Identical triplets and twins developed from isolated blastomeres of 8- and 16-cell mouse embryos supported with tetraploid blastomeres.

We studied the developmental potential of single blastomeres from early cleavage mouse embryos. Eight- and sixteen-cell diploid mouse embryos were disaggregated and single blastomeres from eight-cell embryos or pairs of sister blastomeres from sixteen-cell embryos were aggregated with 4, 5 or 6 tetraploid blastomeres from 4-cell embryos. Each diploid donor embryo gave eight sister aggregates, which later were manipulated together as one group (set). The aggregates were cultured in vitro until the blastocyst stage, when they were transferred (in sets) to the oviducts of pseudopregnant recipients. Eighteen live foetuses or pups were obtained from the transfer (11.0% of transferred blastocysts) and out of those, eleven developed into fertile adults (one triplet, one pair of twins and four singletons). In all surviving adults, pups and living foetuses, only diploid cells were detected in their organs and tissues as shown by analysis of coat pigmentation and distribution of glucose phosphate isomerase isoforms. In order to explain the observed high rate of mortality of transferred blastocysts, in an accompanying experiment, the diploid and tetraploid blastomeres were labelled with different fluorochromes and then aggregated. These experiments showed the diploid cells to be present not only in the inner cell mass (ICM) but also in the trophectoderm. The low number of diploid cells and the predominance of tetraploid cells in the ICM of chimaeric blastocysts might have been responsible for high postimplantation mortality of our experimental embryos.