Photo of an embryo
A three-day-old embryo with an in-
correct number of chromosomes
was capable of undergoing "a dy-
namic process of genetic normali-
sation"; © panthermedia.net /
Npine B.J. Lee

The findings have significant implications not just for preimplantation genetic screening (PGS) during fertility treatment, but also for future, cell-based, stem cell treatments for conditions ranging from haematological disorders to neurological damage.

Kearns, who is an associate professor in the department of gynecology and obstetrics, at the Johns Hopkins Medical Institutions in Baltimore (USA) explained: "There has been mounting evidence that PGS using cells from the outer layer of the blastocyst, the trophectoderm, at day five results in higher pregnancy rates than evaluating a blastomere at the cleavage stage on day three. There are also good data to suggest that there are a substantial number of genetically different cells existing in many cleavage stage embryos – a condition known as 'mosaicism'.

Additionally, tests performed on pregnant women to determine if genetic abnormalities exist in the developing foetus show that aneuploid cells of placental origin are relatively common in foetal blood. These observations led to the hypothesis that mechanisms may exist in the developing embryo that could cause mosaic embryos to marginalise abnormal aneuploid cells and preserve normal cells, and thus 'genetically correct' to a genetically normal embryo.

"Preimplantation genetic screening (PGS) refers to the removal of a cell from a developing embryo and evaluating this cell for all chromosome abnormalities. If the results of this screening show that the embryo is normal, then either it undergoes uterine transfer or is frozen for future use. In cases where PGS evaluation yields a biopsied cell that is chromosomally abnormal, standard practice is to discard the corresponding embryo."


MEDICA.de; Source: European Society of Human Reproduction and Embryology