TUNEL Assay Protocol: DNA Fragmentation, Apoptosis Detection and Quantification

TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labelling) detects accessible 3′-OH DNA ends generated by DNA fragmentation. The method is widely used to identify apoptotic cells in cultured-cell preparations and tissue sections, but TUNEL positivity is not uniquely specific to apoptosis: necrosis, severe DNA damage and some sample-preparation artefacts can also generate strand breaks. Interpretation should therefore combine morphology, biological controls and, where possible, an orthogonal apoptosis marker such as active caspase-3.

Principle

Terminal deoxynucleotidyl transferase (TdT) catalyses addition of labelled nucleotides to free 3′-OH DNA ends. The incorporated label is detected directly by fluorescence or indirectly using an enzyme-linked detection reagent. Increased nuclear TUNEL signal indicates increased accessible DNA strand breaks, but the biological origin of those breaks must be interpreted in context.

Materials

• Fixed cultured cells or tissue sections.
• PBS and fixation/permeabilisation reagents appropriate for the sample.
• TdT enzyme and labelled dUTP/nucleotide mix.
• Reaction buffer supplied or validated for the TdT system.
• DNase I for a positive-control preparation.
• Detection reagents for the chosen fluorescent or chromogenic system.
• Nuclear counterstain where required.
• Fluorescence or bright-field microscope.

Before starting

• Prepare a DNase-treated positive control to demonstrate the labelling reaction is working.
• Prepare a no-TdT negative control to assess non-enzymatic/background signal.
• Include a biological non-apoptotic control and, where possible, a known apoptosis-positive treatment.
• Use sample-specific fixation/permeabilisation conditions because DNA accessibility strongly affects signal.

General TUNEL procedure

1. Fix the specimen. Paraformaldehyde fixation is widely used for cultured cells and many tissue preparations. Use the conditions validated for the chosen kit and specimen.
2. Wash thoroughly. Remove residual fixative before permeabilisation.
3. Permeabilise. Use Triton X-100, proteinase treatment or another kit-compatible approach appropriate to the specimen. Under-permeabilisation lowers sensitivity; over-treatment damages morphology and increases non-specific access.
4. Set up the positive control. Treat a separate control with DNase I according to the assay instructions to create DNA strand breaks.
5. Set up the negative control. Omit TdT from a parallel sample while keeping the other detection components unchanged.
6. Prepare TdT labelling mix. Prepare immediately before use according to the assay chemistry.
7. Incubate with TdT and labelled nucleotide. Incubate under the time and temperature specified by the kit; many systems use approximately 37°C for around 60 minutes.
8. Wash. Remove unincorporated label without allowing the sample to dry.
9. Detect incorporated label. Image directly for fluorescent dUTP systems or apply the enzyme-linked/antibody detection step required for biotin, BrdU or digoxigenin formats.
10. Counterstain if required. A nuclear counterstain allows total nuclei to be counted and helps evaluate morphology.
11. Image consistently. Use identical exposure/threshold settings across experimental groups where quantitative comparison is intended.

How to quantify TUNEL

• Percentage of TUNEL-positive nuclei among total nuclei.
• TUNEL-positive cells per unit tissue area.
• Mean nuclear fluorescence intensity where the assay has been validated for intensity-based comparison.
• Co-localisation with an orthogonal apoptosis marker such as active caspase-3.

Interpretation warnings

• TUNEL detects DNA breaks, not apoptosis as an exclusive biochemical state.
• Necrotic cells and strongly damaged DNA can be TUNEL-positive.
• Fixation and tissue processing can influence DNA accessibility and background.
• Late-stage apoptosis may show very strong signal and fragmented nuclear morphology.
• Use morphology and at least one mechanistically independent marker for high-confidence conclusions.

