Both blastomeres of the mouse 2-cell embryo contribute to the embryonic portion of the blastocyst.

To track the lineage of both blastomeres of 2-cell embryos during mouse preimplantation development, each cell was injected with dextran solutions conjugated with different fluorochromes. The fate of the progeny of the first two blastomeres was followed with confocal microscopy during cleavage and during the formation of the blastocyst. We observed that in most of cleaving embryos the cells derived from the two first blastomeres intermingled in both the trophectoderm and the inner cell mass (ICM) and did not form two discrete groups. We conclude that embryonic parts of blastocysts contain descendants of both blastomeres of 2-cell embryo.