Overview
Metachromatic staining exploits the ability of certain aniline dyes to change colour when they bind to specific tissue components, a phenomenon called metachromasia. Instead of staining a tissue element the same colour as the dye itself (orthochromasia), the dye polymerises on the tissue and produces an entirely different colour.
This lesson covers the mechanism of metachromasia, the common metachromatic dyes, the factors that enhance the reaction, and two classic amyloid-demonstration techniques — crystal violet stain and Congo red stain — both of which rely on this colour-changing property.
Learning Objectives
After this lesson you will be able to…- Define metachromasia and explain why it occurs
- List the common metachromatic dyes used in histology
- Describe the factors that enhance metachromasia
- Perform the crystal violet stain for amyloid
- Perform and interpret the Congo red stain for amyloid, including polarised light findings
Clinical Story
Why This MattersA 62-year-old man with long-standing rheumatoid arthritis presents with proteinuria and an enlarged tongue. The nephrologist suspects secondary (AA) amyloidosis and sends a renal biopsy to histopathology. The technologist must stain the section with Congo red and examine it under polarised light — a false-negative or poorly filtered stain could delay a life-changing diagnosis.
Core Concepts
Metachromasia is the phenomenon in which certain basic aniline dyes stain a tissue component a different colour to the dye's own colour. It occurs because the dye molecules polymerise when bound closely together on strongly anionic (negatively charged) tissue sites, such as sulphated mucopolysaccharides. For example, toluidine blue (which is normally blue) stains mast cell granules pink.
The most frequently used metachromatic dyes are methylene blue, methyl violet, thionin, crystal violet, and toluidine blue. Thionin and toluidine blue are particularly popular for rapid staining of frozen sections because of their quick metachromatic reaction with nucleus and cytoplasm.
Metachromasia is enhanced whenever intermolecular distances between dye molecules are reduced, allowing van der Waals forces to promote polymerisation. Key enhancing factors include increasing dye concentration, decreasing temperature, appropriate pH, and the polar nature of water, which supports the van der Waals interactions holding dye molecules together.
Laboratory Principle
Metachromatic dyes are cationic (positively charged) and react with strongly anionic groups in tissue, such as the sulphate groups of mucopolysaccharides (e.g. heparin in mast cell granules) or the beta-pleated sheet structure of amyloid fibrils. When enough dye molecules bind in close proximity, they polymerise, and the polymerised dye absorbs light at a different wavelength than the free dye — producing the shifted, or metachromatic, colour.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Crystal violet working solution | 10 ml stock (14g/100ml alcohol) + 300 ml water + 1 ml conc. HCl | Staining amyloid purple-violet | Prepare fresh |
| Congo red solution | 1.0 g Congo red in 100 ml distilled water | Amyloid demonstration | Room temperature |
| Saturated lithium carbonate | 1.3 g lithium carbonate in 100 ml distilled water | Differentiation step | Room temperature |
Step-by-Step Procedure
Bring paraffin sections down to water through xylene and descending alcohols before either amyloid stain.
Flood the section for 1–2 minutes and check colour development under the microscope before proceeding.
Rinse in tap water, then mount in water or a water-soluble medium and seal the coverslip edges with nail polish (do not let it dry first).
Alternatively, pour Congo red solution over the section for 20 minutes, then differentiate in saturated lithium carbonate for 1.5 minutes.
Counterstain with haematoxylin for 5 minutes, differentiate in 1% acid alcohol, dehydrate, clear, mount in DPX, and examine amyloid-positive areas under polarised light for apple-green birefringence.
Flow Diagram
Quality Control
Always include a known amyloid-positive tissue section as a positive control run alongside the test slide. Amyloid should stain purple-violet with crystal violet, and show bright red staining with apple-green birefringence under polarised light with Congo red.
Participation in an external quality assurance scheme for special/histochemical stains helps verify inter-laboratory consistency of amyloid detection, particularly the correct identification of birefringence which is prone to technical error.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Purple-violet staining with crystal violet | Positive for amyloid deposition | Correlate clinically; consider Congo red confirmation |
| Apple-green birefringence under polarised light | Confirms amyloid (diagnostic finding) | Report as amyloid-positive; further typing (AA/AL) as indicated |
| No birefringence despite red staining | May be a technical artefact or false negative | Repeat with fresh filtered Congo red and verify section thickness |
Common Errors & How to Avoid Them
Cause: Sections cut too thin or too thick, unfiltered Congo red solution, or tissue not fixed in formalin.
Prevention: Cut sections at 8–10 microns, filter solutions through glass wool, and confirm fixation history before reporting a negative result.
Cause: Old or contaminated dye solutions, prolonged differentiation, or dirty glassware.
Prevention: Prepare working solutions fresh, filter before use, and use clean, dedicated staining jars.
Cause: Nail polish sealant allowed to dry before the section is mounted properly, trapping air bubbles.
Prevention: Mount sections promptly in water-soluble media and seal edges while the mountant is still wet.
Laboratory Tips from the Bench
Always keep a positive amyloid control on hand — the metachromatic colour change can be subtle and is best judged by comparison.
Filter all crystal violet and Congo red solutions through glass wool rather than paper, since paper filters can remove the fine particulate dye needed for birefringence.
Remember 'MET-a-chroma' — the dye MET(amorphoses) its colour on the tissue, unlike ordinary orthochromatic staining.
Important Notes
Tissues fixed in solutions other than formalin may show false-positive birefringence with Congo red, so fixation history should always be checked before reporting.
Congo red birefringence is thickness-dependent; sections must be cut at 8–10 microns for reliable polarised light examination.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer. Use arrow keys to navigate in single-card mode.
Clinical Case Study
Apply Your KnowledgePresents with longstanding rheumatoid arthritis, new-onset proteinuria, macroglossia, and periorbital purpura. Renal biopsy is submitted for amyloid workup.
The combination of purple-violet crystal violet staining and apple-green birefringence on Congo red under polarised light confirms amyloid deposition in the renal parenchyma, consistent with secondary (AA) amyloidosis related to chronic inflammatory disease.
- →Congo red with polarised light is the gold-standard confirmatory stain for amyloid.
- →Section thickness of 8–10 microns is essential for reliable birefringence.
- →Chronic inflammatory conditions like rheumatoid arthritis are classic causes of secondary (AA) amyloidosis.
Frequently Asked Questions
Because mast cell granules contain highly sulphated heparin, which causes the dye to polymerise and shift colour from blue to pink/red — this is metachromasia in action.
Both demonstrate amyloid, but Congo red is preferred because it also allows confirmation via polarised light birefringence, which is more specific and widely accepted diagnostically.
Paper filters remove some of the fine dye particulates required for the birefringence effect, whereas glass wool preserves them, ensuring accurate results.
Quick Revision
10-Minute ReviewKey Takeaways
- Metachromasia occurs when cationic dyes polymerise on strongly anionic tissue sites.
- Mast cell granules, amyloid, mucin, and cartilage are classic metachromatic tissue elements.
- Crystal violet and Congo red are the two principal stains for amyloid.
- Congo red's apple-green birefringence under polarised light is diagnostic for amyloid.
- Non-paper filtration and correct section thickness are essential for reliable results.
- Tissue fixed in non-formalin fixatives may give false-positive birefringence.
Competency Checklist
Track Your MasteryReferences
- NIOS Histology and Cytology Practical Manual, Lesson 12: Metachromatic Staining.
- Bancroft JD, Gamble M. Theory and Practice of Histological Techniques. 7th ed.
- Kiernan JA. Histological and Histochemical Methods: Theory and Practice. 5th ed.