Tissue Processing & Fixation Flashcards

Which fixative is considered the gold standard for routine histological tissue processing?

Answer: 10% neutral buffered formalin

10% neutral buffered formalin (NBF) is the standard fixative because it preserves tissue morphology, is compatible with most staining techniques, and is cost-effective.

What is the primary chemical mechanism by which formalin fixes tissue?

Answer: Cross-linking of proteins via methylene bridges

Formalin fixes tissue by forming methylene bridges (cross-links) between adjacent protein molecules, stabilizing cellular structures.

The recommended fixation time for a core needle biopsy specimen before processing is:

Answer: 6–72 hours

CAP/ASCO guidelines recommend 6–72 hours of formalin fixation for core needle biopsies to ensure adequate fixation without over-fixation artifacts.

Which reagent is used in the clearing step of tissue processing, replacing the dehydrant before paraffin infiltration?

Answer: Xylene

Xylene (or xylene substitutes) is used as the clearing agent because it is miscible with both alcohol and paraffin, making tissue transparent and ready for wax infiltration.

What is the purpose of the dehydration step in tissue processing?

Answer: To remove water so the clearing agent can penetrate

Dehydration removes water from the tissue using a graded series of alcohols, allowing the non-aqueous clearing agent to penetrate in the next step.

Over-fixation in formalin most commonly results in:

Answer: Poor nuclear staining and masked antigens

Over-fixation creates excessive protein cross-linking that masks antigenic epitopes and reduces nuclear hematoxylin staining quality, affecting both morphology and IHC results.

Which fixative is preferred for preserving glycogen in tissues for PAS staining?

Answer: Alcoholic formalin

Alcoholic formalin preserves glycogen better than aqueous formalin because the alcohol prevents glycogen diffusion and dissolution that can occur in aqueous fixatives.