Previous studies of tetraploid↔diploid mouse chimaeras and mosaics have revealed that tetraploid cells do not contribute equally to all tissues of the conceptus. In this study we have shown that, within 30 h of aggregating cleavage stage embryos, tetraploid cells were non-randomly distributed among different tissues of the early blastocyst. They were preferentially allocated to the mural trophectoderm regardless of cell size at the time of aggregation. This early effect may underlie the restricted distribution of tetraploid cells at later stages. We have demonstrated for the first time that ploidy can influence the relative position of blastomeres in the preimplantation embryo.

Ischyodus dolloi Leriche 1902, the youngest record of the species, and Chimaera seymourensis sp. nov. are described from the La Meseta Formation, Late Eocene of Seymour Island, Antarctic Peninsula.

Studies with intact preimplantation mouse embryos and some types of chimaeric aggregates have shown that the most advanced cells are preferentially allocated to the inner cell mass (ICM) rather than the trophectoderm. Thus, differences between 4-cell and 8-cell stage embryos could contribute to the tendency for tetraploid cells to colonise the trophectoderm more readily than the ICM in 4-cell tetraploid[harr ]8 cell diploid chimaeras. The aim of the present study was to test whether 4-cell stage embryos in 4-cell diploid[harr ]8-cell diploid aggregates contributed equally to all lineages present in the E12.5 conceptus. These chimaeras were compared with those produced from standard aggregates of two whole 8-cell embryos and aggregates of half an 8-cell embryo with a whole 8-cell embryo. As expected, the overall contribution of 4-cell embryos was lower than that of 8-cell embryos and similar to that of half 8-cell stage embryos. In the 4-cell[harr ]8-cell chimaeras the 4-cell stage embryos did not contribute more to the trophectoderm than the ICM derivatives. Thus, differences between 4-cell and 8-cell embryos cannot explain the restricted tissue distribution of tetraploid cells previously reported for 4-cell tetraploid[harr ]8-cell diploid chimaeras. It is suggested that cells from the more advanced embryo are more likely to contribute to the ICM but, for technical reasons, are prevented from doing so in simple aggregates of equal numbers of whole 4-cell and whole 8-cell stage embryos.

Studies of tetraploid[harr ]diploid (4n[harr ]2n) mouse chimaeras have demonstrated unequal contributions of 4n cells to different tissues of the midgestation conceptus. Such a pattern has also been reported in chimaeras as early as E3.5d, which show an enhanced contribution of 4n cells to the mural trophectoderm (Everett & West, 1996). In this study, sectioned 4n[harr ]2n and 2n[harr ]2n control chimaeric blastocysts were digitised and reconstructed in 3 dimensions (3-D). The 3-D images revealed only limited mixing of cells from the 2 contributing embryos of individual blastocysts in both chimaera groups. Consequently, the distribution pattern of the 2 cell types was dependent on the spatial relationship between the orientation of the blastocyst and the boundary between the 2 clusters of cells. The distribution patterns observed were not strikingly different for 4n[harr ]2n and 2n[harr ]2n chimaeras, each showing some transgenic positive cell contribution in all 3 identifiable developmental lineages. It was notable, however, that in all 4n[harr ]2n blastocysts at least some 4n cells were located adjacent to the blastocyst cavity. Such a consistent pattern was not evident in 2n[harr ]2n chimaeras. This study has demonstrated the value of 3-D reconstructions for the analysis of spatial relationships of 2 cell populations in chimaeric mouse blastocysts.