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PA25.7 | Lung Disease & Tumour Morphology — Practical — SDL Guide

Learning Objectives

Identify and describe the gross and microscopic features of lobar pneumonia and bronchopneumonia
Recognise the morphological stages of tuberculosis including caseating granuloma, Ghon complex, and cavitation
Describe the gross and microscopic appearances of emphysema and major pneumoconioses
Identify and classify primary lung carcinomas — SCLC, squamous cell, adenocarcinoma — by their histological hallmarks
Apply a systematic gross-to-micro reading strategy to unknown respiratory specimens

INSTRUCTIONS

Lung practical specimens span the full spectrum of pulmonary disease — infection, obstruction, environmental injury, and neoplasia. Pattern recognition at the bench depends on reading gross architecture first, then zooming into the microscopic signature. This module walks you through each category with the same systematic framework you will use in the histopathology practical hall and in clinical case discussions. Mastering these patterns now will pay dividends in medicine, surgery, and oncology postings.

References

Robbins & Cotran Pathologic Basis of Disease, 10th ed., Ch 15 (The Lung) (textbook)
Harsh Mohan Textbook of Pathology, 8th ed., Ch 16 (Pathology of Lung) (textbook)

Version 2.0 | NMC CBUC 2024

CLINICAL SCENARIO

You are handed a glass slide and told: 'Tell me what you see.' The section shows sheets of small hyperchromatic cells with virtually no cytoplasm, nuclei moulding against each other, and scattered crush artefact. Is this infection? Inflammation? Or one of the most aggressive lung cancers? The answer lives in pattern — gross shape, microscopic architecture, cellular detail. This module gives you the reading framework to crack every lung specimen in 60 seconds.

WHY THIS MATTERS

Lung pathology is one of the highest-yield practical topics in Year-2 PA examinations. Spotters routinely include pneumonia, TB, emphysema, and carcinoma. Clinicians who trained in pathology labs are measurably faster at interpreting chest CT reports and histopathology results. The patterns you learn today translate directly into understanding radiological terms like 'consolidation', 'cavitation', and 'mass lesion' that you will encounter daily on wards.

RECALL

Before we start, activate what you already know:

What are the two arms of the innate immune response to bacterial infection — cellular and humoral?
Describe a granuloma in one sentence from your Year-1 general pathology.
Name three cell types in normal alveolar walls (you covered lung histology in AN).
What is the difference between a consolidation and a cavity on a chest X-ray?

Keep these answers in mind — they are the scaffolding for every block that follows.

The Systematic Reading Framework

A three-panel medical diagram summarizes the four-step lung pathology reading framework with gross lung patterns, microscopic pattern categories, and the early lobar pneumonia timeline. — **Panel A:** Four diagnostic steps: Gross architecture, Microscopic pattern, Infective vs neoplastic vs reactive, Name + subtype; includes lung specimen, microscope, diagnostic fork, and diagnosis tag icons.. **Panel B:** Gross architecture patterns: Normal aerated lung, Consolidation, Emphysema, Cavitation, Mass lesion; leader lines to cut-surface lung changes.. **Panel C:** Microscopic patterns: Alveolar filling, Interstitial wall thickening, Vascular lesion, Architectural destruction; bottom inset shows Congestion, Red hepatisation, Grey hepatisation, and Resolution in lobar pneumonia..

Every lung specimen — biopsy, resection, or autopsy lobe — should be read in four steps:

Gross architecture first — Is the parenchyma airless (consolidation), destroyed (emphysema), cavitated, or occupied by a mass?
Microscopic pattern — Is the abnormality alveolar (filling), interstitial (wall thickening), vascular, or architectural destruction?
Infective vs. neoplastic vs. reactive — Inflammation implies exudate + intact architecture; tumour implies loss of architecture + abnormal cytology.
Name + subtype — Combine gross + micro + context to assign a specific diagnosis.

This four-step approach works for every category covered in this module. We will apply it explicitly to each condition.

Pneumonia — Lobar vs. Bronchopneumonia

A four-panel educational diagram compares lobar pneumonia and bronchopneumonia by lung distribution, lobar pneumonia stages, histology, and key distinguishing features. — **Panel A:** Anterior lung cutaway showing uniform lobar consolidation, patchy bronchiole-centred consolidation, affected lobe, bronchi, bronchioles, and normal aerated lung.. **Panel B:** Lobar pneumonia temporal stages: Congestion day 1-2, Red hepatisation day 2-4, Grey hepatisation day 4-8, and Resolution.. **Panel C:** Microscopic comparison showing intact alveolar walls, alveolar neutrophil exudate, fibrin mesh, RBCs, bronchiole-centred inflammation, adjacent alveoli, and patchy consolidation.. **Panel D:** Summary comparison of distribution, gross appearance, microscopic centre, and key distinction between lobar pneumonia and bronchopneumonia..

Lobar pneumonia is a classic bacterial consolidation (usually Streptococcus pneumoniae) affecting an entire lobe or segment in a predictable temporal sequence:

Stage	Gross	Microscopic
Congestion (day 1–2)	Heavy, red, boggy	Vascular engorgement, serous exudate
Red hepatisation (day 2–4)	Liver-like, red, airless	Alveoli packed with RBCs + fibrin + early neutrophils
Grey hepatisation (day 4–8)	Grey, dry, firm	RBCs lysed; neutrophil + fibrin mesh; no airspace
Resolution	Returns to spongy	Macrophage clearance; architecture restored

The defining microscopic feature is alveolar neutrophil exudate filling air spaces with intact alveolar walls — a key distinguishing feature from destructive processes.

