What is Fluorescence In Situ Hybridization (FISH)?
The FISH technique (Fluorescence In Situ Hybridization) is a molecular diagnostic method that enables the identification and localization of specific DNA sequences in chromosomes. Its main goal is to detect chromosomal abnormalities such as aneuploidies, duplications, deletions, and translocations.
To detect these abnormalities, fluorescent probes —short DNA fragments— are used. These probes bind to specific DNA sequences and emit fluorescence when observed under a fluorescence microscope.
This technique is widely used in medicine. In the field of assisted reproduction, it is primarily used for sperm analysis, preimplantation genetic testing (PGT), and fetal cell studies.
The FISH technique consists of three fundamental steps:
- Fixation of the sample (sperm, embryonic cells, fetal cells, etc.) on a slide.
- Addition of fluorescent probes that bind to the target chromosomes.
- Analysis using fluorescence microscopy.
How Are Chromosomal Abnormalities Detected?
- Aneuploidies: These are abnormalities in the number of chromosomes. One of the most well-known examples is Down Syndrome, where three copies of chromosome 21 are present instead of two. In this case, FISH will show three signals for chromosome 21 instead of two.
- Deletions and duplications: These are detected by the absence or excess of fluorescence in specific chromosomal regions.
- Translocations: A translocation is a genetic change in which a chromosome fragment breaks off and attaches to another chromosome. FISH identifies these changes by detecting fluorescence signals in non-corresponding chromosomes.
FISH is a rapid, specific, and highly versatile diagnostic technique, suitable for both gamete and embryo analysis. However, it has limitations, including that it can only analyze a limited number of chromosomes (between 5 and 9), and its destructive nature, meaning the analyzed cells cannot be used for future treatments.
When Is This Technique Indicated?
- In patients with implantation failure in previous cycles when using conventional ICSI.
- In cases of recurrent miscarriage.
- Advanced paternal age.
- History of chromosomal abnormalities in previous pregnancies.
- Patients who have undergone chemotherapy or radiotherapy.
- Unexplained infertility.