Tunel

TUNEL, or Terminal deoxynucleotidyl transferase dUTP nick end labeling, is a laboratory technique used to detect DNA fragmentation that occurs during apoptosis. It was introduced in 1992 by Gavrieli, Sherman, and Ben-Sasson to visualize DNA strand breaks in fixed cells and tissue sections.

The assay uses the enzyme terminal deoxynucleotidyl transferase to add labeled dUTP to exposed 3'-OH ends at DNA breaks. The incorporated nucleotides carry detectable tags such as fluorophores, biotin, or digoxigenin, enabling visualization by fluorescence or enzymatic colorimetric detection in situ.

Applications include quantifying apoptotic cells in histological sections, evaluating treatment responses in cancer and neurodegenerative disease models, and studying developmental biology. The method is widely used in pathology and biomedical research to map apoptotic patterns.

Limitations include potential labeling of necrotic or mechanically damaged DNA, fixation artifacts, and false positives. Results require appropriate positive and negative controls and, when possible, corroboration with additional apoptosis markers such as caspase activity. TUNEL results should be interpreted in context.

Variants such as fluorescent TUNEL (F-TUNEL) and enzyme-based TUNEL with digoxigenin or biotin detection exist, and the method is often combined with counterstaining or other apoptotic assays; related approaches include in situ end labeling (ISEL) for DNA fragmentation detection.