Bronchopneumonia (lobular pneumonia) is a patchy, multifocal consolidation centred on bronchioles and adjacent alveoli. Gross: multiple grey-yellow foci 1–3 cm scattered across dependent lobes. Micro: acute bronchiolitis + surrounding alveolar neutrophil exudate, but with intervening aerated lung — the patchiness is the hallmark.

KEY DISTINCTION: Lobar = whole lobe airless, fibrinous exudate, preserved architecture. Bronchopneumonia = focal, purulent, centred on airways.

SELF-CHECK

A 65-year-old man has a right lower lobe that is uniformly airless, liver-like in consistency, and grey on cut section. Microscopically, alveoli are packed with neutrophils and fibrin mesh with intact alveolar walls. What stage of lobar pneumonia does this represent?

A. Congestion

B. Red hepatisation

C. Grey hepatisation

D. Resolution

Reveal Answer

Answer: C. Grey hepatisation

Grey hepatisation (days 4–8) shows grey-white, firm, airless lung on gross. Microscopically the alveoli are filled with neutrophils, fibrin, and cellular debris, while RBCs have been lysed — hence the grey colour. Red hepatisation is earlier (days 2–4) and shows RBC-rich exudate giving a red liver-like appearance.

Tuberculosis — Caseating Granuloma to Cavity

Diagram showing the microscopic layers of a caseating tuberculosis granuloma, gross progression from Ghon focus to cavitary TB, and Ziehl-Neelsen staining of acid-fast bacilli. — **Panel A:** Central caseation necrosis, epithelioid macrophage palisade, Langhans giant cell with horseshoe/peripheral wreath nuclei, peripheral lymphocyte cuff, outer fibrous rim.. **Panel B:** Ghon focus, subpleural caseating nodule, ipsilateral hilar lymph node caseation, Ghon complex, upper-lobe cavitary TB, liquefied caseation draining into bronchus, thick-walled cavity with caseous lining.. **Panel C:** Ziehl-Neelsen stain, red acid-fast bacilli, blue counterstained background, note that few AFB may be present and negative ZN does not exclude TB..

Tuberculosis produces one of pathology's most recognisable microscopic signatures: the caseating granuloma.

Microscopic anatomy of the TB granuloma:
• Central caseation necrosis — granular, eosinophilic, acellular material (ghost cells, lipid from mycobacterial wall). Looks like cottage cheese on gross.
• Surrounding epithelioid macrophages — plump, pale, kidney-shaped nuclei arranged in a palisade.
• Langhans giant cells — large multinucleated cells (nuclei arranged in a horseshoe or peripheral wreath) formed by macrophage fusion. Langhans ≠ Langerhans (islets). Hallmark of TB granuloma.
• Peripheral lymphocyte cuff — chronic inflammatory infiltrate.
• Outer fibrous rim — attempted walling off.

Gross progression:
1. Ghon focus — subpleural parenchymal caseating nodule (usually right upper lobe).
2. Ghon complex — Ghon focus + ipsilateral hilar lymph node caseation (primary TB).
3. Cavitary TB — post-primary, upper lobe; caseation liquefies and drains into bronchus leaving a thick-walled cavity with caseous lining. Cavity walls harbour actively replicating bacilli.

Special stain — ZN (Ziehl-Neelsen): AFB (acid-fast bacilli) stain red against blue background. Mycobacterium tuberculosis is the archetype AFB. Important: TB granulomas may have few AFB; a negative ZN does not exclude TB.

Annotated H&E-style histology diagram of a caseating tuberculous granuloma showing central caseation necrosis, epithelioid macrophages, a Langhans giant cell, and a lymphocyte cuff. — **Panel A:** High-power H&E view of caseating TB granuloma with central acellular caseation necrosis, surrounding epithelioid macrophages, Langhans giant cell, peripheral horseshoe nuclei, and lymphocyte cuff.. **Panel B:** Magnified Langhans giant cell showing pale eosinophilic cytoplasm and peripherally arranged horseshoe nuclei.. **Panel C:** Simplified concentric zonal map showing central caseation necrosis, epithelioid macrophage zone, and outer lymphocyte cuff..

SELF-CHECK

On a ZN-stained section of lung, you see red-staining bacilli within an area of granulomatous inflammation. The finding is most consistent with:

A. Streptococcus pneumoniae — ZN-positive cocci

B. Fungal hyphae identified by ZN carbol-fuchsin

C. Haemosiderin-laden macrophages staining red on ZN

D. Acid-fast bacilli, consistent with mycobacterial infection

Reveal Answer

Answer: D. Acid-fast bacilli, consistent with mycobacterial infection

ZN stain (Ziehl-Neelsen) uses hot carbol-fuchsin; acid-fast organisms (mycobacteria) retain the stain and appear red/pink against a blue or green counterstain. Bacteria like Streptococcus are Gram-positive but not acid-fast, so they do not stain red on ZN. Fungi are identified by PAS or GMS stains. Haemosiderin appears golden-brown on H&E and does not stain red on ZN.