15.2 Microtomy, Sectioning Artifacts, Hematoxylin and Eosin Staining, and Special Stains

Microtomy, Sectioning Artifacts, Hematoxylin and Eosin Staining, and Special Stains

After embedding, the paraffin block must be cut into thin, flat, artifact-free sections and stained so the pathologist can read them. Staining carries 20% of the histopathologic techniques blueprint and tissue processing 40%, so microtomy and staining together account for much of this subject. This section covers microtome types and knife geometry, flotation and adhesives, troubleshooting sectioning artifacts, the chemistry of hematoxylin and eosin staining, and the special stains used to show connective tissue, carbohydrates, pigments, microorganisms, and lipids. Cytology is covered in the next section.

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1. Microtomy and Instrumentation

Microtomy is the mechanical process by which embedded tissue blocks are sectioned into ultra-thin slices (3 to 5 micrometers [$\mu\text{m}$]) suitable for light transmission microscopy.

                         ┌──────────────────────────────────────────────┐
                         │          Microtomy Knife Angle Setup         │
                         └──────────────────────┬───────────────────────┘
                                                │
             Block Travel Axis (Vertical)       │  Clearance Angle: ~5° to 10°
                         │                      │  • Too small: scraping, compressed sections
                         ▼                      │  • Too large: chatter, thick-and-thin bands
                   ┌──────────┐                 │
                   │ Paraffin │                 │  Wedge Bevel Angle: 27° to 32°
                   │  Block   │                 │  • Ground cutting facet on knife edge
                   └────┬─────┘                 │
                        │   /|                  │  Water Bath Operations:
                        │  / | Knife Wedge      │  • Temperature: 45°C to 50°C
                        │ /  |                  │    (5°C to 10°C below wax melting point)
                        │/   |                  │  • Adhesives: Mayer's egg albumin,
                        │/───┘ Clearance Angle  │    poly-L-lysine, 0.1% gelatin
                        │ (~5°-10°)             │  • Drying Oven: 60°C for 30 to 60 minutes

Instrument Types

1. Rotary Microtome (Minot Type): The standard instrument for routine paraffin sections. Operated by rotating a handwheel that drives a vertical block advance mechanism governed by a precision micrometer screw and pawl-and-ratchet gear, advancing the block toward a stationary knife by 3 to 5 $\mu\text{m}$ per stroke.
2. Freezing Microtome and Cryostat: A specialized rotary microtome enclosed within a hermetically sealed, refrigerated cabinet maintained at -20°C to -25°C. It cuts fresh, unfixed tissues frozen rapidly in OCT (Optimal Cutting Temperature) embedding matrix. Provides STAT intraoperative consultations within 10 to 15 minutes, preserving labile cellular enzymes and neutral lipids that would otherwise be destroyed by alcohol and clearing solvents.
3. Sliding Microtome: The knife travels horizontally across a fixed block; used primarily for large celloidin-embedded blocks or whole-organ brain sections.
4. Ultramicrotome: Utilizes diamond or freshly broken glass knives to cut ultrathin sections (50 to 90 nanometers [$\text{nm}$]) for transmission electron microscopy.

Knife Geometry and Operating Angles

• Bevel Angle: The angle formed between the two ground facets that create the cutting edge, standardized between 27° and 32°.
• Clearance Angle (Rake / Tilt Angle): The crucial functional angle formed between the posterior facet of the microtome knife and the vertical cutting plane of the paraffin block face. Histotechnology texts commonly quote a clearance angle of about 5° to 10°; the working setting depends on the blade type and holder geometry, so start from the manufacturer's recommendation and adjust in small steps:
  • Clearance Angle Too Small: The knife facet rubs, scrapes, and compresses the block face, failing to cut a section on one stroke and producing a thick, jammed section on the next.
  • Clearance Angle Too Large: The knife edge digs into the block, inducing severe vibration, chatter marks, and thick-and-thin horizontal ribbons.

Water Bath Operations and Section Adhesives

• Water Bath Temperature: Maintained rigidly at 45°C to 50°C—calculated as 5°C to 10°C below the melting point of the paraffin wax (56°C–58°C). The surface tension of warm water flattens out wrinkles and folds in the paraffin section without melting the block wax.
• Section Adhesives: Added to prevent sections from detaching during subsequent acidic, alkaline, or high-temperature staining cycles:
  • Mayer's Egg Albumin: Egg white (albumin) mixed with equal parts glycerin (to retain moisture) and a crystal of thymol (preservative/bacteriostat).
  • Gelatin (0.1% to 0.5%): Added directly to the floating bath water.
  • Poly-L-Lysine: Synthetic polymer imparting positive surface charges that bind negatively charged tissue components; essential for immunohistochemistry and in situ hybridization.
• Slide Drying: Slides are drained vertically and placed in a drying oven at 60°C for 30 to 60 minutes (or 37°C overnight) to melt the paraffin onto the glass and cross-link adhesives.

