Specimen Procurement, Handling, and Fixation
Why fixation matters
Fixation preserves morphology by cross-linking proteins (formalin) and inactivating autolytic enzymes, preventing autolysis and putrefaction. The variables that determine adequacy are fixative type, concentration, pH, osmolality, temperature, time, tissue thickness, and volume ratio. Routine fixative is 10% neutral buffered formalin (NBF): 10% of 37–40% stock formaldehyde gives about 4% formaldehyde, phosphate-buffered to pH 7.0–7.4. Unbuffered formalin forms acid formalin pigment and compromises staining.
Practical rules
Use at least a 10:1 fixative-to-tissue ratio, slice tissue 3–4 mm thick, and remember penetration is roughly 1 mm per hour. Cold and warm ischemia both matter; fixation begins at collection, not at grossing. Overfixation can harden tissue and mask immunohistochemical epitopes; underfixation leaves central autolysis. Know special fixatives: Bouin's (picric acid, yellow), B5 (mercury), Zenker's, alcohols for cytology/nucleic acids, and glutaraldehyde for electron microscopy. Decalcification with acid is fast but damages DNA; EDTA is slow and gentle.
Traps and a worked example
Common distractors confuse “10% formalin” with 10% formaldehyde, assume fixation starts in the grossing room, or ignore container labeling and time records. Questions may ask why a centrally necrotic specimen stains poorly.
Worked example: A 1 × 1 × 1 cm biopsy is placed in NBF at 9:00 a.m. Because fixation advances about 1 mm/hour from each surface, the center is reached in roughly 5 hours; 6–8 hours is minimal and overnight is typical. A 4-mm-thick slice fixes far faster than a 1-cm cube.