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PA3.4 | Inflammation Morphology — Practical — SDL Guide

Learning Objectives

Identify the dominant inflammatory cell type in an H&E section and classify the inflammation as acute, chronic, or granulomatous
Recognise the microscopic patterns of acute inflammation: suppurative/abscess, fibrinous, serous, ulcer, and phlegmon
Describe the microscopic features of chronic inflammation including mononuclear infiltrate, fibrosis, and granulation tissue
Identify a granuloma and name its key cellular components: epithelioid histiocytes, Langhans giant cells, lymphocyte cuff, and caseation
Match gross appearances (abscess, fibrinous surfaces, chronic ulcer, granulomatous lymph nodes) to their microscopic counterparts
Apply a systematic low-to-high-power reading strategy for inflammation specimens

INSTRUCTIONS

This practical module trains your eye for the most common pattern-recognition task in pathology: deciding, from a glass slide, whether inflammation is acute, chronic, or granulomatous. The skill directly underlies PA3.4 and is assessed in every university practical examination. Work through each section actively — pause at each micro_quiz before reading the explanation.

References

Robbins & Cotran Pathologic Basis of Disease, 10th ed., Ch 3 (Acute and Chronic Inflammation) (textbook)
Harsh Mohan: Textbook of Pathology, 8th ed., Ch 4 (Inflammation) (textbook)

Version 2.0 | NMC CBUC 2024

CLINICAL SCENARIO

The registrar slides a haematoxylin and eosin (H&E) stained section under the microscope and says: 'Tell me — acute or chronic?' You have 30 seconds. The dominant cell is everything. This module gives you the pattern library to answer that question instantly, every time.

RECALL

Before you start, briefly recall from your Year-1 physiology:
• What are the five cardinal signs of acute inflammation and their vascular basis?
• Name the first cell to arrive at an acute inflammatory site.
• What is a monocyte's tissue name once it has emigrated?

Jot your answers — the microscopy will map directly onto these responses.

WHY THIS MATTERS

Every surgical specimen — an appendix, a lymph node, a lung lobe — comes to pathology with the question: what kind of inflammation is this, and how active is it? Your answer drives the clinician's choice between antibiotics, steroids, anti-TB therapy, or surgery. Pattern recognition on H&E is not an academic exercise; it is the first diagnostic act.

Reading Strategy: Low Power → High Power

A four-panel histology teaching diagram shows the required low-power to high-power microscopy sequence and how dominant inflammatory cells classify acute, chronic, or granulomatous inflammation. — **Panel A:** Low-power tissue architecture; alveolar spaces; pale eosinophilic exudate; tissue destruction; discrete collections or nodules; 4x-10x objective. **Panel B:** Medium-power inflammatory infiltrate; neutrophils; mononuclear cells; epithelioid clusters; dominant cell type; 20x objective. **Panel C:** High-power cell morphology; neutrophil with multilobed nucleus; macrophage with kidney-shaped or oval nucleus; plasma cell with clock-face chromatin; lymphocyte with small dark round nucleus; 40x objective. **Panel D:** Dominant cell decision strip; neutrophils to acute inflammation; lymphocytes, plasma cells, and macrophages to chronic inflammation; epithelioid histiocyte clusters to granulomatous inflammation.

A disciplined reading sequence prevents missed diagnoses.

1. Low power (4×–10×): Assess architecture. Is the tissue normal? Is there destruction, exudate, or fibrosis? Identify any discrete collections or nodules.

2. Medium power (20×): Identify dominant cell type in the infiltrate (neutrophils vs. mononuclear cells vs. epithelioid clusters).

3. High power (40×): Confirm cell identity by nuclear morphology. Count: multilobed = neutrophil; kidney-shaped/oval = macrophage; clock-face chromatin = plasma cell; small dark round = lymphocyte.

The single most important question: what is the dominant inflammatory cell?
• Neutrophils → acute inflammation
• Lymphocytes, plasma cells, macrophages → chronic inflammation
• Epithelioid histiocytes in clusters → granulomatous inflammation

This low→high sequence is required in university practicals. Examaminers deduct marks for jumping straight to 40×.

SELF-CHECK

On low power you see a tissue section with diffuse pale eosinophilic material filling alveolar spaces plus small, dark-staining cells with multilobed nuclei. What is the dominant cell type and what category of inflammation does this represent?

