2. Fixation and Simple Fixatives

Fundamentals of Fixation and Tissue Preservation

    Fixation:

         •The aim is to preserve the tissue in as lifelike a manner as possible

        •Tissue proteins are stabilized = resistant to further change

        •Convert soluble contents into insoluble (prevent loss during subsequent processing)

        •There are many different fixatives

        •Each has advantages and disadvantages

      Aims of Fixation:

        •Prevent Autolysis: Destruction of cells by own enzymes

        •Specialized cells are affected first eg. brain, kidney

  • Putrefaction (or Putrefaction):

    • Defined as the breakdown and disbursement of cellular tissue caused by microbial action.

    • Microorganisms responsible include normal body flora as well as external microbes.

    • Can be reduced using antiseptic techniques, but is best prevented entirely through chemical or physical fixation.

  • Autolysis:

    • The self-destruction of tissues by endogenously released intracellular enzymes following ischemic cell death.

  • Primary Objectives and Aims of Fixation:

    • Stops both autolysis and putrefaction completely.

    • Irreversibility: Fixation cannot undo or reverse damage that has already occurred prior to immersion; improper or delayed fixation cannot be fixed post-hoc.

    • Tissue Hardening: Soft, squishy tissues are hardened by fixation, making them significantly easier to section thinly during microtomy.

    • Chemical Protection: Protects tissue architecture from damage during exposure to harsh downstream processing chemicals (dehydration reagents, clearing agents, embedding media).

    • Prevention of Over-Hardening: Over-fixation must be avoided, as it causes excessive shrinkage, extreme brittleness, and tissue fracturing/shattering.

    • Improvement of Optical Contrast: Enhances microscopic visibility of cellular details by increasing the refractive index of cellular components.

    • Influence on Staining: Alters tissue chemistry to make components more receptive to specific dyes; can act as a mordant (a binding agent between tissue structures and dyes).

    • Solidification of Cellular Constituents: Solidifies semi-fluid intracellular components, anchoring them in their true in vivo spatial positions to prevent displacement artifacts.

Factors Influencing Fixation

  • Passive Diffusion Mechanism:

    • Fixation relies fundamentally on the passive diffusion of liquid fixative into the tissue matrix.

  • Temperature:

    • Increased Temperature: Accelerates the kinetic energy of molecules and increases the rate of fixation. However, excessively high temperatures cook the tissue, causing thermal protein coagulation and tissue destruction.

    • Decreased Temperature: Reduces autolysis, enzyme activity, and microbial growth, but simultaneously decreases the rate of passive fixative diffusion.

    • Routine Standard: Standard routine fixation is carried out at room temperature (typically around 22 ∘C22\,^\circ\text{C}).

    • Special Applications: Refrigeration at 4 ∘C4\,^\circ\text{C} is utilized when preserving specific sensitive elements such as enzymes, antigens (for immunohistochemistry), or fine ultrastructure (for electron microscopy).

  • Agitation:

    • Agitating the fixative liquid speeds up diffusion and penetration rates throughout the tissue without causing thermal or structural damage.

Specimen Handling, Preparation, and Special Tissue Types

  • Fixative Volume Requirements:

    • The volume of fixative liquid must be 15×15\times to 20×20\times (15−20×15-20\times) the physical volume/size of the tissue specimen.

    • Requires large, adequately sized containers to prevent liquid spillage and specimen overcrowding.

  • Tissue Type and Density Variations:

    • High-density or fibrous tissues (e.g., bone, brain, fibrous connective tissue capsules) fix very slowly due to passive diffusion barriers.

    • Kidneys possess a dense fibrous capsule that acts as a physical barrier to fixative penetration.

  • Handling Procedures for Specific Tissue Morphologies:

    • Thick Fibrous Tissues: Must be sliced open or dissected to expose internal surfaces directly to the fixative.

    • Hollow Organs (e.g., Stomach, Intestines): Must be sliced open and pinned flat, or inflated/filled with fixative solution so the inner mucosal lining is directly fixed.

    • Lungs: Fixative can be instilled directly through the bronchus into the alveolar network under controlled pressure to maintain structural integrity without mechanical disruption.

    • Large Solid Organs/Specimens: Partial sectioning or incomplete serial cuts ("bread-loafing" or "enveloping") must be performed to allow fixative to reach interior regions before deep tissue autolysis occurs.

  • Specimen Thickness Limits:

    • Tissues placed into standard cassettes for routine processing must be trimmed to 3−4 mm3-4\,\text{mm} (3−4 mm3-4\,\text{mm}) in thickness.

    • Allows rapid, multi-directional diffusion from all surfaces. Tissues must not completely crowd the cassette, allowing free movement of fixative fluid around the specimen.

  • Timing and Cold Ischemia:

    • Tissues must be placed into fixative as rapidly as possible post-excision to halt ischemic damage.

    • Cold Ischemic Time: The time elapsed from tissue excision from the body to immersion in fixative. Must be maintained at less than 30 min30\,\text{min} (<30 min< 30\,\text{min}).

  • Osmolality:

    • Solutions should ideally be isotonic relative to tissue cells.

