Fixation is the first and most critical step in tissue preservation. It stops cellular decay, stabilizes macromolecules, and hardens the tissue for subsequent processing and sectioning. The quality of every downstream technique — H&E staining, IHC, molecular analysis — depends on adequate and appropriate fixation.
Goals of Fixation
The primary goals of fixation are to prevent autolysis (self-digestion by intracellular enzymes) and putrefaction (bacterial decomposition); to preserve tissue morphology as close to the living state as possible; to harden tissue for cutting without distortion; to stabilize cellular components (proteins, nucleic acids, carbohydrates) for staining; and to render cells permeable to dyes and antibodies. No single fixative achieves all goals optimally — the choice depends on the intended downstream application.
Formalin Chemistry
10% neutral buffered formalin (NBF) is the universal fixative in histopathology. Commercial formalin is a 37% aqueous solution of formaldehyde gas — 10% NBF is a 1:10 dilution (3.7% formaldehyde). The neutral buffer (sodium phosphate monobasic and dibasic, pH 7.0-7.2) prevents formation of formic acid (which causes acid hematin pigment) and maintains optimal cross-linking conditions.
Formaldehyde cross-links proteins by forming methylene bridges (-CH2-) between adjacent amino groups, particularly the epsilon-amino group of lysine and the amide groups of peptides. This creates a three-dimensional gel network that stabilizes tissue structure. Cross-linking is temperature-dependent — it proceeds approximately twice as fast at 37°C as at room temperature. Routine fixation at room temperature requires 6-24 hours for small biopsies and 24-48 hours for larger specimens. The standard rule is 1 hour per mm of tissue thickness.
Factors Affecting Fixation
Fixative volume must be at least 10-20 times the tissue volume. Inadequate volume exhausts the fixative before penetration is complete. Tissue thickness — sections should be no thicker than 4-5 mm for adequate penetration; thicker sections fix poorly in the center. Temperature — higher temperature accelerates fixation but increases autolysis before the fixative penetrates; room temperature is optimal for routine use. Agitation — gentle rotation improves fixative circulation around the specimen. Delay to fixation — warm ischemia time (time from vascular clamping to specimen removal) and cold ischemia time (time from removal to fixation) must be minimized. Prolonged cold ischemia degrades RNA, proteins, and phosphoproteins (affecting IHC, particularly ER and phospho-specific markers).
Types of Fixatives
Cross-linking (aldehyde) fixatives — formaldehyde, glutaraldehyde (for EM), glyoxal (a less toxic alternative). Glutaraldehyde provides superior ultrastructure preservation but penetrates slowly and causes more cross-linking, which can mask antigens for IHC.
Coagulative fixatives — ethanol, methanol, acetone. These precipitate proteins by disrupting hydration shells rather than cross-linking. They preserve enzyme activity and nucleic acids but cause more shrinkage and harden tissue rapidly. Alcohol fixation is used for PCR and molecular testing.
Combination fixatives — Bouin’s solution (picric acid, formaldehyde, acetic acid) provides excellent nuclear detail and is used for endocrine tissues, testicular biopsies, and gastrointestinal biopsies. B5 fixative (mercuric chloride, formaldehyde) was historically used for lymph nodes but is now rarely used due to mercury toxicity. Zinc formalin is a mercury-free alternative for lymphoid antigen preservation.
Non-cross-linking alternatives — molecular fixatives (PAXgene, HOPE) preserve nucleic acids and proteins for molecular analysis but provide less optimal morphology than formalin. They are used primarily in biobanking and research settings.
Post-Fixation Processing
After fixation, specimens are transferred to 70% ethanol for storage if processing is delayed. Tissues can remain in 70% ethanol for weeks without adverse effects. Prolonged storage in formalin (>72 hours) causes over-fixation that increases antigen masking and DNA fragmentation. For long-term preservation, formalin-fixed tissue can be transferred to ethanol after 24-48 hours.
Quality Control
Fixation is monitored through fixative freshness (solutions changed regularly), fixation time (documented on the requisition), appearance (well-fixed tissue is firm and uniformly pale), and downstream staining quality (poor fixation manifests as poor nuclear detail, pallor, or edge effects on H&E). Quality assurance programs include regular review of fixation adequacy indicators.
