Chromosomal mosaicism in the human preimplantation embryo in vitro

It has previously been demonstrated that a large percentage of in vitro generated human embryos are chromosomally mosaic. The current thesis investigated this mosaicism in greater detail. It characterized the mosaicism present at each stage of preimplantation development in vitro. It examined the relevance of the different forms of the observed mosaicism to preimplantation embryo wastage, implantation failure, and fetal and placental mosaicism. Finally, it addressed the identification of the chromosomally mosaic embryos during preimplantation genetic diagnosis (PGD). For each of the studies presented within the thesis, blastomeres from "spare" in vitro generated embryos were assessed for chromosomal content using multi-color fluorescence in situ hybridization (FISH) DNA probes. Mosaicism was detected at all stages of preimplantation development, from the 2-cell stage to the blastocyst stage; it comprised of diploid, aneuploid, "chaotic", haploid, and polyploid chromosome patterns. Compared to blastocysts, arrested embryos or embryos at the earlier stages of development, showed a much higher incidence of mosaicism involving "chaotic" imbalances for multiple chromosomes and/or high percentages of abnormal cells. These results indicate that extensive post-zygotic abnormalities impair embryonic development to the blastocyst stage. The presence of mosaicism was not predicted by embryo morphology. Mosaicism may therefore contribute to the low rates of blastocyst formation in vitro and to the high rates of implantation failure following cleavage stage embryo transfer. Probe mixtures comprising of three autosomes, of one autosome and gonosomes, or of five autosomes could be applied for the identification of the mosaic embryos during cleavage stage PGD. Culture of isolated blastomeres from cleavage stage embryos for genetic diagnosis increases the number of cells available for analysis; however, the presence of nuclear defects and mosaicism among the cultured cells indic