Troubleshooting

References and primary methods

1. Gavrieli Y, Sherman Y, Ben-Sasson SA. Identification of programmed cell death in situ via specific labeling of nuclear DNA fragmentation. J Cell Biol. 1992;119:493–501.
2. Negoescu A et al. TUNEL apoptotic cell detection in tissue sections: critical evaluation and improvement. J Histochem Cytochem. 1998;46:317–327. doi:10.1177/002215549804600306.
3. Loo DT. In situ detection of apoptosis by the TUNEL assay: an overview of techniques. Methods Mol Biol. 2011;682:3–13. doi:10.1007/978-1-60327-409-8_1.
4. Do TUNEL and Other Apoptosis Assays Detect Cell Death in Preclinical Studies? 2020. PMID:33260475.

Applying TUNEL to cultured cells versus tissue sections

Cultured cells usually require relatively mild permeabilisation because the specimen is thin and directly accessible. Fixed paraffin-embedded tissue may need deparaffinisation, rehydration and enzymatic or detergent pretreatment before TdT can access DNA breaks. Frozen sections often require less aggressive retrieval. These differences mean a condition optimised for monolayer cells should not be transferred directly to FFPE tissue.

Designing positive and negative controls

The most important technical positive control is a specimen treated with DNase I to create accessible DNA breaks. It confirms that TdT labelling and detection chemistry are functional. A no-TdT control reveals background from the labelled nucleotide or downstream detection reagents. Biological controls should include a condition expected to produce apoptosis and an untreated/healthy comparator wherever practical.

Combining TUNEL with other apoptosis markers

TUNEL becomes much more informative when paired with a mechanistically independent marker. Active caspase-3 immunostaining can support caspase-dependent apoptosis, while nuclear morphology can distinguish condensed or fragmented apoptotic nuclei from diffuse damage. Annexin V/viability-dye flow cytometry provides an orthogonal cell-based readout of phosphatidylserine exposure and membrane integrity.

When double-labelling tissue, confirm that the antigen-retrieval, permeabilisation and TUNEL chemistry are compatible with the antibody stain. Order of operations can materially change signal.

Quantification strategy

1. Define the region of interest before looking at treatment identity where possible.
2. Use a nuclear counterstain to identify total nuclei.
3. Set the TUNEL-positive threshold using negative controls, not by visually adjusting each image.
4. Apply identical acquisition settings to groups being compared.
5. Report both the number of specimens/biological replicates and the number of fields/cells analysed.
6. For tissue, consider reporting TUNEL-positive nuclei per area as well as percentage positive if tissue cellularity varies substantially.

What a good TUNEL experiment should show

• DNase-treated positive control gives strong nuclear labelling.
• No-TdT control shows minimal signal.
• Nuclear counterstain remains morphologically interpretable.
• Untreated biological control has plausible baseline positivity.
• Differences between groups are reproducible across biological replicates, not driven by one field.

Frequently asked questions

Does TUNEL only label apoptotic cells?

No. TUNEL labels accessible DNA strand breaks. Apoptosis often produces abundant DNA fragmentation, but necrosis, severe genotoxic injury and tissue-processing artefacts can also be positive.

Why is the DNase-positive control important?

Without it, a completely negative experiment cannot distinguish true absence of DNA fragmentation from failed TdT labelling or poor sample accessibility.

Can TUNEL be quantified by fluorescence intensity?

It can be, but percentage-positive nuclei or positive nuclei per area are usually easier to interpret. Intensity-based comparison requires carefully standardised staining and imaging and may be affected by the number of strand breaks per nucleus.

Can TUNEL be combined with IHC or IF?

Yes, but compatibility must be tested. Fixation, retrieval and enzyme steps can affect antibody epitopes or fluorophores, so include single-stain controls and confirm the order of operations.

Explore more with ABMIUM

For apoptosis studies, combine TUNEL with a protein-level marker rather than relying on DNA fragmentation alone. Explore caspase-3 antibodies, use Find your MATCH, or compare related immunofluorescence and IHC workflows.