Overview
A microtome (Greek: mikros = small, temnein = to cut) is a tool used to cut extremely thin slices of material, known as sections. It is one of the most precise and important instruments in the histopathology laboratory, directly determining the quality of every section examined under the microscope.
This lesson covers the major types of microtomes — rotary, sledge, cryomicrotome, ultramicrotome, vibrating, saw, and laser — their applications, and the microtome knife itself, including its care, sharpening, and the disposable blades increasingly used in modern practice.
Learning Objectives
After this lesson you will be able to…- Define a microtome and describe its major applications
- Explain the principle and advantages of the rotary microtome
- Differentiate sledge, cryomicrotome, ultramicrotome, vibrating, saw and laser microtomes
- Describe the microtome knife, its types, and sharpening techniques (honing and stropping)
- Compare traditional reusable knives with disposable blades
- Describe correct care and storage of the microtome knife
Clinical Story
Why This MattersA pathologist urgently needs a frozen section result during surgery to determine whether a breast lump is malignant before the surgeon decides how much tissue to remove. The histotechnologist uses a cryostat — a rotary microtome housed in a refrigerated cabinet — to freeze and cut the specimen in minutes rather than the 16 hours a paraffin-embedded section would require, delivering a diagnosis to the operating theatre while the patient is still under anaesthesia.
Core Concepts
The most commonly used microtome for routine histopathology. A rotary action of the hand wheel actuates the cutting movement; the knife is fixed horizontally, and the block holder moves up and down on a steel carriage advanced by a micrometer screw. Advantages: heavy and stable (minimal vibration), can obtain serial sections/ribbons, cutting/knife angle adjustable, and can cut celloidin sections with a special holder. Typical cut thickness is 1–60 µm; can achieve 0.5 µm 'semi-thin' sections in hard resin-embedded samples. Auto-cut microtomes have a built-in motor drive with foot/hand control.
The sample sits in a fixed holder (shuttle) on a linear bearing, allowing many coarse sections to be cut readily. Used for large samples such as those embedded in paraffin for biological preparations. Typical cut thickness: 10–60 microns.
Many rotary microtomes can be adapted to cut in a refrigerated (liquid nitrogen or mechanically cooled) chamber. Reduced temperature increases sample hardness (via glass transition), enabling semi-thin sections; both sample and knife temperature must be controlled to optimise thickness. Cryosectioning is much faster than traditional paraffin histology (about 15 minutes vs 16 hours) and is used for rapid intraoperative diagnosis and some immunohistochemistry.
Functions like a rotary microtome but with extremely tight mechanical tolerances; linear thermal expansion of the mounting provides very fine thickness control. Used for TEM (40–100nm sections) and SBFSEM (30–50nm), and sometimes for light-microscopy survey sections up to 500nm. Diamond knives (preferred) or glass knives are used, with sections floated on liquid and picked up on grids.
The vibrating microtome uses a vibrating blade requiring less cutting pressure, ideal for difficult biological samples (cut thickness 10-500µm). The saw microtome uses a recessed rotating saw for very hard materials like teeth or bone (minimum ~30µm thickness). The laser microtome performs contact-free slicing using a focused infra-red laser via photo-disruption, requiring no prior embedding, freezing, or fixation, and can also cut very hard materials such as bone, teeth, and some ceramics (10-100µm thickness).
Made of high-carbon steel, tempered at the tip; a plain wedge (C-type) knife, 100-350mm long, is used for routine work. Honing removes nicks and irregularities using coarse and fine abrasives. Stropping removes the 'burr' formed during honing and polishes the cutting edge. Diamond and glass knives (expensive) are used for ultramicrotomy. Disposable blades are now widely used: low-profile blades for small/soft biopsies (kidney, liver); high-profile blades for tougher tissue (myometrium, breast, skin). Advantages of disposable blades: no honing/stropping needed, resistant to corrosion and heat, comparable hardness to steel; disadvantages: relatively expensive and less rigid than steel knives.
Laboratory Principle
A rotary microtome cuts sections through a repeating four-phase cycle: the sample holder descends, the knife shears a section from the block face, the section remains on the knife edge, and at the top of the rotary stroke the holder advances by precisely the thickness of the next intended section before the cycle repeats. The flywheel's mass smooths this motion, preventing the sample from stalling mid-cut and ensuring uniform section thickness.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Xylene | AR grade | Cleaning the microtome knife before and after use | Ventilated storage |
| Light machine oil / grease | Light grade | Protecting stored knives against corrosion | Cool, dry storage |
| Honing abrasive compound | Coarse & fine grades | Removing nicks and irregularities from knife edge | Dry storage |
Step-by-Step Procedure
Choose rotary (routine paraffin), cryostat (frozen/rapid), sledge (large blocks), or ultramicrotome (EM) based on the specimen and clinical need.
Fix the paraffin/resin block firmly into the block holder, ensuring correct orientation.
