How Histology Slides Are Prepared

If you’re involved in biological research, chances are, at some stage, you’ve submitted tissue specimens to a histology lab. Somehow they magically produced beautiful slides for you—each containing thin sections of your specimens, ready for microscopic evaluation.

Have you ever been curious about the process a histology technician follows for histology slide preparation? Although the exact process depends on different tissue types, there are five basic steps. Read on for the five important stages in histology slide preparation.

The Five Steps of Histology Slide Preparation

1. Tissue fixation

Slide preparation begins with the fixation of your tissue specimen. This is a crucial step in tissue preparation, and the purpose of the fixation process is to prevent tissue autolysis and putrefaction. For best results, your biological tissue samples should be transferred into fixative immediately after collection.

Although there are many types of fixative, most specimens are fixed in a 10% neutral buffered formalin solution. The optimum formalin-to-specimen volume ratio should be at least 10:1 (e.g., 10ml of formalin per 1 cm³ of tissue). This will allow most tissues to become adequately fixed within 24-48 hours. Formalin containers should be capped, leak-proof, and appropriately labeled.

2. Specimen Transfer to Cassettes

After fixation, specimens are trimmed using a scalpel to enable them to fit into an appropriately labeled tissue cassette. Specimens should not be so big that they fill the cassette – they are trimmed so as not to touch the edges.

Additionally, they must not be too thick (ideally, they should be less than 4 mm); otherwise, they risk being “waffled” when the cassette lid is closed. The filled tissue cassettes are then stored in formalin until processing begins.

3. Tissue Processing

Processing tissues into thin microscopic sections is usually done using a paraffin block, as follows:

1. Dehydration, which involves immersing your specimen in increasing concentrations of alcohol to remove the water and formalin from the tissue.
2. Clearing, in which an organic solvent such as xylene is used to remove the alcohol and allow infiltration with paraffin wax.
3. Embedding, where specimens are infiltrated with the embedding agent – usually paraffin wax. The tissue becomes surrounded by a large block of molten paraffin wax, creating what is now referred to as the “block”. Once the block solidifies, it provides a support matrix that allows very thin sectioning.

4. Sectioning

Your tissue specimen is now ready to be cut into sections that can be placed on a slide.

1. Wax is removed from the surface of the block to expose the tissue.
2. Blocks are chilled on a refrigerated plate or ice tray for 10 minutes before sectioning.
3. A microtome is used to slice extremely thin tissue sections off the block in the form of a ribbon.

The microtome can be pre-set to cut at different thicknesses, but most tissues are cut at around 5 µm. You can discover more ways to slice tissue sections here.

Once cut, the tissue ribbons are carefully transferred to a warm water bath. Here they are allowed to float on the surface, and can then be scooped up onto a slide placed under the water level.

Charged glass slides work best for this process—they improve tissue adhesion to the glass, and help reduce the chance of sections washing off the slide during staining.

Slides should be clearly labeled and then allowed to dry upright at 37°C for a few hours to gently melt the excess paraffin wax, leaving the tissue section intact.

5. Staining

Most cells are transparent and appear almost colorless when unstained. Histochemical stains, such as hematoxylin and eosin, which is the routine stain, are therefore used to provide contrast to tissue sections, making tissue structures more visible and easier to evaluate.

Following the staining process, a cover slip is mounted over the tissue specimen on the slide, using optical grade glue, to help protect the specimen for microscopic examination.

What Can Go Wrong: Troubleshooting Histology Slide Preparation

Even with careful technique, histology slide preparation has multiple points of failure. Here are the most common problems at each stage and what to do about them.

Fixation Problems

Symptom: Poor nuclear staining, tissue appears pale and washed out under H&E Cause: Under-fixation. The tissue wasn’t in formalin long enough, or the formalin-to-tissue ratio was too low. Thick specimens (>4mm) are particularly prone — formalin penetrates at roughly 1mm per hour, so a 10mm biopsy needs far longer than the standard 24-hour window. Fix: For thick specimens, trim to 3-4mm before fixation or extend fixation time. If under-fixed tissue has already been processed, there is no recovery — the slide is lost. Prevention is the only option.

Symptom: Tissue is hard and brittle, sections crumble during cutting Cause: Over-fixation. Tissue left in formalin for days or weeks becomes over-crosslinked. This is common with specimens that arrive from external sites after long transit or get forgotten in the fixative. Fix: Nothing fully reverses over-fixation. Rehydrating the block overnight in water before sectioning can improve ribbon quality slightly. For future specimens, transfer to 70% ethanol after 48 hours if processing is delayed — it stops further fixation while preserving tissue morphology.

Symptom: Tissue autolysis — cells appear disrupted, nuclei are indistinct, cytoplasm is granular Cause: Delayed fixation. Autolytic enzymes begin degrading tissue immediately after excision. Even 30-60 minutes at room temperature before reaching formalin can cause visible autolysis, particularly in tissues with high enzyme content like liver and kidney. Fix: There is no fix for autolysed tissue — the damage is irreversible. Ensure specimens reach fixative within 30 minutes of collection and that surgical teams understand why this matters.

Cassetting Problems

Symptom: Tissue is “waffled” — compressed with a grid pattern imprinted on the surface Cause: Specimen was too thick when the cassette lid was closed. The lid perforations compress anything above 4mm, distorting tissue architecture. Fix: Re-embed in a fresh block if enough tissue remains. Prevent by trimming specimens to less than 4mm before cassetting — this is the most commonly skipped step in busy labs.

