ESCs injected into the 8-cell stage mouse embryo modify pattern of cleavage and cell lineage specification.

During mouse embryogenesis initial specification of the cell fates depends on the type of division during 8- to 16- and 16- to 32-cell stage transition. A conservative division of a blastomere creates two polar outer daughter cells, which are precursors of the trophectoderm (TE), whereas a differentiative division gives rise to a polar outer cell and an apolar inner (the presumptive inner cell mass - ICM) cell. We hypothesize that the type of division may depend on the interactions between blastomeres of the embryo. To investigate whether modification of these interactions influences divisions, we analyzed the pattern of blastomere division and cell lineage specification in chimeric embryos obtained by injection of a different number of mouse embryonic stem cells (ESCs) into 8-cell embryos. As the ESCs populate only the ICM of the resulting chimeric blastocysts, they emulated in our model additional inner cells. We found that introduction of ESCs decreased the number of inner, apolar blastomeres at the 8- to 16-cell stage transition and reduced the number of ICM cells of host embryo-origin during formation of the blastocyst. Moreover, we showed that the proportion of inner blastomeres and their fate (EPI or PE) in chimeric blastocysts was dependent on the number of ESCs injected. Our results suggest the existence of a regulative mechanism, which links number of inner cells with a proportion of conservative vs. differentiative blastomere divisions during the cleavage and thus dictates their developmental fate.