyloid, apple-green birefringence), Prussian blue (iron), PAS (glycogen, basement membranes), Ziehl-Neelsen (acid-fast bacilli).

- IHC shortcuts: Cytokeratin/EMA (epithelial), vimentin (mesenchymal), CD45 (lymphoid), CD20 (B-cell, rituximab target), CD3 (T-cell), CD15+CD30 (Reed-Sternberg), chromogranin/synaptophysin/CD56 (neuroendocrine), S100/HMB-45/Melan-A (melanoma), c-KIT/DOG-1 (GIST), ER/PR/HER2 (breast), PSA (prostate).

- Adaptations: hyperplasia (↑number), hypertrophy (↑size), atrophy (↓size), metaplasia (cell type swap — Barrett's), dysplasia (pre-malignant), anaplasia (no differentiation).

- Apoptosis: programmed, caspase-mediated, no inflammation. Intrinsic = mitochondrial (cytochrome c, Bax vs Bcl-2). Extrinsic = Fas/TNF death receptors.

- Necrosis: coagulative (ischaemia — most common), liquefactive (brain, abscess), caseous (TB), fat (pancreatitis, breast), fibrinoid (vasculitis).

- Acute inflammation = neutrophils, TNF/IL-1. Chronic = lymphocytes/macrophages; granuloma = TB, sarcoid, Crohn's, foreign body.

- Healing: labile and stable cells regenerate; permanent cells (neurons, cardiac, skeletal muscle) scar.

- Grading = differentiation; Staging = spread (TNM).

- Cytology: FNA + exfoliative. Reports use 1–5 categories (B, C, Thy, P, U, M, R): 1 inadequate → 5 malignant.