Mosaicism in Preimplantation Human Embryos

Since the very first publications on preimplantation genetic testing, researchers have faced a serious problem — a high mosaicism level in the preimplantation human embryos obtained by means of in vitro fertilization cycles. The nature of this mosaicism and its high impact on embryo development draws attention to this issue. In this research we studied the cells from different parts of preimplantation human embryos with mosaicism in the trophectoderm cells detected using Next-generation Sequencing (NGS). Six human blastocysts with mosaicism in their trophectoderm cells were each sectioned in three parts: two containing only trophectoderm cells and one predominantly inner cell mass. These parts were then analyzed individually. Our data indicate that the proportion of aneuploid cells in bioptate taken for preimplantation genetic testing does not necessarily reflect the true chromosomal status of the whole embryo and cannot be extrapolated to that in the embryoblast cells. The results of our study strongly suggest that mosaicism revealed in blastocyst reduces the likelihood of finding the euploid chromosome set in the other parts of the embryo. Karyotypes of cells from different parts of mosaic embryos show low concordance. Chromosomal abnormalities in mosaic embryos are unpredictably diverse, which may lead not only to loss of conception, but also to the development of genetic disease in the offspring. According to our data, the mosaic rate tends to increase in the samples containing trophectoderm adjacent to the embryoblast, which may have physiological significance for the implantation. Comparative studies focused on the concordance of mosaicism level of and the type of chromosomal abnormalities detected in different parts of preimplantation human embryos will improve clinical recommendations regarding the transfer of mosaic embryos.