Formation by aggregation of viable chimaeras between ICM and eight-cell mouse embryos with different genetic backgrounds.

The author studied the difference between immunosurgical and microsurgical isolation of inner cell masses (ICMs) from fully expanding blastocysts. The stage after which no further cell exchange between ICM and trophoblast can take place may involve the closure of certain gene pathways. The process of the aggregation of ICMs and eight-cell embryos with different genetic backgrounds was also studied. The results seem to lend strong support to the opinion that the microsurgical technique for isolating the differentiating ICM from the trophoblast is better than immunosurgery. The author also supports the view that the cells of the ICM become committed in the fully expanding blastocyst, while the cells of the eight-cell embryo are still labile and can differentiate to either ICM or trophoblast cells. Cell position during the aggregation process plays a role in cell determination and differentiation.