A. Lymphocytes — chronic inflammation

B. Plasma cells — chronic inflammation

C. Neutrophils — acute inflammation

D. Epithelioid histiocytes — granulomatous inflammation

Reveal Answer

Answer: C. Neutrophils — acute inflammation

Multilobed (segmented) nuclei are the hallmark of neutrophils. Neutrophil dominance places this firmly in the acute inflammation category. The pale eosinophilic material in the alveoli is likely fibrinous or serous exudate — features of acute lobar pneumonia.

Acute Inflammation — Core Microscopic Features

A four-panel histology diagram shows the core microscopic features of acute inflammation: neutrophil infiltrate, oedema, vascular congestion, and fibrin deposition. — **Panel A:** Low-power acute inflammation overview showing neutrophil-rich infiltrate, pale oedematous spaces, congested venules, collagen fibres, and focal fibrin-rich exudate.. **Panel B:** High-power neutrophil-rich infiltrate showing multilobed neutrophil nuclei, pale pink cytoplasm, karyorrhexis, pyknosis, and necrotic cellular debris.. **Panel C:** Oedema and vascular congestion showing pale acellular interstitial fluid, widened spaces between fibres, dilated venule, packed red blood cells, and endothelial lining.. **Panel D:** Fibrin in acute exudate showing eosinophilic amorphous-to-fibrillar fibrin strands, tissue fluid, nearby neutrophils, and a visual distinction from homogeneous hyaline material..

Regardless of pattern, all acute inflammatory sections share four microscopic signatures:

Neutrophil-rich infiltrate — multilobed nuclei, pink cytoplasm, often degenerating (karyorrhexis/pyknosis at the centre of necrosis)
Oedema — pale, acellular space widening between cells and fibres (eosinophilic fluid in alveoli or interstitium)
Vascular congestion — dilated, red-cell-packed capillaries and venules
Fibrin — eosinophilic, amorphous to faintly fibrillar material; stains red-pink; distinct from homogeneous glass-like hyaline

These four features occur together in varying proportions across all acute patterns.

IMPORTAN: Karyorrhexis (nuclear fragmentation) in neutrophils indicates a very active, destructive process — note it when present.

Acute Patterns: Suppurative / Abscess

A four-panel medical diagram shows the gross and microscopic structure of a suppurative abscess, contrasts it with tuberculous caseation, and lists common clinical sites. — **Panel A:** Gross abscess cut-section showing pus-filled cavity, creamy-yellow pus, pyogenic membrane, firm fibrous wall in chronic abscess, and surrounding inflamed tissue.. **Panel B:** Microscopic abscess zones showing central liquefactive necrosis, degenerate and viable neutrophils, cellular debris, basophilic bacterial clumps, macrophages, granulation tissue, new capillaries, fibroblasts, and fibrous capsule.. **Panel C:** Comparison of abscess liquefactive necrosis with neutrophil-rich pus versus tuberculous caseation with amorphous cheese-like necrosis and granulomatous inflammation.. **Panel D:** Clinical relevance examples showing appendiceal abscess, breast abscess, and cerebral abscess sharing the same basic morphology..

Suppurative inflammation is characterised by the formation of pus — a creamy mixture of dead and dying neutrophils, liquefied necrotic tissue, and tissue fluid.

Abscess = a localised collection of pus, walled off by a pyogenic membrane (inner zone of neutrophils and necrotic debris → outer zone of granulation tissue → outermost fibrous capsule in chronic abscess).

Microscopic hallmarks:
• Central zone: liquefactive necrosis with neutrophils (both viable and degenerate), cellular debris, and bacteria (may be visible as basophilic clumps)
• Rim: viable neutrophils, then macrophages beginning debris clearance
• Periphery: granulation tissue (new capillaries + fibroblasts) if the abscess is more than a few days old

Gross: pus-filled cavity with creamy-yellow content; walls may be firm in chronic abscess.

Clinical relevance: appendiceal abscess, breast abscess, cerebral abscess — all share this morphology.

IMPORTAN: Do not confuse liquefactive necrosis in an abscess with caseation in tuberculosis. Caseation is amorphous, cheese-like, and granuloma-associated; abscess necrosis is more cellular-ghost-rich and neutrophil-surrounded.