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2. Microtomy Sectioning Artifacts and Troubleshooting Matrix

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3. Routine Hematoxylin and Eosin (H&E) Staining

H&E is the universal diagnostic staining system in anatomical pathology, providing an exceptional balance between crisp blue-purple nuclear chromatin and differential shades of pink-red cytoplasmic structures.

Hematoxylin Chemistry and Mordanting

• Botanical Origin: Extracted from the heartwood of the logwood tree Haematoxylum campechianum. Crude hematoxylin is non-staining and lacks dye properties.
• Oxidation (Ripening): Hematoxylin must be oxidized to its active quinonoid chromophore, hematein:
  • Natural Ripening: Exposure to atmospheric oxygen and sunlight in unsealed carboys; slow (takes 3 to 6 months), but yields long-lasting staining solutions (e.g., Ehrlich's hematoxylin).
  • Chemical Ripening: Rapid chemical oxidation using oxidizing agents: sodium iodate ($NaIO_3$) in Mayer's and Gill's hematoxylin; mercuric oxide ($HgO$) in Harris hematoxylin; potassium permanganate ($KMnO_4$) in MacCallum's.
• Mordanting Mechanism: Hematein is an anionic dye with minimal direct affinity for tissue. To bind cellular structures, it requires a polyvalent metal cation known as a mordant (primarily aluminum, $Al^{3+}$, or ferric iron, $Fe^{3+}$). The mordant forms a coordination complex (chelate lake) with hematein, imparting a net positive charge that binds tenaciously to the negatively charged phosphate polyanions of nuclear DNA and RNA.

Progressive vs. Regressive Staining

• Progressive Staining: Tissue sections are incubated in a mild hematoxylin solution (e.g., Mayer's) just long enough to stain nuclear chromatin to the desired intensity without overstaining the cytoplasm. Decolorization is unnecessary.
• Regressive Staining: Tissue sections are intentionally overstained with a concentrated, highly aggressive hematoxylin formula (e.g., Harris). Excess dye is subsequently extracted using a chemical differentiating agent:
  • Differentiation: Immersion in acid alcohol (1% hydrochloric acid in 70% ethyl alcohol) for several seconds, stripping hematoxylin from the cytoplasm and uncharged structures while leaving nuclear chromatin intact.
  • Bluing: Acid alcohol turns the hematoxylin lake red. The section is washed in an alkaline solution—Scott's tap water substitute ($MgSO_4$ and $NaHCO_3$), 0.2% ammonium hydroxide, or saturated lithium carbonate—shifting the pH above 8.0, which converts the red hematoxylin-aluminum lake into an insoluble, crisp deep blue-purple complex.

Eosin Y Counterstaining

• Mechanism: Eosin Y is a synthetic xanthene acidic dye (tetrabromofluorescein). In an acidic solution (pH 4.6 to 5.0 adjusted with glacial acetic acid), eosin's anionic carboxylate groups bond ionically to positively charged basic amino acid residues ($NH_3^+$ of lysine, arginine, and histidine).
• Differential Color Shading: Proper eosin staining produces three distinct shades of pink/red:
  1. Erythrocytes (RBCs): Brilliant, deep crimson-red or intense pink.
  2. Muscle and Cytoplasm: Intermediate pink.
  3. Collagen and Basement Membranes: Pale pink.

Standard H&E Technical Sequence

$\text{Deparaffinization (Xylene }\times 3) \longrightarrow \text{Rehydration (100\%, 95\%, 80\%, 70\% Alcohols }\to \text{ Water)}$ $\longrightarrow \text{Harris Hematoxylin} \longrightarrow \text{Rinse} \longrightarrow \text{Acid Alcohol (Differentiation)}$ $\longrightarrow \text{Rinse} \longrightarrow \text{Scott's Bluing / Alkaline Wash} \longrightarrow \text{Rinse} \longrightarrow \text{Eosin Y Counterstain}$ $\longrightarrow \text{Dehydration (95\%, 100\% Alcohols }\times 2) \longrightarrow \text{Clearing (Xylene }\times 3) \longrightarrow \text{Mounting}$

• Mounting Media: Synthetic resins (DPX, Permount) dissolved in xylene; refractive index matches high-grade optical glass ($1.51\text{ to }1.54$).

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4. Special Histochemical Stains Panel