Unbiased contribution of the first two blastomeres to mouse blastocyst development.

Several recent studies have proposed a model that the organization of the mouse blastocyst is determined by the pattern of early cleavages: the plane of first cleavage divides the two-cell embryo into embryonic (Em) and abembryonic (Ab) halves, while the timing of the second cleavages specifies which blastomere becomes the Em half. This model is still controversial because of conflicting observations in various studies. Here, we investigated the possibility that the difference between mouse strains contributed to the discrepancy of the findings of different experiments regarding the relationship between the first two cleavages and the blastocyst axial pattern. First, we showed by using a lipophilic, fluorescent tracer that the plane of the first cleavage bears no consistent spatial relationship to the Em-Ab axis of the blastocyst regardless of the genotypic background. Secondly, the order of the second cleavage does not correlate with the Em-Ab polarity of the blastocyst. This was demonstrated by tracing the lineage of the early- and later-dividing two-cell stage blastomeres in the whole embryo as well as by comparing the developmental potential of isolated early- and later-dividing blastomeres and chimeras made entirely of early- or later-dividing blastomeres. These results suggest that contrary to recent studies, the differences between the early- and later-dividing blastomeres of the two-cell embryo are not functionally evident and do not define the Em-Ab polarity of the blastocyst. The significance of these findings is discussed in relation to human assisted reproduction and preimplantation genetic diagnosis.