Caltech biologists find second source for mouse placenta

New map of implantation process

Caltech biologists produced the most detailed picture yet of implantation in mouse embryos. Researchers tracked genetic function within over 22,000 distinct cells throughout and following the incident. The team filmed living embryos as they attached to the uterine wall roughly six days after fertilization. This work overturns a long-held textbook view that the placenta grows from a single group of cells.

Border mural cells travel upward

Researchers found several cells from the other side of the embryo that move to assist in constructing the structure. These border mural cells often number just two to four per embryo and behave differently from their neighbors. They switch on genes associated with placenta-forming cells before joining the ectoplacental cone which builds much of the mouse placenta. When researchers removed most of these mural cells from early embryos grown in the lab, the structures failed to form properly.

Implications for human pregnancy loss

Human embryos appear to have similar cells that help explain why some pregnancies fail very early. The results further reveal gaps in current lab-grown embryo models built from stem cells. These artificial structures cannot implant or make a working placenta because they lack the specific border mural cells. Magdalena Zernicka-Goetz, who serves as Bren professor of biology and biological engineering, performed the research in her laboratory.