Human embryos grown for two weeks in lab.

Nature has followed history’s longest-cultured human embryos:

Developmental biologists have grown human embryos in the lab for up to 13 days after fertilization, shattering the previous record of 9 days. The achievement has already enabled scientists to discover new aspects of early human development, including features never before seen in a human embryo.

“It’s really embarrassing at the beginning of the twenty-first century that we know more about fish and mice and frogs than we know about ourselves,” says Ali Brivanlou, a developmental biologist at the Rockefeller University in New York City and lead author of the study in Nature. “This is a bit difficult to explain to my students.”

Magdalena Zernicka-Goetz, a developmental biologist at the University of Cambridge, UK, and her colleagues developed the culture technique using mouse embryos. Many scientists have attempted to simulate conditions in the womb by growing embryos on a layer of maternal cells, but Zernicka-Goetz’s group chose instead to use a gel matrix with higher levels of oxygen. The mouse embryos survived past gastrulation — the stage at which they form layers of cells that will become organs.

In Nature Cell Biology, she and her colleagues describe how they adapted the technique to work for human embryos donated by an in vitro fertilization (IVF) clinic.

The teams watched as the cells in the embryos began to differentiate — and reveal features that are unique to human development. For instance, Brivanlou and his colleagues have identified a group of cells that shows up in the embryo around day 10 and disappears around day 12.

The scientists don’t yet know the function of the cell cluster, which, at its peak, forms 5–10% of the embryo. But it seems to be a transient organ, akin to the tails that human embryos grow much later in development and then lose before birth. “This is like discovering a new organ in your body,” Brivanlou says.