Unit 3: Section cutting
Histotechnology-I notes · PTU syllabus (BMLS504-18)
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Unit summary
Thin, even sections are the heart of histology. This unit covers sectioning equipment, microtome knives and sharpening, types of microtome, freezing microtomes and cryostats, faults in paraffin sectioning and their remedies, and mounting sections on slides.
After this unit you can
- Describe microtomes and knives
- Sharpen knives by honing and stropping
- Cut frozen sections with a cryostat
- Identify sectioning faults and mount sections
PTU syllabus topics
- Equipment for sectioning
- microtome knives and sharpening (honing, stropping)
- types of microtome
- freezing microtome and cryostats
- faults in paraffin section cutting and remedies
- mounting sections to glass slides
Rotary
Routine paraffin sections
3–5 µm
Sliding
Large or hard blocks
Variable
Cryostat (freezing)
Frozen sections during surgery
5–10 µm
Ultramicrotome
Electron microscopy
Under 0.1 µm
Topic 1
Equipment for sectioning
- Microtome, knives or disposable blades, floatation water bath (about 5–10 °C below wax melting point, 40–45 °C), slide warmer or oven, forceps, brushes, adhesive-coated slides.
Topic 2
Microtome knives and sharpening
- Plane-concave
- Celloidin and paraffin (biconcave variant for paraffin)
- Plane wedge
- Paraffin and frozen sections
- Tool edge (C profile)
- Hard tissue
- Disposable blades
- High and low profile — most common now
Purpose
Removes nicks; forms the cutting edge
Polishes and removes burrs; final sharpening
Tool
Hone (stone or glass plate) with abrasive and oil or water
Leather strop
Direction
Heel to toe, edge first
Toe to heel, edge last (back first)
- Bevel angle about 27–32°; clearance angle 3–8° between knife and block.
Topic 3
Types of microtome
Rotary
Wheel turns; block moves vertically past a fixed knife
Routine paraffin sections — most common
Base sledge
Knife fixed; block moves
Large blocks
Sliding
Knife slides over a fixed block
Celloidin sections
Rocking
Block rocks in an arc
Simple, older
Freezing microtome and cryostat
Cut frozen tissue
Rapid diagnosis
Ultramicrotome
Glass or diamond knife
Electron microscopy (60–90 nm)
Topic 4
Freezing microtome and cryostat
- Cryostat: a microtome inside a refrigerated cabinet (−20 °C to −25 °C); tissue frozen onto a chuck in OCT compound, cut at 5–10 µm, picked on a slide and stained rapidly (H&E, toluidine blue).
- Uses: intraoperative diagnosis (margins, malignancy), lipids, enzymes, immunofluorescence (kidney and skin biopsies). Drawbacks: freezing artefact, fatty tissue difficult, infection risk (decontaminate).
Topic 5
Faults in paraffin section cutting and remedies
Sections fail to form a ribbon
Wax too hard; knife blunt; room cold
Breathe on block, softer wax, new blade
Thick and thin sections alternating
Loose block, knife or clamp; clearance angle wrong
Tighten everything; adjust angle
Sections compressed or wrinkled
Blunt knife; warm block
New blade; cool block on ice
Splits or lines across sections
Nicked knife; hard particles (calcium, sutures)
Move blade; decalcify
Crumbling, holes
Poor processing — incomplete dehydration or infiltration
Reprocess
Sections curl
Blunt knife; clearance angle too large
New blade; adjust angle
Ribbon curved
Block edges not parallel
Trim block edges parallel
Topic 6
Mounting sections on slides
- 1Float the ribbon on the water bath to flatten
- 2Separate sections with forceps
- 3Pick up on a clean, coated slide (albumin, poly-L-lysine or charged slides)
- 4Drain and label
- 5Dry on a hot plate or in an oven at 56–60 °C for 30–60 minutes
- Adhesives prevent sections floating off during staining, especially after decalcification or heat-retrieval steps.
Key terms
- Rotary microtome
- Most common microtome for paraffin sections
- Honing
- Grinding a knife edge on a hone
- Stropping
- Polishing a knife on leather
- Cryostat
- Refrigerated microtome for frozen sections
- Clearance angle
- Angle between knife facet and block face
Quick revision
- Sectioning equipment; water bath temperature.
- Knife profiles; honing vs stropping; angles.
- Rotary, sledge, sliding, rocking, cryostat, ultramicrotome.
- Frozen section uses and drawbacks.
- Faults and remedies; mounting and adhesives.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.Which microtome is used routinely?
- Q2.Distinguish honing and stropping.
- Q3.What is OCT compound?
- Q4.Give two uses of frozen sections.
- Q5.What causes alternate thick and thin sections?
- Q6.Why are coated slides used?
Long-answer questions
- Q1.Describe the types of microtome.
- Q2.Describe the cryostat and frozen section technique.
- Q3.Describe faults in paraffin section cutting and their remedies.
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