WHAT IS A MOSAIC EMBRYO?

Embrión mosaico

More and more couples or women decide to genetically analyze their embryos, with the aim of finding out which ones present alterations and transferring those that are healthy. Thanks to analyses focused on detecting chromosomal aneuploidies in embryos, such as preimplantation genetic screening (PGS or PGT-A) by NGS, it is possible to know which embryos present some type of genetic alteration.

The human being has 46 chromosomes (23 pairs) in which all the genes present in the genome are distributed. Of these 46 chromosomes, half comes from our father and a half from our mother. If the number of chromosomes is not adequate (there are chromosomes in excess or in deficiency) the embryo will not develop correctly and as a consequence, its ability to implant in the mother’s uterus drastically decreases, resulting in abortion or a baby with a genetic defect in those cases in which the pregnancy could continue.

Embryonic mosaicism consists in the fact that the embryo has a mixture of cells whose number of chromosomes is normal and other cells that present alterations in that number for one or several chromosomes. The causes do not seem to be related to paternal or maternal factors, but rather to a failure during embryonic division. NGS (Next-generation sequencing) validation, known as massive sequencing, is capable of detecting gains and losses of genetic material more precisely. This test allows simultaneous analysis of all 24 chromosomes (1-22, X and Y) with higher resolution. After this test, it is possible to know if the embryo is mosaic or not and what is the degree of mosaicism that it presents.

ADVANTAGES OF PERFORMING THE PGT-A

The PGT-A is a recommended test for any assisted reproduction process (IVF, egg donation), as it increases the chances of pregnancy with a healthy embryo. Although it is especially aimed at elderly patients who have suffered implantation failure, repeated abortions or have a diagnosis with altered semen parameters.

IS IT POSSIBLE TO TRANSFER A MOSAIC EMBRYO?

Although a mosaic embryo could give rise to a healthy baby, the risks or limitations that exist when transferring these embryos are variables depending on the type of mosaicism. These embryos could be considered transferable only in those cases in which normal (euploid) embryos are not available. Likewise, it is essential to follow selection criteria and protocols before deciding which embryos to transfer, and to provide adequate genetic counseling prior to transfer.

The chances of implantation of a mosaic embryo compared to a euploid (normal) embryo are lower, but, in turn, higher than the implantation rate of a non-analyzed embryo.

Any doubts? Ask us without obligation. Our embryology team will be happy to answer all your questions.

international@ovoclinic.net

Information sources:

Treff, Net et al (2021). L’embrione “a mosaico”: idee sbagliate e interpretazioni errate nei test genetici preimpianto per l’aneuploidia. Sterile fertile.;116(5):1205-1211

Besser, A et al (2021). Gestione basata sull’evidenza del mosaicismo cromosomico preimpianto: lezioni dalla clinica. Sterile fertile.;116(5):1220-1224

Abhari, S et al (2021). Gravidanza e risultati neonatali dopo il trasferimento di embrioni a mosaico: una rassegna. J Clin Med.;10(7):1369

Takeuchi K. (2020) Test genetici pre-impianto: passato, presente, futuro. Reprod Med Biol.;20(1):27-40

Lledó, B et al (2017) Potenziale di impianto di embrioni a mosaico, Systems Biology in Reproductive Medicine, 63:3, 206-208

Greco, E. et al. (2015) Bambini sani dopo il trasferimento intrauterino di blastocisti aneuploidi a mosaico. N Inglese J Med 373:2089-2090

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