A study on the origin of the mouse placental trophoblast lineage

The mouse placenta forms primarily from cells in the trophectoderm (TE) of the blastocyst. Complicated lineage specification in placental trophoblast cells finally forms the functional placenta. However, at which stage the fate of mouse trophoblast lineages is determined key open questions. Lineage tracing is the most reliable approach to study cell fate determination. Herein, we took advantage of a fluorescent Cre recombinase reporter mouse for our lineage tracing experiments. Cell labeling experiments were performed by microinjection of Cre mRNA into single blastomere respectively at the two-, four- and eight-cell stage, and increasing bias of the labeled cell distribution was observed in the E12.5 fetus and placenta, which was also supported by the single cell sequencing results.Together, our data provide direct evidence supporting the idea that the first cell fate choice of early blastomeres, toward the inner cell mass (ICM) or trophoblast fate, has become determined at the four-cell stage, and three different cell fates may exist among the four blastomeres.Compared with previous studies, it is the first study to trace the cell lineages from early stages over such a long developmental period. Overall design: We administered a microinjection of Cre mRNA into a single blastomere of the mTmG mouse at 4- or 8-cell stages. We isolated Labelled cells at the blastocyst stage and perform candidate based gene expression analyses to validate expression patterns of some key regulators that are implicated in TE vs. ICM lineage bias.