After fixation, tissue must be infiltrated with a supporting medium — typically paraffin wax — to enable thin sectioning. Automated tissue processors carry the tissue through sequential reagent baths over 8-24 hours, replacing water with paraffin through three stages: dehydration, clearing, and infiltration.
The Tissue Processor
Modern tissue processors are enclosed, ventilated instruments that move baskets of tissue cassettes through a series of reagent stations. Carousel-type processors (Leica ASP300, Thermo Excelsior) rotate the basket through individual reagent containers. Dip-and-rotate processors lower the basket into a single retort that sequentially fills and drains with each reagent. Both types include a vacuum/pressure cycle during paraffin infiltration that forces wax into tissue cavities and removes residual air.
Processors hold 100-400 cassettes per run. A typical overnight (14-16 hour) protocol processes all routine biopsies and small resections. A shorter (4-8 hour) “rapid” protocol is used for same-day processing of urgent specimens, frozen section remnants, and small biopsies. The protocol is selected based on tissue type and size.
Dehydration
Water is immiscible with paraffin and must be completely removed. Dehydration uses graded ethanols (70%, 80%, 95%, 100%) in increasing concentration. Starting with 70% ethanol prevents osmotic shock and excessive shrinkage. The 100% ethanol step is the most critical — any residual water will prevent paraffin infiltration and produce brittle, difficult-to-section blocks. Absolute ethanol contains <1% water; it should be changed frequently (every 50-100 runs) to maintain effectiveness.
Dehydration artifacts: incomplete dehydration (grainy, soft blocks), over-dehydration (excessive shrinkage and hardening of small biopsies), and carryover of water from ethanol to clearing agent (cloudy clearing reagents).
Clearing
Clearing agents are miscible with both ethanol and paraffin. They replace ethanol in the tissue and render the specimen translucent (hence “clearing” — the tissue appears clear as the refractive index matches the clearing agent). Xylene is the universal clearing agent — it performs well but requires handling as a hazardous solvent (neurotoxic, flammable). Substitutes include isopropanol, limonene-based clearing agents (HistoClear, Citrisolv), and dibutyl phthalate. Xylene substitutes clear more slowly and may leave residual clearing agent in the tissue.
Clearing artifacts: incomplete clearing (opaque, white patches in the block — the tissue still contains ethanol or water) and over-clearing (excessive hardening, particularly of fatty tissues).
Paraffin Infiltration
The clearing agent is gradually replaced with molten paraffin wax (56-60°C melting point). The processor moves the basket through two to three paraffin baths, each 1-2 hours, under vacuum. Vacuum infiltration (15-25 inches Hg) removes residual clearing agent and air from tissue cavities, ensuring complete penetration. Fresh paraffin at the final station produces the cleanest blocks.
Paraffin quality affects sectioning. Paraffin additives include DMSO (improves infiltration), resins (improve sectioning of hard tissues), and plastic polymers (improve ribbon continuity). The ideal paraffin has a narrow melting range, good adhesion to tissue, and minimal crystal formation when cooled.
Protocol Selection
Routine protocol (overnight, 14-16 hours) — suitable for most biopsies and small resections (up to 4 mm thickness). Reagent times: 70% ethanol (1 h), 80% ethanol (1 h), 95% ethanol (1 h × 2), 100% ethanol (1.5 h × 2), xylene (1.5 h × 2), paraffin (2 h × 2 with vacuum).
Large specimen protocol (24 hours) — for fatty, dense, or >4 mm specimens. Longer dehydration steps (2 h each), additional xylene steps (total 6-8 hours), and extended paraffin infiltration (4 h × 2). Fatty tissue requires extended clearing because fat dissolves slowly in xylene.
Rapid protocol (4-6 hours) — for small urgent biopsies. Uses higher temperatures (60°C), shorter times, and stronger dehydrants (isopropanol). May produce harder blocks but adequate morphology.
Troubleshooting Processing Problems
Soft or mushy blocks — incomplete dehydration or inadequate paraffin infiltration. Re-process the tissue by melting the paraffin and repeating from 100% ethanol.
Brittle blocks — over-dehydration, excessive heat, or prolonged xylene exposure. Reduce dehydration times or decrease oven temperature. Add a drop of wetting agent to the water bath during sectioning.
Tissue shrinkage — normal processing causes 10-15% linear shrinkage. Excessive shrinkage results from high ethanol concentrations applied too early — ensure graded dehydration starting at 70%.
White, chalky areas in the block — incomplete clearing (residual water or ethanol). Re-process from the clearing stage.
Quality Control
Monitor processing quality by checking block appearance (uniform, translucent paraffin without white patches), sectioning quality (smooth ribbons without chatter or tearing), and H&E quality (crisp nuclear detail without excessive pallor). Reagent purity is checked by specific gravity (ethanol), refractive index, or test tissue processing runs. Regular maintenance of the processor — cleaning reagent lines, replacing seals, calibrating temperature and vacuum — prevents batch failures. Document all reagent changes and maintenance in the quality assurance log.
