ASCP Tissue Fixation and Decalcification 1 — Questions and Answers
Question 1: What is the primary purpose of tissue fixation in histology?
- To dehydrate tissue before embedding
- To prevent autolysis and putrefaction (Correct answer)
- To stain cellular components for visibility
- To harden tissue for paraffin infiltration
Correct answer: To prevent autolysis and putrefaction
Fixation prevents autolysis (self-digestion by intracellular enzymes) and putrefaction (bacterial decomposition) by stabilizing tissue proteins and halting enzymatic activity.
Question 2: Which fixative is considered the standard routine fixative used in most histopathology laboratories?
- Glutaraldehyde 2.5%
- Carnoy's fixative
- 10% neutral buffered formalin (Correct answer)
- Bouin's solution
Correct answer: 10% neutral buffered formalin
10% neutral buffered formalin (NBF) is the universal standard fixative because it preserves tissue morphology well and is compatible with most staining and molecular applications.
Question 3: What is the approximate formaldehyde concentration in a solution labeled '10% neutral buffered formalin'?
- 10% formaldehyde
- 37–40% formaldehyde
- 3.7–4% formaldehyde (Correct answer)
- 1% formaldehyde
Correct answer: 3.7–4% formaldehyde
10% formalin is a 1:10 dilution of the 37–40% formaldehyde stock solution, yielding an actual formaldehyde concentration of approximately 3.7–4%.
Question 4: What is the primary chemical mechanism by which formalin fixes tissue?
- Coagulation of proteins through dehydration
- Formation of methylene bridges that cross-link proteins (Correct answer)
- Precipitation of nucleic acids by pH change
- Removal of membrane lipids to expose proteins
Correct answer: Formation of methylene bridges that cross-link proteins
Formalin reacts with amino groups of proteins to form methylene bridges, creating a stable cross-linked protein network that preserves tissue architecture.
Question 5: Bouin's fixative is particularly well-suited for which type of tissue specimen?
- Bone and calcified specimens
- Tissues for electron microscopy
- Testicular and gastrointestinal biopsies (Correct answer)
- Frozen section specimens
Correct answer: Testicular and gastrointestinal biopsies
Bouin's fixative (picric acid, formalin, glacial acetic acid) provides excellent cytological detail, especially for testicular and gastrointestinal biopsies.
Question 6: According to CAP guidelines, what is the recommended fixation time range for standard surgical resection specimens in 10% NBF?
- 1–2 hours
- 6–72 hours (Correct answer)
- 3–5 hours
- 1–2 weeks
Correct answer: 6–72 hours
CAP guidelines recommend a minimum of 6 hours and a maximum of 72 hours of fixation in 10% NBF to ensure adequate protein cross-linking, particularly important for breast cancer biomarker testing.
Question 7: Which fixative is specifically required for optimal ultrastructural preservation in electron microscopy?
- 10% neutral buffered formalin
- Zenker's fixative
- Glutaraldehyde (Correct answer)
- Carnoy's fixative
Correct answer: Glutaraldehyde
Glutaraldehyde (2–4%) is the fixative of choice for electron microscopy because its bifunctional aldehyde groups provide superior cross-linking and ultrastructural preservation.
ASCP Tissue Fixation and Decalcification 1
What is the primary purpose of tissue fixation in histology?