Microtomy (section cutting)

Principle

• A microtome advances a paraffin block by a set thickness and cuts it against a sharp blade, producing a ribbon of thin sections.
• The rotary microtome is standard for paraffin blocks; routine sections are about 3–5 µm thick.

Specimen

• Paraffin blocks from processing.

Reagents and equipment

• Rotary microtome, disposable steel blades (low- or high-profile), ice tray or cold plate, flotation water bath (about 40–45 °C, several degrees below wax melting point), adhesive or charged slides, slide drying oven or hot plate, forceps and brushes.

Procedure

• Clamp the block and blade securely with the correct clearance angle (a few degrees, as recommended).
• Face (trim) the block at a thicker setting until the full tissue surface is exposed, avoiding excessive trimming of small biopsies.
• Chill the block face on ice or a cold plate.
• Cut at the required thickness with a smooth, even hand-wheel motion to form a ribbon.
• Float the ribbon on the water bath to flatten wrinkles, separate sections and pick up on labeled slides.
• Drain and dry (e.g. about 60 °C for a short period or 37 °C overnight) so sections adhere.
• Lock the hand-wheel and cover the blade when not cutting.

Quality control and pitfalls

• Chatter (parallel thick/thin bands): dull or loose blade, loose block, hard or over-dehydrated tissue, or wrong clearance angle.
• Knife lines or scratches: nicked blade or grit in tissue.
• Thick and thin alternating sections: loose parts or wrong clearance angle.
• Wrinkles and folds: water bath too cool or poor floating technique; bubbles under sections: poor pickup.
• Holes: over-trimming or poorly processed tissue.
• Floaters (contaminating tissue): skim the water bath between blocks.
• Check slide labels match the block.

Clinical use

• Produces sections for H&E, special stains, IHC and in situ hybridization; kidney biopsies often need thinner sections (about 2–3 µm).
• Frozen sections are cut on a cryostat instead.