Model of chromosome mosaicism reveals normal development of chimeric mammalian embryos

The majority of human embryos display mosaic aneuploidy, containing a mixture of euploid and aneuploid cells before implantation. An understanding of how this aneuploidy affects embryonic development is therefore critical for clinicians striving to improve the success rates of in vitro fertilization (IVF) treatment. Here, we directly assess the developmental fate of aneuploid cells and the consequences of chromosome mosaicism for pregnancy success. We show that the fate of aneuploid cells is dependent upon lineage: in the fetal lineage abnormal cells are eliminated by apoptosis, while in the placental lineage these cells show severe proliferative defects. By modulating the ratios of euploid and aneuploid cells within the embryo, we find that mosaic embryos have full developmental potential, provided they contain a minimum number of euploid cells. These data prompt a re-evaluation of embryo selection methods during IVF procedures, suggesting that cleavage-stage biopsy might be an inaccurate predictor of embryo success.