Histotechnology: frozen sections and cryostat

6 Kerala PSC Lab Technician Gr II practice questions on Histotechnology: frozen sections and cryostat, part of Pathology. The first 6 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Medium

   Compared with a freezing microtome, a cryostat:

   1. ACan cut only formalin-fixed tissue
   2. BNeeds tissue embedded in paraffin first
   3. CKeeps knife and block in a cold cabinet
   4. DFreezes tissue with jets of CO2 gas

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   Answer: C · Keeps knife and block in a cold cabinet

   A cryostat is a rotary microtome housed in a refrigerated cabinet, usually at about -20 °C, so the knife, block and sections all stay frozen. It gives thin serial sections of good quality, and the closed cabinet can be disinfected. The freezing microtome freezes the tissue on its stage with carbon dioxide or a thermoelectric module while the knife stays near room temperature, so sections are thicker and harder to handle. Both cut fresh tissue without embedding.

   • A: Cryostats usually cut fresh, unfixed tissue.
   • B: Frozen sections are cut without paraffin embedding.
   • D: Freezing the tissue with CO2 on the stage is the freezing microtome's method.

2. Question 2 · Easy

   What is the main use of frozen sections in a hospital laboratory?

   1. ARapid diagnosis during surgery
   2. BPermanent archiving of tissue
   3. CPreparing electron microscopy
   4. DDecalcifying bone quickly

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   Answer: A · Rapid diagnosis during surgery

   Frozen sections can be cut, stained and reported within minutes, so the surgeon can decide during the operation whether a lesion is malignant or a margin is clear. They are also used where processing would destroy what is sought, such as fat, enzymes and antigens for immunofluorescence. Morphology is poorer than in paraffin sections. The final diagnosis is confirmed on paraffin sections of the same tissue.

   • B: Paraffin blocks, not frozen sections, are the permanent archive.
   • C: Electron microscopy needs glutaraldehyde fixation and resin embedding.
   • D: Freezing does not decalcify; calcified tissue cannot be cut in a cryostat.

3. Question 3 · Hard

   During surgery for a breast lump in a 45-year-old woman, a cryostat at -15 °C cuts the lymph node well, but fatty breast tissue smears and crumbles. What is the best correction?

   1. AProcess the tissue into paraffin wax
   2. BLower the temperature to about -25 to -30 °C
   3. CWarm the block to about -5 °C and recut
   4. DFix the breast tissue in formalin first

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   Answer: B · Lower the temperature to about -25 to -30 °C

   Different tissues need different cutting temperatures. Fat stays soft at -15 °C and smears, so fatty tissue is cut colder, at about -25 °C or below, by cooling the chuck with a freezing aid or lowering the cabinet temperature. Cellular tissue such as lymph node cuts well at -15 to -20 °C, and becomes brittle and cracks if it is too cold. Formalin fixation or paraffin processing would delay the answer far beyond what the surgeon can wait.

   • A: Paraffin processing takes hours, too long for an intraoperative answer.
   • C: Warming makes the fat even softer and the smearing worse.
   • D: Formalin fixation delays the result and does not stop fat smearing.

4. Question 4 · Easy

   Fresh tissue is attached to the cryostat chuck and supported during cutting with:

   1. AHot molten paraffin wax
   2. BDPX mountant
   3. CCelloidin solution
   4. DOCT embedding compound

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   Answer: D · OCT embedding compound

   OCT (optimal cutting temperature) compound is a water-soluble mixture of glycols and resins that is liquid at room temperature and freezes into a firm support around the tissue. It holds the tissue on the chuck and gives an even consistency for cutting at about -15 to -25 °C. It dissolves away during staining. Paraffin wax and celloidin need dehydrated tissue, so they cannot be used for fresh frozen sections.

   • A: Paraffin needs fully processed, dehydrated tissue.
   • B: DPX is a mountant for finished, stained sections.
   • C: Celloidin needs dehydrated tissue and days of infiltration.

5. Question 5 · Medium

   Why are frozen sections, rather than paraffin sections, used to demonstrate fat and tissue enzymes?

   1. AParaffin wax reacts with fat stains
   2. BFrozen sections are thinner than paraffin sections
   3. CFat and enzymes stain only at low temperature
   4. DProcessing removes lipid and destroys enzymes

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   Answer: D · Processing removes lipid and destroys enzymes

   Paraffin processing passes tissue through alcohol and xylene, which dissolve lipids, and through hot wax, which inactivates most enzymes. Frozen sections skip these steps, so fat stains such as Oil Red O and enzyme methods can be done. Frozen sections are also used for immunofluorescence and rapid diagnosis during surgery.

   • A: The problem is loss of lipid during processing, not a reaction with wax.
   • B: Frozen sections are usually thicker, not thinner, than paraffin sections.
   • C: The stains are applied at room temperature after cutting.

6. Question 6 · Medium

   Fresh muscle biopsies are snap-frozen in isopentane cooled by liquid nitrogen mainly to prevent:

   1. AIce crystal artefact
   2. BDrying of the knife edge
   3. CLoss of tissue enzymes
   4. DOverheating of the cryostat

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