Overview
Freshly cut tissue sections are colourless, and their component structures cannot be distinguished under the microscope. Staining with different coloured dyes that have specific affinities for particular tissue components makes identification and study of morphology possible. Hematoxylin and Eosin (H&E) is by far the most frequently used stain in histology worldwide.
This lesson explains the chemistry of hematoxylin (which is not itself a dye), the properties and preparation of Harris's and Mayer's hematoxylin, the role of eosin as a counterstain, and the full step-by-step H&E staining procedure used in every diagnostic histopathology laboratory.
Learning Objectives
After this lesson you will be able to…- Describe hematoxylin, its source, and its preparation from the bloodwood tree
- Explain why hematoxylin has no intrinsic staining property and how mordants create the hematoxylin lake
- Differentiate progressive and regressive staining, and describe differentiation and blueing
- Describe the preparation of Harris's and Mayer's hematoxylin
- Describe eosin and its role as a cytoplasmic counterstain
- Perform the complete H&E staining method step by step
Clinical Story
Why This MattersA pathologist reviewing a suspicious colon polyp biopsy depends entirely on the quality of the H&E stain to distinguish normal glandular architecture from dysplastic or malignant change. If the hematoxylin under-stains the nuclei or the eosin over-stains the cytoplasm, subtle features like nuclear hyperchromasia and loss of polarity — the very findings that separate a benign polyp from early adenocarcinoma — can be missed entirely. H&E remains the foundation stain because, done correctly, it reliably reveals exactly this kind of nuclear and cytoplasmic detail.
Core Concepts
Hematoxylin is extracted from the bark of the bloodwood tree, Haematoxylon campechianum. Freshly logged bark chips are boiled in water, producing an orange-red solution that turns yellow, then black, on cooling; the water is evaporated to yield crude hematoxylin, which is then purified. Solutions must be oxidized to retain staining ability — naturally by exposure to light for 3-4 months, or chemically using sodium iodate or mercuric oxide (chemical oxidation is near-instant but the product has a shorter shelf life). Hematoxylin itself is neither a dye nor has colouring properties. It must be oxidized to hematin, a weak anionic purple dye, which alone has no affinity for nucleic acids. A metallic salt mordant (ammonium/potassium alum, ferric salts, chrome alum, or phosphotungstic acid) is combined with hematoxylin to give it a positive charge, allowing the resulting cationic dye-metal complex — called a hematoxylin lake — to bind the anionic nuclear chromatin.
Progressive staining leaves tissue in the stain just long enough to reach the correct endpoint, checking slides at intervals. Regressive staining deliberately over-stains the tissue, then removes ('differentiates') the excess dye with acid-alcohol until the correct endpoint is reached — this is how Harris's hematoxylin, a regressive stain, is used. The hematoxylin-alum complex initially gives a reddish hue due to acidic pH; converting this to the final blue colour by raising to alkaline pH (using tap water or ammonium hydroxide) is called blueing.
Harris's hematoxylin is prepared by dissolving hematoxylin in absolute alcohol and ammonium alum in hot water, mixing and boiling, then adding mercuric oxide and cooling rapidly; glacial acetic acid, if added to the working solution, gives brisk nuclear staining but reduces shelf life. Mayer's hematoxylin dissolves hematoxylin in distilled water with gentle heat, then adds ammonium alum, sodium iodate, citric acid (reduces pH) and chloral hydrate (preservative) in sequence.
Eosin stains the cytoplasm a rose colour and acts as the counterstain to hematoxylin's nuclear staining. The most widely used form is Eosin Y ('Y' for yellowish), available in water-soluble or alcohol-soluble forms; most laboratories use the water-soluble form dissolved in an alcohol-water solution, with a small amount of glacial acetic acid added to increase staining intensity. A properly prepared working solution should appear slightly cloudy.
Laboratory Principle
H&E staining exploits differences in the acid-base chemistry of tissue components. Hematoxylin, once oxidized to hematin and combined with a metal mordant to form a cationic 'lake', binds electrostatically to the acidic (anionic) phosphate groups of DNA/RNA in nuclear chromatin, staining nuclei blue-purple. Eosin, an acidic (anionic) dye, binds to basic (cationic) proteins in the cytoplasm and extracellular matrix, staining these components varying shades of pink to red. Together, this simple two-dye combination reveals both nuclear and cytoplasmic detail with excellent contrast.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Harris's hematoxylin | 5g hematoxylin, 50ml abs. alcohol, 100g ammonium alum, 2.5g mercuric oxide, 40ml glacial acetic acid | Regressive nuclear stain | Room temperature, filtered before use |
| Mayer's hematoxylin | 1g hematoxylin, 50g ammonium alum, 0.2g sodium iodate, 1g citric acid, 50g chloral hydrate/L | Progressive nuclear stain | Room temperature |
| Eosin Y (water soluble) | 1g eosin Y, 80ml water, 320ml 95% alcohol, 0.4ml glacial acetic acid | Cytoplasmic counterstain | Room temperature, in dark bottle |
| 1% HCl in 70% alcohol | Acid-alcohol working solution | Differentiation (removing excess hematoxylin) | Room temperature |
| Ammonium hydroxide / tap water | Alkaline solution | Blueing — converts red hematoxylin hue to blue | Room temperature |
Step-by-Step Procedure
Deparaffinize sections in xylene (10-20 min), then rehydrate through 100% and 95% alcohol (1-2 min each), then rinse in tap and distilled water.
