Human Embryo the Size of a Poppy Seed Offers Rare Glimpse Into Early Development

The donation of a miniscule human embryo the size of a poppy seed has given scientists an extremely rare glimpse into an early stage of human development that has long been difficult to study, the Associated Press reported. The embryo was in its third week after fertilization, a time period that practical and ethical concerns have made tricky to scrutinize.

A woman who ended her pregnancy donated a single embryo between 16 to 19 days old to European researchers. Most women aren't aware that they're pregnant so soon after conception, and global rules in place for decades until recently prohibited labs from growing human embryos past 14 days, the AP reported.

Because those factors make it challenging for scientists to gain access to embryos at this stage of development, the rare donation provided the researchers with a unique chance to examine it. A study published Wednesday by Nature detailed a process called "gastrulation," which begins roughly 14 days after fertilization and lasts a little over a week.

Shankar Srinivas, a developmental biology expert at the University of Oxford, was the lead investigator in the study, collaborating with researchers in the U.K. and Germany. He described gastrulation as "a process by which you have this kind of explosion of cell diversity."

"It's during gastrulation that the different cells emerge, but they also start to be positioned in different places in forming the body so that they can carry out their functions and form the correct organs," Srinivas said.

This developmental stage in humans had never been fully charted before, according to Oxford University officials.

For more reporting from the Associated Press, see below.

Donated Embryo Aids Researchers
Research released in the journal Nature provides a rare glimpse of an early stage of human development. This image from Oxford University shows a human embryo 16 to 19 days after fertilization. University of Oxford via AP

For decades, the so-called "14-day rule" on growing embryos in the lab has guided researchers, with some places, including the U.K., writing it into law. Others, including the U.S., have accepted it as a standard guiding scientists and regulators.

Earlier this year, the International Society for Stem Cell Research recommended relaxing the rule and allowing researchers to grow embryos past two weeks under limited circumstances and after a tough review process. But the rule remains law in the U.K.

This research was not subject to the law because the embryo wasn't grown in a lab. But it is an example of the types of things scientists expect to learn more about if rules are relaxed. Researchers found various types of cells, including red blood cells and "primordial germ cells" that give rise to egg or sperm cells. But they didn't see neurons, Srinivas said, meaning embryos aren't equipped at this stage to sense their environment.

The authors said they hope their work not only sheds light on this stage of development but also helps scientists learn from nature about how to make stem cells into particular types of cells that can be used to help heal damage or disease.

Robin Lovell-Badge, a stem cell expert at London's Francis Crick Institute who chaired the group behind the guidelines, said being able to culture human embryos beyond 14 days "would be incredibly important to understand not just how we develop normally but how things go wrong."

It's very common for embryos to fail during gastrulation or shortly afterwards, he said. "If things go even slightly wrong, you end up with congenital abnormalities, or the embryo miscarries."

Dr. Daniel Sulmasy, director of the Kennedy Institute of Ethics at Georgetown University, said "those of us who are morally conservative" always thought the 14-day rule was somewhat arbitrary, "but at least it was some recognition of the humanness of the embryo."

With the new recommendation, there will be more research on older embryos, he said. "Part of what science does is to always try to go forward and learn things that are new. And that continues to be a pressure. But the mere fact that we can do something is not sufficient to say that we ought to do it."

Oxford Researcher
Shankar Srinivas, a developmental biology expert at the University of Oxford, was the lead investigator in a study examining an early stage of human development that has long been difficult to scrutinize. Honorands and senior university members take part in the annual Encaenia ceremony at Oxford, west of London, on September 22, 2021. Tolga Akmen/AFP via Getty Images