Expression Of Oct4, Cdx2 And Yap1 During Blastocyst Formation In The Marsupial, Monodelphis Domestica

The marsupial blastocyst first forms as a simple epithelium securely adherent to the zona pellucida. It has no inner cell mass (ICM), which in the eutherian (mouse) blastocyst contains the embryonic stem cells at this stage. The marsupial ICM equivalent, the pluriblast, is co-planar with the trophoblast. By contrast, the mouse trophoblast encloses the ICM. Oct4, Cdx2 and Yap1 play crucial roles in allocating mouse blastocyst cells to either trophoblast or ICM fates. Because these genes are found in the opossum genome, we hypothesized that they may have a similar role in cell allocation between pluriblast and trophoblast in the opossum blastocyst. In both mouse and opossum, Oct4 is expressed in all embryonic cells prior to blastocyst formation. During mouse blastocyst formation, Cdx2 is upregulated as Oct4 is downregulated in the nascent trophoblast. In the opossum, a patch of cells in the unilaminar blastocyst epithelium (pluriblast) undergoes the same switch in gene expression. In both types of embryos, Yap1 is translocated to the nuclei of putative trophoblast cells but remains in the cytoplasm of cells fated to be ICM or pluriblast. Our results indicate that the roles of Oct4, Cdx2 and Yap1 in allocating cells to the trophoblast lineage are evolutionarily conserved between marsupials and eutherians, despite the topological differences between their blastocysts. Our results also suggest that cell position plays less of a role in mammalian trophoblast differentiation as has been long believed