What is embryo mosaicism? 0

Human beings have 46 chromosomes (23 pairs) and these chromosomes contain all the genes in the genome. Half of the 46 chromosomes come from our father and the other half come from our mother. Right from the embryo stage, humans need the number of chromosomes to be normal in order for them to develop correctly. If the number of chromosomes is not correct (there are too many or too few), the embryo does not develop appropriately and, as a result, it does not implant in the mother’s uterus, it leads to a pregnancy loss or the mother gives birth to a child with malformations.

If an embryo has developed following in vitro fertilisation, we are able to avoid situations such as these by using comprehensive embryo chromosome diagnosis techniques (PGS/PGT-A/CCS). The powerful technology that we currently have at our disposal (array CGH and next generation sequencing) means that we are able to identify abnormalities in the number of any of the chromosomes, ensuring that we only transfer embryos that are chromosomally normal. Diagnosis of this kind is performed using an embryo biopsy. It entails removing between 5 and 10 cells from the external layer of the embryo (the future placenta) between days 5 and 7 of embryo development. This procedure does not impede the embryo from developing and implanting correctly.

A phenomenon that occurs in human embryos known as chromosomal mosaicism has been described since techniques such as these for embryo chromosome diagnosis have been in use.

Embryo mosaicism means that the embryo has a mixture of chromosomally normal and abnormal cells (in one or several chromosomes). This abnormality is the result of poor separation of the chromosomes during embryo division. It does not appear to have any connection to any factor in the mother or in the father. It is estimated that 20% of human embryos have chromosomal mosaicism. The abnormal cells in a given chromosome can go from 20% to 80% of the total number of cells in the embryo.

It has been demonstrated that the ability of mosaic embryos to implant and lead to a successful pregnancy is only slightly lower than in embryos that are not mosaic. Nonetheless, this difference needs to be taken into account. Around 30% of these embryos lead to a viable pregnancy. It would appear that these embryos are somehow able to correct the abnormal cells or simply divide more slowly than the rest and disappear. The result is an entirely normal embryo.

With a view to transfer of mosaic embryos, these embryos should only be taken into account if no normal embryos are available. The type of mosaicism should also always be taken into account when deciding whether or not to transfer an embryo. For example, transfer of mosaic embryos with trisomies in chromosomes 13, 18 and 21 is not advised since these chromosomes are linked to serious syndromes in the foetus. Again, transfer of mosaic embryos is not advised when the percentage of abnormal cells is in excess of 50%.

If a woman does get pregnant following transfer of a mosaic embryo, an antenatal diagnosis (amniocentesis) must be performed in order to confirm that the foetus’ karyotype is normal.

At Instituto Bernabeu, we have the most advanced available technology for chromosomal diagnosis of the embryo and an interdisciplinary team of doctors and geneticists who can provide specialist guidance in cases such as these.

Dr Ruth Morales, a molecular biologist at Instituto Bernabeu.

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