Mouse Embryo Films Reveal a Second Cellular Source for the Placenta

The deletion experiments make the strongest case. When researchers removed most mural cells from early embryos in the lab, those embryos never formed the early placenta precursor. In the uterus, very few developed at all.

"For decades, textbooks have told us the placenta comes from one place," Zernicka-Goetz said. "Watching embryos implant showed us that a few cells from the other side of the embryo make the journey too, and that without them the placenta's foundations don't form properly." Her hypothesis: first contact with the uterine wall is what primes these cells to migrate and invade.

Does this apply to humans?

Only maybe. Human embryos implant differently from mice, an important caveat. But when the team filmed human blastocysts in culture, they saw a similar group of border cells moving toward the embryo. A few switched on the same marker genes found in mice. Mining published datasets, the researchers also found cells with that signature among the invasive cells anchoring the human placenta into the uterine wall. A comparable population plausibly exists in people. Plausibly is the operative word.

What does this change for fertility research?

Implantation is the stage scientists understand least — and the stage when most pregnancies are lost. The road map caught other hard-to-see events too:

• Body-forming cells pass through a brief, distinct state between two known stem-cell stages, and a gene called Wt1 — best known from kidney development and a childhood kidney cancer — switches on sharply, then off.
• The first molecular signs of the head-to-tail body axis appear earlier than previously reported. No current stem-cell line fully captures the Wt1 transition.

The findings also expose a gap in lab-grown embryo models, the embryo-like structures built from stem cells. Those models cannot implant properly or make a working placenta. The new results suggest a reason: they lack correctly formed mural cells. "We now hope to be able to establish methods to create these cells in our embryo models with the hope to improve their implantation," Zernicka-Goetz said. "If we succeed, these studies might shed new light on why human embryo implantation fails so often."

Hope, hedged twice. Still, the team has made its full dataset freely available through an interactive website, so others can stress-test the claim. That's how a textbook gets rewritten — slowly, and in the open.