Learning objectives
1. Orientation principles
Embedding determines what the pathologist or researcher sees first. The tissue surface that must be sectioned should contact the mold base and lie flat without tilt, folds, or trapped air.
| Specimen | Preferred orientation | Reason |
|---|---|---|
| Skin ellipse | Epidermis perpendicular to the blade; all epidermal edges at the same level. | Shows epidermis, dermis, margins, and lesion depth. |
| GI biopsy | On edge with mucosa facing the same direction. | Displays crypt-villus architecture rather than a flat mucosal surface. |
| Tubular structure | Cross-section unless longitudinal relationships are specifically required. | Shows lumen and concentric wall layers. |
| Cyst wall | Wall on edge. | Preserves epithelial lining and wall thickness. |
| Lymph node | Cut surface flat; capsule represented. | Shows capsule, subcapsular sinus, cortex, and medulla. |
| Muscle | Include transverse and longitudinal orientation when needed. | Different disorders and artifacts are easier to assess in different planes. |
| Kidney core | Straight and flat without bending. | Maximizes cortex and glomerular yield across levels. |
| Multiple tiny pieces | Centralize, align, and use biopsy paper, mesh, or sponge as validated. | Prevents loss and unequal depth. |
Orientation rule: think about the plane of section before the tissue touches molten paraffin.
2. Building a good paraffin block
- Use paraffin only hot enough to remain fully molten; avoid unnecessary heat exposure.
- Select a mold that provides support without excessive paraffin around the tissue.
- Flatten the required surface at the mold base and hold it until the first paraffin film forms.
- Attach the cassette securely and cool the block rapidly and evenly on a cold plate.
- Avoid cracks, bubbles, tissue tilt, incomplete cassette attachment, and mixed specimen identifiers.
- For re-embedding, preserve the entire specimen and document the reason when required.
Block preparation
Trim excess paraffin from the back and edges, expose a complete tissue face without wasting tissue, and cool the block appropriately before sectioning. Small biopsies and limited research specimens should be faced conservatively.
3. Microtomy controls
| Variable | Effect | Practical approach |
|---|---|---|
| Blade | Dull or damaged areas cause compression, scoring, tearing, and knife lines. | Use a clean sharp segment; inspect blade holder and specimen clamp. |
| Clearance angle | Too small can scrape or compress; too large can chatter. | Start with the instrument/blade recommendation, often near 3–5°, then adjust minimally. |
| Block temperature | Warm wax compresses; very cold wax may crack or chatter. | Cool consistently and match block hardness to tissue. |
| Cutting speed | Excess speed increases compression and vibration. | Use a smooth controlled stroke, especially for hard or heterogeneous tissue. |
| Section thickness | Affects morphology, stain intensity, and quantitative analysis. | Use the validated target, commonly about 3–5 µm for routine paraffin sections. |
| Water bath | Too cool leaves wrinkles; too hot causes expansion, distortion, and analyte loss. | Use clean water below the paraffin melting point and validate locally, often around 40–45 °C. |
4. Artifact troubleshooting
| Artifact | Common causes | Corrective direction |
|---|---|---|
| Compression | Warm block, dull blade, fast cutting, soft wax. | Cool block, move to a sharp blade area, slow the stroke, verify processing. |
| Chatter or “venetian blind” | Hard/brittle tissue, excessive clearance angle, loose clamp, vibration. | Secure components, reduce angle slightly, cool or condition appropriately, review processing. |
| Knife lines | Blade nick, hard debris, calcification, contaminated wax. | Move or replace blade; inspect tissue and blade holder; recut after removing debris. |
| Folds and wrinkles | Poor ribbon handling, low bath temperature, damaged section edge. | Improve ribbon technique and use validated flotation time and temperature. |
| Section lifting | Dirty slide, inadequate adhesive, aggressive retrieval, poor drying. | Use validated charged slides/adhesive and drying conditions; avoid touching tissue area. |
| Holes or tearing | Incomplete processing, brittle inclusions, rough facing, static. | Review processing and block condition; adjust cutting and environmental controls. |
| Floaters | Dirty bath, forceps, brush, blade, or shared surfaces. | Clean between cases, skim or change bath water, and investigate contamination source. |
5. Cryostat sectioning
A common starting chamber temperature is near −20 °C, but optimal temperature depends on tissue composition. Fatty tissue generally requires colder conditions; very cellular or aqueous tissue may cut better warmer.
- Treat unfixed human tissue as potentially infectious and follow exposure-control procedures.
- Use appropriate PPE and mechanical tools; never retrieve tissue near the blade with fingers.
- Lock the handwheel and guard or remove the blade during cleaning and specimen changes.
- Follow the manufacturer and facility decontamination procedure; wiping visible surfaces alone is not a complete decontamination process.
- For RNA, DNA, or other sensitive workflows, use dedicated clean tools, fresh blades, controlled surfaces, and contamination monitoring.
6. Quality control and safety
- Verify microtome thickness settings and mechanical condition according to the maintenance program.
- Check water-bath and oven temperatures with calibrated or verified devices.
- Document preventive maintenance, blade-holder cleaning, loose components, and corrective actions.
- Use cut-resistant practices for blade handling and dispose of blades in approved sharps containers.
- Label slides before or immediately after section pickup using a controlled workflow that prevents case mix-up.
- For quantitative image analysis, standardize section thickness, level, tissue orientation, staining, and scanning.
Practice quiz
Best: —References and scope
- ASCP Board of Certification. Current HT and HTL examination content guidelines and suggested reading list.
- Carson FL, Cappellano CH. Histotechnology: A Self-Instructional Text. 5th ed. ASCP Press; 2020.
- Brown RW, ed. Histologic Preparations: Common Problems and Their Solutions. College of American Pathologists.
- Follow current microtome/cryostat manuals, local SOPs, infection-control procedures, and safety data.
Independent educational content. Not affiliated with or endorsed by ASCP or the ASCP Board of Certification.