    • Hypertonic Solutions: Draw water out of cells, causing cellular collapse, tissue shrinkage, and brittleness.

    • Hypotonic Solutions: Push water into cells, causing swelling and cellular lysis.

    • Isotonic conditions are maintained by incorporating non-reactive background salts into the fixative formulation.

Microwave-Assisted Fixation

  • Mechanism:

    • Microwave irradiation generates heat, increasing kinetic energy and accelerating passive diffusion rates while enhancing chemical reactions that cross-link and stabilize proteins.

  • Equipment Requirements:

    • Requires specialized, highly controlled histology microwaves that deliver uniform heating throughout the cavity.

    • Domestic/commercial microwaves (e.g., from Best Buy or The Brick) are unacceptable due to uncontrolled hot spots that scorch or unevenly fix tissues.

    • Histology microwaves represent significant financial investments.

  • Advantages:

    • Rapid Speed: Reduces fixation time from hours down to minutes (ideal for urgent biopsy interpretation).

    • Morphological Preservation: Excellent preservation of fine cellular details and subsequent staining characteristics.

    • Artifact Reduction: Minimizes exposure artifacts associated with long-term chemical immersion.

    • Versatility: Compatible with multiple chemical fixative types.

  • Pathologist Caveat:

    • Traditional pathologists may harbor skepticism toward microwave fixation, fearing it "cooks" tissues. While slight thermal effects occur, specialized protocols prevent morphological degradation.

Consequences of Improper Fixation

  • Irreversibility: Autolytic and putrefactive breakdown cannot be undone once initiated.

  • Primary Causes of Fixation Failure:

    • Insufficient Volume: Using less than the required 15−20×15-20\times fixative-to-tissue ratio leads to incomplete immersion and incomplete fixation (e.g., placing a large bowel or placenta specimen into a small container containing only a few drops of liquid).

    • Capsular Barriers: Intact fibrous capsules (such as the kidney capsule) block penetration, leading to internal liquefaction and autolysis while outer layers appear intact.

    • Insufficient Time: Routine fixation requires 12−72 hr12-72\,\text{hr} (typically overnight; complete chemical stabilization can take up to 72 hr72\,\text{hr}). Shorter exposures lead to unfixed centers.

    • Single-Sided Contact / Compression: Tissues resting directly against container walls or bottoms cannot fix on contacting surfaces (e.g., placing a soft whole brain at the bottom of a container squishes two sides against the plastic, blocking diffusion; resolved by suspending the brain by the brainstem).

  • Microscopic Histology of Autolyzed / Poorly Fixed Tissue:

    • Nuclei show karyorrhexis (fragmentation), karyolysis (fading), or complete absence.

    • Cytoplasm appears cloudy, loosened, displaced, and exhibits loss of staining affinity.

    • Complete loss of crisp cellular boundaries and architectural detail.

  • Microscopic Histology of Well-Fixed Tissue (e.g., Kidney):

    • Crisp, well-defined blue nuclear dots.

    • Preservation of cellular detail across structures such as the glomerulus and surrounding convoluted tubules.

    • Retention of normal pink foamy intracellular protein content.

Classification and Chemical Action of Fixatives

  • Physical Fixation vs. Chemical Fixation:

    • Physical Fixation: Includes heat fixation (e.g., heat-fixing bacterial Gram stains over a Bunsen burner, which coagulates proteins similarly to frying an egg), microwaving, freezing, freeze-drying, and freeze-substitution. Histology relies primarily on chemical fixation.

    • Chemical Fixation: Relies on chemical reagents dissolved in solutions.

  • Classification Categories of Chemical Fixatives:

    • Simple vs. Compound Fixatives: Simple fixatives contain a single chemical compound in solution; compound fixatives contain two or more chemical reagents combined.

    • Coagulant vs. Non-Coagulant Fixatives:

    • Coagulants: Compete with bound water molecules on cellular proteins, stripping hydration shells. Active protein groups then cross-bind each other, collapsing tertiary structures into aggregates/clumps and causing mild tissue shrinkage.

    • Non-Coagulants: Fix tissue without stripping hydration shells or causing protein clumping.

    • Additive vs. Non-Additive Fixatives:

    • Additives: Chemically combine and cross-link with protein molecules via ionic or covalent bonds, building a stabilizing chemical scaffold (often causes slight tissue swelling).

    • Non-Additives: Alter tissue structures mechanically or physically without forming chemical bonds with tissue components.

    • Tolerant vs. Intolerant Fixatives:

    • Tolerant Fixatives: Tissues can remain immersed indefinitely without structural damage, brittleness, or degradation.

    • Intolerant Fixatives: Rapidly damage tissues upon prolonged exposure; tissues become extremely brittle, fragile, or shatter during sectioning if left beyond specified timeframes.

Histological Artifacts and Pigments

  • Artifact Definition:

    • Any feature, alteration, or structure introduced into a tissue specimen that was not present in life.

    • Can be introduced at any processing stage: excision, fixation, dehydration, clearing, embedding, microtomy sectioning, staining, or coverslipping.

