HT Specimen Handling & Fixation 3 — Questions and Answers
Question 1: Which of the following is a MAJOR safety concern when working with Zenker's fixative?
- High flammability
- Mercury toxicity and disposal requirements (Correct answer)
- Strong alkalinity causing skin burns
- Release of cyanide gas when heated
Correct answer: Mercury toxicity and disposal requirements
Zenker's fixative contains mercuric chloride, a heavy metal that is highly toxic and requires special disposal procedures per environmental regulations.
Question 2: A specimen is received in Hollande's fixative. This fixative is MOST similar in composition to:
- Zenker's fixative
- Bouin's fixative (Correct answer)
- Carnoy's fixative
- Michel's fixative
Correct answer: Bouin's fixative
Hollande's fixative is a modified Bouin's solution that substitutes copper acetate for some picric acid, offering similar preservation properties.
Question 3: What is the purpose of adding acetic acid to a fixative such as Carnoy's solution?
- To increase protein cross-linking
- To preserve nucleic acids and prevent chromosome shrinkage (Correct answer)
- To enhance lipid preservation
- To neutralize the pH of the fixative
Correct answer: To preserve nucleic acids and prevent chromosome shrinkage
Acetic acid in Carnoy's fixative penetrates rapidly and fixes nucleic acids, counteracting the tissue shrinkage caused by ethanol.
Question 4: Which fixative is recommended for preserving tissue intended for immunofluorescence studies of renal biopsies?
- 10% neutral buffered formalin
- Michel's transport medium (Correct answer)
- Glutaraldehyde
- Bouin's fixative
Correct answer: Michel's transport medium
Michel's medium preserves antigens for immunofluorescence at room temperature, allowing transport of renal biopsies without freezing.
Question 5: The process of tissue shrinkage during fixation is MOST associated with which type of fixative?
- Formalin-based fixatives
- Coagulative fixatives such as ethanol (Correct answer)
- Glutaraldehyde
- Osmium tetroxide
Correct answer: Coagulative fixatives such as ethanol
Coagulative fixatives like alcohols precipitate proteins and cause significant cellular shrinkage, unlike cross-linking fixatives such as formalin.
Question 6: When a skin punch biopsy for direct immunofluorescence (DIF) arrives, the CORRECT action is to:
- Place immediately in 10% NBF
- Place in Zeus or Michel's transport medium (Correct answer)
- Freeze in liquid nitrogen
- Fix in glutaraldehyde for EM
Correct answer: Place in Zeus or Michel's transport medium
DIF requires unfixed tissue transported in Zeus or Michel's medium to preserve immunoglobulin and complement antigens at the dermal-epidermal junction.
Question 7: What is the role of sodium phosphate buffer in 10% neutral buffered formalin (NBF)?
- To increase fixation speed
- To maintain pH near 7.0 and prevent formic acid formation (Correct answer)
- To enhance protein cross-linking
- To act as a preservative against microbial growth
Correct answer: To maintain pH near 7.0 and prevent formic acid formation
Phosphate buffer maintains the pH of NBF near neutrality, preventing autoxidation of formaldehyde to formic acid which would create an acidic fixative.
HT Specimen Handling & Fixation 3
Which of the following is a MAJOR safety concern when working with Zenker's fixative?