New research provides glimpse into early human evolution
Scientists were able to get a rare glimpse into a crucial early stage of human development by analyzing an embryo in its third week after fertilization – a moment in time that is difficult to study due to practical and ethical considerations.
European researchers looked at a single fetus between 16 and 19 days old, donated by a woman who had terminated her pregnancy. Experts said researchers so far lack a complete understanding of this stage of development because human embryos at this stage are difficult to obtain. Most women don’t yet know they’re pregnant at this point, and global guidelines for decades until recently banned growing human embryos in the lab for more than 14 days.
The study, published online Wednesday in the journal Nature, looked at “gastrulation,” which begins about 14 days after fertilization, when an embryo is still about the size of a poppy seed, and lasts just over a week.
“It’s a process where you have this kind of explosion in cell diversity,” said lead researcher Shankar Srinivas, an expert in evolutionary biology at the University of Oxford who worked with colleagues in the UK and Germany on the research. “During the gastrulation process, different cells appear, but they also begin to be localized in different places in the body composition so that they can do their jobs and form the right organs.”
For decades, the so-called “14-day rule” about developing embryos in the lab has guided researchers, with some places, including the UK, writing it into law. It has been accepted by others, including the United States, as a guiding standard for scholars and regulators.
Earlier this year, the International Society for Stem Cell Research recommended loosening the rule and allowing researchers to grow embryos within the past two weeks under limited conditions and after a difficult review process. But the rule remains law in the UK.
This research was not subject to law because the fetus was not grown in a laboratory. But it’s an example of the kinds of things scientists would expect to learn more about if the rules were relaxed. The researchers found different types of cells, including red blood cells and “primitive germ cells” that give rise to egg or sperm cells. But Srinivas said they did not see the neurons, which means that fetuses are not equipped at this point to sense their environment.
Oxford University officials said this stage of development has not been fully identified in humans before.
The authors said they hope their work will not only shed light on this stage of development, but also help scientists learn from nature about how to turn stem cells into specific types of cells that can be used to help treat damage or disease.
Being able to transplant human embryos after 14 days “would be very important for understanding not only how we develop normally but how things go wrong,” said Robin Lovell-Badge, a stem cell expert at the Francis Crick Institute in London who chaired the group behind the guidelines.
It is very common for fetuses to fail during gastrulation or shortly thereafter, he said. “If it goes wrong, you end up with birth defects, or a miscarriage.”
“Those of us who are morally conservative” have always thought the 14-day rule was somewhat arbitrary, said Dr. Daniel Solmacy, director of the Kennedy Institute of Ethics at Georgetown University, “but at least it was an acknowledgment of the humanity of the fetus.”
With the new recommendation, he said, there will be more research on older fetuses. “Part of what science does is always try to move forward and learn new things. And that is still stressful. But just the fact that we can do something is not enough to say that we have to do it.”
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