    • Clinical Impact: Can obscure true micro-architecture or mimic pathological states, making diagnosis challenging.

  • Pigments in Histology:

    • Endogenous / Natural Pigments: Biologically occurring substances such as melanin, hemosiderin, and lipofuscin (can be normal or pathological).

    • Artifactual Pigments: Chemical precipitates formed by reactions between fixatives and tissue constituents (must be recognized or chemically removed so as not to mimic pathology).

Comprehensive Analysis of Specific Simple Fixatives

  • Formalin:

    • Classification: Simple, tolerant, additive, non-coagulant fixative.

    • Diffusion vs. Fixation Rate: Penetrates tissues rapidly at approximately 0.4 cm/hr0.4\,\text{cm/hr} (4 mm/hr4\,\text{mm/hr}), but fixes slowly (chemical cross-linking reactions begin between 8−12 hr8-12\,\text{hr} and reach completion around 72 hr72\,\text{hr}).

    • Tissue Effects: Causes minimal shrinkage and slight, desirable tissue hardening.

    • Chemical Nature: Formaldehyde is a gas. Formalin is an aqueous solution of formaldehyde gas dissolved in water. A saturated stock solution of formalin contains 37−40%37-40\% formaldehyde gas by weight.

    • Routine Laboratory Working Solution: Standard 10%10\% formalin solution is actually a 10%10\% dilution of the saturated stock, meaning it contains approximately 4%4\% actual formaldehyde gas.

    • Polymerization & Stabilization: High-concentration formaldehyde polymerizes into an insoluble white paraformaldehyde precipitate. Polymerization is prevented by adding up to 10%10\% methanol as a stabilizing agent.

    • Universal Standard: 10%10\% Neutral Buffered Formalin (NBF) is the primary, universal fixative used across 99.9%99.9\% of routine pathology laboratories for light microscopy and immunohistochemistry (IHC).

  • Glutaraldehyde:

    • Chemical Structure: A dialdehyde containing two functional aldehyde groups (unlike formaldehyde, which contains one).

    • Classification: Additive, non-coagulant, tolerant fixative.

    • Mechanism & Pitfalls: Only one aldehyde group cross-links with tissue proteins; the second aldehyde group remains unreacted and free. Free aldehyde groups react with subsequent aldehyde-demonstrating stains (e.g., Schiff reagent), producing severe false-positive staining results. Unsuited for routine histology.

    • Primary Application: Exclusively used for Electron Microscopy (EM).

    • Toxicity & Odor: Extremely toxic; possesses a distinct odor resembling fresh-cut grass.

  • Mercuric Chloride (HgCl2\text{HgCl}_2):

    • Classification: Additive, coagulant, simple, highly intolerant fixative.

    • Mechanism: Reacts specifically with histidine amino acid residues in cellular proteins.

    • Artifact Formation: Leaves a dense, black mercury pigment precipitate throughout the specimen (requires subsequent chemical removal).

    • Safety & Disposal: Extremely hazardous waste with severe environmental toxicity; discontinued in virtually all modern non-specialized pathology laboratories.

  • Ethyl Alcohol (Ethanol):

    • Classification: Non-additive, strongly coagulant, simple, intolerant fixative.

    • Properties: Highly flammable, controlled substance.

    • Lab Regulation: Requires specific government licenses, strict storage logs, and periodic audits. Laboratory ethanol is denatured/poisoned with foul-tasting or toxic additives to prevent human consumption.

  • Picric Acid (Trinitrophenol):

    • Chemical Nature: Trinitrophenol; chemically closely related to trinitrotoluene (TNT).

    • Classification: Additive, coagulant, moderately intolerant fixative.

    • Explosive Hazard: High explosive when dry! Must always be maintained under a layer of liquid. Splashes on jar lids that dry out can detonate upon opening. (Historical Context: The 19171917 Halifax Harbour explosion involved a ship carrying picric acid, detonating and throwing its anchor 2 miles2\,\text{miles}).

    • Artifact Formation: Produces a bright yellow tissue pigment artifact; easily removed by washing/rinsing with water.

  • Acetic Acid:

    • Chemical Nature: Concentrated/glacial acetic acid (CH3COOH\text{CH}_3\text{COOH}; concentrated vinegar).

    • Classification: Non-additive, non-coagulant, tolerant pickling agent.

    • Effects: Lyses red blood cells (erythrocytes); provides exceptional preservation of nuclear detail.

    • Tissue Swelling & Combination Use: Causes marked tissue swelling. Never used as a single fixative alone; combined into compound fixatives to balance out the severe shrinkage caused by coagulants like mercuric chloride.

  • Potassium Dichromate (K2Cr2O7\text{K}_2\text{Cr}_2\text{O}_7):

    • Classification: Intolerant, highly toxic, carcinogenic, environmental hazard.

    • pH-Dependent Action:

    • Low pH (<3.4< 3.4): Acts as a coagulative fixative.

    • Higher pH (>3.4> 3.4): Acts as a non-coagulative fixative.

    • Artifact Formation: Produces an insoluble yellow-brown chrome pigment artifact.

    • Current Status: Phased out of standard laboratories due to extreme disposal hazards.