MLT GlobeHistopathology: Tissue processing & embedding – page 2
39 Histopathology MCQs on Tissue processing & embedding with answers and explanations.
Why is paraffin impregnation carried out in two or more changes of molten wax?
Xylene carried into the first wax bath contaminates it; later fresh baths ensure pure wax and firm blocks.
In a graded ethanol dehydration schedule, which step is usually repeated two or three times?
Several changes of absolute alcohol ensure all water is removed before clearing; the lower grades are single steps.
A tissue processor is set for a delayed start over the weekend. In which reagent should tissue wait?
Tissue waits in fixative so fixation continues; prolonged alcohol, xylene or hot wax would harden or damage it.
How often reagents are changed on an automated processor mainly depends on:
Reagents become diluted and contaminated in proportion to tissue throughput, so rotation is based on cassette counts or monitoring.
Why does a processor basket pause above a reagent before moving on?
Draining minimises contamination of the next reagent; drying tissue would damage it.
If clearing is incomplete and alcohol remains in the tissue, what happens during wax infiltration?
Paraffin is immiscible with alcohol, so residual alcohol blocks infiltration. The result is poorly supported tissue that crumbles or tears on sectioning.
To show mucosa, submucosa, muscularis and serosa in one section, how should a piece of small intestine be embedded?
Tubular and layered specimens are embedded on edge so each section passes through all wall layers. Flat embedding gives only a single layer.
As the melting point of an embedding paraffin increases, the wax becomes:
Higher-melting waxes are harder and support dense tissues and thin sections, but may ribbon less readily in cool rooms.
Compared with 55–58 °C paraffin, a wax melting at 50–52 °C will:
Low-melting wax is softer: it ribbons readily but supports dense tissue poorly and makes very thin sections harder to obtain.
Celloidin-embedded blocks should be stored:
Celloidin blocks harden and shrink if they dry out, so they are kept in 70–80% alcohol. Paraffin blocks are stored dry.
Which embedding medium is hygroscopic, so its blocks must be stored with a desiccant?
Water-soluble waxes such as Carbowax absorb moisture from air and soften. Paraffin and epoxy resin are not hygroscopic; celloidin is stored in alcohol.
Fresh tissue is quenched at about −160 °C, then its ice is removed by sublimation under vacuum at about −40 °C before embedding. This technique is called:
Freeze-drying (lyophilization) removes water as vapour by sublimation under vacuum. Freeze-substitution instead dissolves the ice in a cold solvent such as acetone.
A laboratory switches from xylene to an aliphatic hydrocarbon clearant. What drawback should it anticipate?
Aliphatic substitutes are less toxic but tolerate little water and do not mix with some mountants, so coverslipping may need compatible media.
During microtomy, many blocks contain hard, brittle, shrunken tissue. The most likely processing fault is:
Paraffin kept well above its melting point overhardens and shrinks tissue. Water in the clearant causes soft, poorly infiltrated tissue instead.
Fine microscopic chatter in sections is commonly caused by tissue that is:
Excess time in absolute alcohol makes tissue hard and brittle, producing chatter. Under-processed tissue is soft and mushy.
H&E sections show patchy, uneven staining with poor nuclear detail. A likely processing cause is:
Water in the clearant prevents complete clearing and paraffin infiltration, giving unevenly processed tissue that stains patchily.
To save time, all fixation, dehydration and clearing steps of a processor are run at 45 °C. What problem is most likely?
Heat accelerates reagents, so small or delicate tissues become over-hardened and brittle, producing microchatter on cutting.
Which reagent can dehydrate tissue and also mix directly with paraffin, so a separate clearing step can be skipped?
Dioxane is miscible with water and paraffin, acting as a dehydrant–clearant, though its toxicity limits use; ethanol needs a clearing agent.
What is the main advantage of Leuckhart (L-shaped) embedding moulds?
Two L-shaped metal pieces on a plate can be moved to form moulds sized to the specimen.