Evidence for selection against tetraploid cells in tetraploid<-->diploid mouse chimaeras before the late blastocyst stage

Tetraploid (4n) cells do not contribute equally to all tissues of midgestation mouse chimaeras and mosaics. Our previous studies of early blastocysts showed that 4n cells are preferentially allocated to the mural trophectoderm of the early blastocyst and this may contribute to the restricted distribution pattern seen at later stages. In this study of later-stage blastocysts we found evidence for selection against 4n cells. The contribution of 4n cells to 4n<-->2n chimaeric blastocysts decreased between E3.5 and E4.5 days, whereas the composition of 2n<-->2n controls changed little over this period. These results suggest that, prior to implantation, blastocysts have already lost some tetraploid cells from their embryonic and extra-embryonic lineages due to a combination of preferential allocation of 4n cells to the mural trophectoderm and selection against 4n cells throughout the embryo.