Allocation of cells to the inner cell mass and trophectoderm of 3/4 mouse embryos

The allocation of cells to the inner cell mass (ICM) and trophectoderm (TE) was investigated at 6-h intervals from 78 h to 102 h after hCG injection in 3/4 mouse embryos to determine the effect of removal of a single blastomere at the 4-cell stage on early differentiation. The procedures used to produce 3/4 embryos had little effect on embryo development. Embryos that had a single blastomere removed and then re-aggregated (RA embryos) had the same total number of cells as untreated (UT) embryos except at 78 h (P less than 0.05) and 102 h (P less than 0.01) post hCG where there were slightly less cells in RA embryos. Three-quarter embryos always had significantly fewer cells than RA embryos (P less than 0.001), with an average of 74% of the total cell number of RA embryos. As expected, 3/4 embryos always had significantly fewer cells in the ICM and TE compared with RA embryos (P less than 0.001). However, the ICM:TE ratio was also significantly lower in 3/4 embryos compared with RA embryos at 84, 96, and 102 h post hCG, indicating that the allocation of cells to the ICM and TE was disturbed. The ICM:TE ratio of 3/4 embryos could not be manipulated if either an early- or late-dividing blastomere was selectively biopsied at the 4-cell stage; this suggests that the known preferential contribution of an early-dividing blastomere to the ICM is not cell autonomous.