Cellular Self-Destruction – Visualizing Apoptosis with AAT Bioquest’s TUNEL Assays

Programmed cell death, the cell’s suicide program, planned cell demise – all these terms describe apoptosis. It is a tightly regulated physiological process that plays a crucial role in the development, maintenance, and aging of multicellular organisms, during which individual cells are selectively removed [1]. As early as 1842, the German-Swiss naturalist Carl Vogt first described this phenomenon while studying the metamorphosis of tadpoles of the common midwife toad [2]. However, the term “apoptosis” was not introduced until more than a century later by John F. R. Kerr, Andrew Wyllie, and Alastair R. Currie, who characterized the process in detail using histology and microscopy [3].

Today, apoptosis is a major focus of research into cancer development and autoimmune diseases. A variety of detection methods are used for this purpose – one well-established classic is the “TdT-mediated dUTP-biotin nick end labeling” method, or TUNEL for short. First described in 1992, this technique employs the enzyme TdT (terminal deoxynucleotidyl transferase) to visualize DNA fragmentation during apoptosis [4]. Our partner AAT Bioquest offers Cell Meter™ TUNEL Apoptosis Assay Kits, providing a highly sensitive, fluorescence-based visualization of apoptotic nuclei – optimized for both live and fixed cells as well as tissue samples.

These topics await you:

1) Apoptosis – the Cell’s controlled Self-Destruction

2) Visualizing Apoptosis with the TUNEL Assay

3) The Advantages of AAT’s Cell Meter™ TUNEL Assays

Apoptosis – the Cell’s controlled Self-Destruction

Programmed cell death is essential for the normal development and function of an organism and serves multiple purposes: it eliminates transformed or potentially harmful cells, regulates cell numbers and thus tissue size, and ensures the selection of genetically intact germ cells [5]. In contrast to necrosis, a pathological process triggered by external damage, apoptosis is actively initiated by the cell itself and therefore represents a regular component of metabolism. While necrotic cells swell and provoke inflammatory responses, apoptosis begins with cell shrinkage. At the same time, activated endonucleases cleave the DNA into characteristic fragments of about 180 to 200 base pairs in length [3]. This internucleosomal DNA fragmentation is considered the main biochemical hallmark of apoptosis – and cells with precisely such DNA strand breaks can be reliably detected in the laboratory using the TUNEL assay [6].

Visualizing Apoptosis with the TUNEL Assay

To detect and quantify apoptotic cells in situ, the TUNEL assay labels the exposed 3´-OH ends of DNA breaks with modified deoxyuridine triphosphate (dUTP) (Fig. 1). This can be achieved either directly with dye-labeled dUTPs or indirectly using 5-bromo-2´-deoxyuridine-5´-triphosphate (BrdUTP) in combination with specific antibody conjugates. In both cases, terminal deoxynucleotidyl transferase (TdT) is required to catalyze the incorporation of the modified dUTPs at the free 3´-hydroxyl ends of fragmented DNA [6]. To allow the reagents to enter the nucleus, the samples must first be fixed and permeabilized (Fig. 1). Initiation of the TUNEL reaction also requires cobalt as a cofactor in the buffer solution [5]. Depending on the type of dUTP used, apoptotic cells can then be detected and quantified by various methods – such as fluorescence microscopy, flow cytometry, or fluorescence-based microplate assays [6].

Figure 1: Workflow of the Cell Meter™ TUNEL Assays from AAT Bioquest. After fixation and permeabilization of the sample, the TUNEL reaction is initiated by adding the reagent. The enzyme TdT then labels the free hydroxyl groups at DNA strand breaks with fluorescence-tagged nucleotides. Following optional counterstaining, apoptotic cells can be visualized by fluorescence microscopy [6].

The Advantages of AAT’s Cell Meter™ TUNEL Assays

Since its introduction, the TUNEL assay has become an established in situ technique for identifying apoptotic cells. Compared to earlier methods, it offers higher sensitivity and enables reliable quantification of apoptotic cells across a broad measurement range. AAT Bioquest’s Cell Meter™ Apoptosis Assays are specifically optimized for fluorogenic detection and are available with fluorescence emission in the green, red, or blue spectrum. In addition, AAT provides flexible assay formats suitable for both live cells and fixed cells or tissue samples.

A key advantage of the Cell Meter™ TUNEL Assays over other commercially available kits is the omission of sodium or potassium cacodylate in the reaction buffer. Cacodylate is a carcinogenic arsenic derivative that, due to its toxicity, can itself induce apoptosis and thereby cause background signals and distorted results. By eliminating this toxic reagent, the assay is not only safer to handle but also delivers reliable and reproducible results with a significantly reduced false-positive rate [7].

Whether you are studying the toxicity and safety of drug candidates or analyzing disease-related cellular changes, AAT Bioquest’s Cell Meter™ TUNEL Assay Kits provide a simple, safe, and highly sensitive solution for detecting apoptotic DNA fragmentation. Be sure to also explore AAT’s full product portfolio and discover additional innovative tools from the fluorescence and luminescence expert.

All Cell Meter™ TUNEL Assays from AAT Bioquest

Sources

[1] https://www.spektrum.de/lexikon/biologie/apoptose/4488, 16.08.2025

[2] C. Vogt: Untersuchungen über die Entwicklungsgeschichte der Geburtshelferkröte (Alytes obstetricans). Jent & Gassmann, Solothurn, Schweiz 1842.

[3] https://de.wikipedia.org/wiki/Apoptose, 16.08.2025

[4] 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 Nov;119(3):493-501.

[5] https://flexikon.doccheck.com/de/Apoptose, 16.08.2025

[6] https://www.aatbio.com/products/tunel-assay, 16.08.2025

[7] https://www.aatbio.com/resources/assaywise/2018-7-1/a-superior-direct-method-for-the-quantitative-analysis-of-dna-fragments-in-late-stage-apoptosis, 16.08.2025

Preview Image: https://images.aatbio.com/universal/image-gallery/22.png, 22.08.2025