Stain with filtered hematoxylin for 3-5 minutes, then wash in tap water.
Differentiate with 1% HCl in 70% alcohol for 1-2 dips, checking under the microscope; return for further differentiation if needed.
Wash slides in running tap water for 15 minutes to convert the red hematoxylin hue to blue.
Stain in eosin for 1-4 minutes to colour the cytoplasm rose pink.
Dehydrate through 95% then 100% alcohol (5-6 dips each), then clear in xylene twice.
Mount slides with a suitable mounting medium (Permount or DPX) and coverslip.
Flow Diagram
Quality Control
Filter hematoxylin before each staining run to remove precipitate; run a known positive control tissue with each batch to confirm nuclear and cytoplasmic staining intensity; change alcohol and xylene baths regularly, as directed by SOP, since carried-over contamination weakens differentiation and clearing.
Participate in external quality assessment programs for H&E staining quality (nuclear detail, cytoplasmic contrast, absence of precipitate/artefact) to benchmark staining performance against other laboratories.
Reference Values
Key Parameters⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals and SOPs.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Pale, washed-out nuclear staining | Under-staining, over-differentiation, or exhausted hematoxylin | Re-stain with fresh filtered hematoxylin; reduce differentiation time |
| Excessively dark, muddy nuclear staining obscuring detail | Insufficient differentiation or over-staining | Increase differentiation time in acid-alcohol; check hematoxylin concentration |
| Weak or absent pink cytoplasmic staining | Exhausted or improperly prepared eosin, or excessive dehydration washing it out | Prepare fresh eosin; verify acetic acid was added to increase intensity |
Common Errors & How to Avoid Them
Cause: Precipitate in unfiltered hematoxylin deposits on sections, obscuring nuclear detail with dark specks.
Prevention: Filter hematoxylin immediately before each staining session, as noted in the standard procedure.
Cause: Drying at any point during H&E staining causes artefactual staining and precipitate that cannot be corrected later.
Prevention: Keep sections wet throughout the entire staining process — moving quickly between water and staining steps.
Cause: Excessive time in acid-alcohol strips too much hematoxylin, leaving nuclei pale and poorly defined.
Prevention: Differentiate for only 1-2 dips, checking microscopically, and stop as soon as background staining is removed while nuclei remain crisp.
Laboratory Tips from the Bench
Always check the differentiation step under the microscope rather than by eye alone — subtle over- or under-differentiation is much easier to spot at higher magnification.
If your eosin working solution looks perfectly clear rather than slightly cloudy, add a few more drops of glacial acetic acid — cloudiness is a simple visual indicator of correct concentration.
Remember H&E's order with 'Blue then Pink' — Hematoxylin stains nuclei blue-purple FIRST, Eosin stains cytoplasm pink SECOND, always in that sequence.
Important Notes
At no stage of H&E staining should the section be allowed to dry — even brief air exposure between steps can cause permanent staining artefacts.
H&E staining, as classically performed with Harris's hematoxylin, is a regressive technique: tissue is deliberately over-stained, then the excess dye is carefully removed (differentiated) to reach the ideal staining intensity.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeA tubular adenoma is removed during screening colonoscopy and submitted for histological assessment of the grade of dysplasia present.
Well-executed H&E staining with correctly timed differentiation and blueing allowed clear visualization of nuclear hyperchromasia, stratification, and loss of polarity — the key morphological features needed to grade the degree of dysplasia in the adenoma.
- →High-quality H&E staining is essential for accurate dysplasia grading
- →Differentiation must be checked microscopically to avoid under- or over-staining nuclei
- →Nuclear detail (hyperchromasia, stratification, polarity) is the key feature H&E reveals for dysplasia assessment
Frequently Asked Questions
In its natural, unoxidized form, hematoxylin has no colouring or staining ability; it must first be chemically oxidized to hematin, which is then combined with a metal mordant to create the actual staining compound.
Progressive staining stops as soon as the correct staining intensity is reached, while regressive staining deliberately over-stains the tissue and then removes the excess dye (differentiation) until the desired endpoint is achieved.
A small amount of glacial acetic acid increases the intensity of eosin staining, but the working solution should remain slightly cloudy — if it becomes perfectly clear, more acetic acid should be added to restore correct staining strength.
Quick Revision
10-Minute ReviewKey Takeaways
- H&E remains the cornerstone stain of diagnostic histopathology worldwide.
- Hematoxylin requires oxidation and mordanting before it can stain nuclear chromatin.
- Regressive staining with careful differentiation achieves ideal, reproducible nuclear detail.
- Blueing is essential to convert hematoxylin's red hue into its familiar blue-purple nuclear colour.
- Eosin's pink cytoplasmic counterstain provides essential contrast to nuclear staining.
- Meticulous, well-timed technique at every step is what makes H&E diagnostically reliable.
Competency Checklist
Track Your MasteryReferences
- Bancroft JD, Layton C. Theory and Practice of Histological Techniques. 8th ed.
- Suvarna SK, Layton C, Bancroft JD. Bancroft's Theory and Practice of Histological Techniques.
- NIOS Vocational Course — Histology and Cytology Module, Lesson 10: Hematoxylin and Eosin Staining.