https://phys.org/news/2018-12-source-stem-cells-proteins.html

"Pluripotent cells are the progenitors of embryonic stem cells, and they are famous because they can become any part of the body," she said. "We have discovered a process that regulates the balance between pluripotent and placenta, and it works by changing the physical location of cells within the embryo's ball of cells."

To form this wondrous ball, the mother packages two closely related proteins into her eggs, which help oversee this decision-making process. The scientific team used genetic tools to eliminate the proteins YAP1 and WWTR1 from mouse eggs and embryos. The team discovered that these two proteins first position these cells into distinct inside and outside locations, which then decides their fate: placenta or pluripotent.