Adjust the micrometer screw to the desired thickness (commonly 3-5µm for routine H&E).
Remove excess wax and rough-cut until the full tissue face is exposed.
Rotate the hand wheel steadily to produce a continuous ribbon of sections, floating them onto a water bath before mounting on slides.
Wipe the knife with xylene after use, and store it in its protective box; apply light oil/grease if stored long-term.
Flow Diagram
Quality Control
Inspect knife edge for nicks before starting a session; verify section thickness periodically using calibrated settings; discard or re-sharpen knives producing chatter marks, tears, or uneven ribbon thickness. Use a dedicated knife for decalcified/hard tissue to protect the main knife edge.
Microtomes should be serviced and calibrated periodically per manufacturer schedules; laboratories may participate in external assessment of section quality (thickness uniformity, absence of artefacts) as part of accreditation.
Reference Values
Key Parameters⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals and SOPs.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Chatter marks / venetian-blind artefact on sections | Dull knife, loose block holder, or vibration during cutting | Re-sharpen/replace knife blade; check block holder tightness |
| Tears or holes in sections | Nicked knife edge or hard/calcified debris in tissue | Inspect and replace knife; confirm complete decalcification |
| Wrinkled or compressed sections | Blunt knife or incorrect cutting angle | Re-sharpen knife; adjust clearance angle |
Common Errors & How to Avoid Them
Cause: A damaged cutting edge produces tears, holes, and inconsistent section thickness.
Prevention: Inspect the knife edge before each session and re-sharpen (hone and strop) or replace disposable blades promptly when damage is noted.
Cause: Hard, incompletely decalcified, or bony tissue rapidly dulls and nicks the knife edge.
Prevention: Reserve a separate knife specifically for hard or bone-containing tissue to protect the main knife.
Cause: Exposed knife edges can be damaged by contact with other objects and are prone to corrosion.
Prevention: Always store knives in their dedicated protective box, cleaned with xylene, and coated with light oil/grease for long-term storage.
Laboratory Tips from the Bench
When cutting a difficult, fatty or friable specimen, briefly chilling the block on ice or a cold plate before sectioning often produces cleaner, more consistent ribbons.
Keep at least one dedicated 'hard tissue' knife or set of disposable blades for bone or calcified specimens, sparing your primary knife from unnecessary nicks.
Remember knife maintenance with 'Honing removes Holes/nicks, Stropping removes Small burrs and Shines' — Honing = coarse correction, Stropping = fine polish.
Important Notes
Microtome knives and blades are extremely sharp; always use the knife guard when not actively cutting, and handle blade changes with the recommended forceps or blade-removal tool, never bare fingers.
Cryosectioning takes about 15 minutes compared to roughly 16 hours for standard paraffin processing, making it indispensable for intraoperative frozen section diagnosis.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeDuring breast-conserving surgery, the surgeon excises a lump and sends it for immediate intraoperative assessment to determine whether wider excision margins are needed.
The cryostat enabled rapid freezing and sectioning of the specimen within minutes, allowing the pathologist to render an intraoperative diagnosis of invasive carcinoma while the patient remained under anaesthesia, guiding the surgeon's decision to proceed with wider excision.
- →Cryostat sectioning provides rapid intraoperative diagnosis unavailable with standard paraffin processing
- →Frozen sections may show more ice-crystal artefact than paraffin sections but remain diagnostically useful
- →Rapid turnaround directly influences real-time surgical decision-making
Frequently Asked Questions
The rotary microtome is heavier and more stable, producing less vibration and allowing thinner, more consistent serial sections and ribbons, which are ideal for routine diagnostic histology.
Disposable blades save time by eliminating honing and stropping and resist corrosion and heat well, but they are relatively more expensive and less rigid than a well-maintained traditional steel knife.
Rapid freezing can create ice crystals within tissue that distort cellular architecture, an unavoidable trade-off for the dramatically faster turnaround time frozen sectioning offers.
Quick Revision
10-Minute ReviewKey Takeaways
- The microtome is the precision instrument that determines final section quality for microscopy.
- Different microtome types (rotary, sledge, cryostat, ultramicrotome, saw, laser) serve specific clinical and research needs.
- The rotary microtome remains the standard tool for routine paraffin histopathology.
- Cryosectioning provides rapid intraoperative diagnostic capability unmatched by paraffin processing.
- Microtome knife sharpness and care (honing, stropping, correct storage) directly affect section quality.
- Disposable blades have largely supplemented traditional reusable knives in many modern laboratories.
Competency Checklist
Track Your MasteryReferences
- Bancroft JD, Layton C. Theory and Practice of Histological Techniques. 8th ed.
- Suvarna SK, Layton C, Bancroft JD. Bancroft's Theory and Practice of Histological Techniques.
- NIOS Vocational Course — Histology and Cytology Module, Lesson 9: Microtome.