Symptom: Small biopsy specimens are lost during processing Cause: Tiny specimens (needle biopsies, polyps) fall through cassette perforations or are not visible when trimming the block face. Fix: Wrap small specimens in lens tissue or place them in a tissue sponge inside the cassette before processing. Mark the cassette with a coloured pen to flag that a small specimen is inside.

Processing Problems

Symptom: Tissue is soft and doesn’t cut cleanly — sections fold or compress Cause: Incomplete dehydration or incomplete paraffin infiltration. Residual water prevents proper wax embedding. This is more common with fatty tissues (breast, skin) which are hydrophobic and resist dehydration. Fix: Run the processor again with fresh reagents if the block hasn’t been cut. For fatty tissues, extend dehydration steps or add an additional xylene step. Check processor reagent quality — alcohols and xylene need regular replacement as they become saturated with water and tissue debris.

Symptom: Bubbles or holes in the paraffin block Cause: Air trapped during embedding, or incomplete clearing leaving residual xylene that evaporates. Also occurs when molten paraffin is poured too quickly or the tissue is placed incorrectly in the mould. Fix: Trim the block face past the bubbles and re-embed if holes extend into the tissue. Prevent by ensuring complete clearing and pouring paraffin gently. Warm the embedding forceps before handling tissue to prevent premature wax solidification around the specimen.

Sectioning Problems

Symptom: Sections have holes or tears — “Swiss cheese” appearance Cause: Calcified tissue, bone spicules, or foreign material (sutures, staples) in the block. The microtome blade hits hard material and tears the surrounding tissue. Fix: Decalcify specimens before processing if calcification is expected. For unexpected calcification found during cutting, soak the block face in decalcifying solution for 30-60 minutes before continuing. Replace the blade — cutting hard material destroys the edge.

Symptom: Sections are thick and thin alternately — “venetian blind” artifact Cause: Vibration in the microtome, usually from a loose chuck, worn anti-roll plate, or the operator applying uneven pressure during cutting. Fix: Tighten all components. Check the blade for nicks — even a small nick causes this pattern. Ensure the block is fully cold before cutting; a warm block compresses unevenly. Cut at a slow, consistent speed rather than rushing.

Symptom: Sections float off the slide during staining Cause: Sections not properly adhered to the slide. Usually caused by using plain rather than charged slides, inadequate drying, or cutting sections that are too thick. Fix: Always use charged (poly-L-lysine or APES-coated) slides for routine work. Dry sections at 60°C for 20-30 minutes rather than overnight at 37°C — higher temperature ensures paraffin melts completely and tissue bonds to the glass. If sections are still lifting, increase drying time or check slide coating batch.

Staining Problems

Symptom: Weak or absent haematoxylin staining — nuclei appear pale or grey rather than deep blue Cause: Haematoxylin solution is exhausted or contaminated. Progressive haematoxylin is particularly sensitive to oxidation and loses strength over time. Also caused by insufficient staining time or inadequate bluing. Fix: Replace haematoxylin solution. Check the bluing agent — insufficient bluing (converting haematoxylin from red to blue) is a common cause of pale nuclear staining. Test staining time on a control slide before processing the full batch.

Symptom: Eosin staining is too dark or too uniform — tissue looks pink with no differentiation between cytoplasmic structures Cause: Over-staining with eosin, or failure to differentiate adequately in alcohol after eosin. Eosin concentration may have increased through evaporation. Fix: Differentiate in 95% alcohol after eosin until the desired intensity is reached — this step is often rushed. If over-stained sections can’t be recovered, de-stain in acid alcohol and re-stain from the eosin step.

Symptom: Background staining — non-specific colour throughout the section, not just in target structures Cause: Incomplete dewaxing before staining, residual paraffin blocking reagent penetration. Also caused by inadequate washing between steps or using degraded reagents. Fix: Extend xylene deparaffinisation steps. Replace reagents. Check that sections were fully dried before staining — incompletely dried sections retain paraffin that resists dewaxing.

Histology Slide Preparation: Get the Poster!

So as you can see, histology slide preparation is no breeze—it’s quite an intricate work of art! Although you may want to learn how to do this to help cut costs in the lab, I’d advise you to think twice about this, and instead send the specimens to a histology laboratory for this purpose – especially if tissue evaluation is an important part of your study.

You’ll save yourself a lot of stress and time accidentally damaging your delicate tissue sections by leaving this job to an experienced histology technician. Plus, your slides will be much easier to evaluate.

While histology is great for lower-resolution imaging of whole tissues, it is limiting if you want to investigate subcellular structural changes in (say) brain tissue and tumors. In this case, a more detailed and high-powered technique, such as brain cancer electron microscopy, is required.

Are you wondering what histological stain will stain the tissue you’re interested in? Download our free, colorful guide to histological stains and pin it up.

Further Reading and Additional Resources

If you found this article useful, you might want to check out some of our related articles below:

• An Introduction To Fixation For Histology: Think Before You Fix!
• Tissue Processing For Histology: What Exactly Happens?
• Special Stains for Histology: An Introduction and Basic Overview
• The A to Z of Histological Stains

Originally published April 4, 2012. Reviewed and updated in November 2020 and December 2023.

Check out the latest episode of The Microscopists podcast to hear from leaders in the microscopy world.

Nicola Parry

Dr Nicola Parry graduated from veterinary school at the University of Liverpool and spent several years working in mixed general practice in the UK before moving to the USA to pursue Anatomical Pathology at the American College of Veterinary Pathologists.

Share this article: