Sperm DNA fragmentation (TUNEL Assay)

Sperm DNA fragmentation is thought to be a very sensitive measure of sperm abnormality, independent of sperm counts or standard sperm tests, and predicts natural fertility as well as fertility with the most advanced forms of assisted reproduction. Sperm with high DNA fragmentation may fertilize an egg and embryo development stops before implantation or may even initiate a pregnancy but there is a significantly higher likelihood that it will result in miscarriage. More importantly what has also been demonstrated is that the degree of DNA fragmentation correlates very highly with the inability of the sperm to initiate a birth regardless of the technology used to fertilize the egg such as insemination, IVF or ICSI. There are several ways to test for sperm DNA fragmentation; the most widely used and statistically robust test is called TUNEL (terminal dUTP nick-end labeling). 
The TUNEL assay quantifies the incorporation of dUTP at single- and double-strand DNA breaks in a reaction catalyzed by the template-independent enzyme terminal deoxynucleotidyl transferase (TdT).
This enzyme incorporates biotinlyated deoxyuridine to 3′-OH of DNA to create a signal, which increases with the number of DNA breaks. Sperm with normal DNA therefore have only background staining/ fluorescence, while those with fragmented DNA (multiple chromatin 3′-OH ends) stain/fluoresce brightly.

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