A new pair of studies have reported raising human embryos outside the womb for almost two weeks, longer than ever before. The studies, published today in Nature, raise questions about a long-standing rule that limits the lifespan of human embryos to two weeks on ethical grounds.
The researchers, from The Rockefeller University in New York and the University of Cambridge, were studying the changes that embryos go through as they move out of their earliest stage of development. In the study, they monitored the development of embryos past the seven-day mark — the period after which they’re supposed to implant themselves in the tissue of the uterus. Embryos are often raised as sources of stem cells, but developing them for longer can help researchers better understand pregnancy and human growth. When early pregnancies fail, for example, it’s often because the embryo wasn’t successfully implanted, and raising embryos in a controlled environment could help researchers figure out what’s going wrong during that crucial period.
Raising embryos outside the womb can teach us how they grow inside it
But so far, it’s been difficult to keep embryos growing in vitro after seven days, when they would ordinarily be implanted, and few studies have pushed past the one-week mark. In the studies, the researchers used a new method that was tested on mice in 2014. In this case, they applied it to donated human blastocysts — the small clusters of stem cells that form after an egg is fertilized. After seven days, the embryos kept growing on the surface of the petri dish they were raised in; after 12 to 13 days, they appeared to still be developing.
What might have happened next is a mystery. The samples were destroyed under an internationally approved policy developed over 30 years ago, barring research on human embryos outside the womb past 14 days. But in a commentary published alongside the studies, three supporters of human development research are urging scientists, ethicists, and policymakers alike to consider revisiting the limit and to reevaluate what it means. "The 14-day rule has been effective for permitting embryo research within strict constraints — partly because it has been technologically challenging to break it," write Insoo Hyun, bioethics professor at Case Western Reserve University School of Medicine; Amy Wilkerson, associate vice-president of research support at Rockefeller University; and Josephine Johnston of the bioethics-focused Hastings Center.
The 14-day rule is a long-standing compromise that’s been debated for the past few decades. Even recently, UK biologist Azim Surani also called for loosening the 14-day restriction. The idea is to place some moral limits on creating and destroying human embryos, while still letting scientists work with embryos in fields like in vitro fertilization and stem cell development, both of which can provide major benefits to fully-grown humans. Proposed in 1979 by the US Department of Health Education and Welfare’s Ethics Advisory Board, the rule gained acceptance through the 1980s and 1990s. Today, it’s law in a dozen countries. In many more, including the US, it’s an accepted scientific guideline. (The United States also places strict limits on government funding for any human embryo research.)
There's no 'bright line' for human life
The 14-day benchmark was proposed because, after about two weeks, human embryos begin to form the so-called primitive streak, a band of cells that work as a centerpoint for the embryos to start developing a symmetrical human body. To supporters of the rule, this means philosophically that the embryo is on its way to becoming one distinct person, no longer holding the possibility of splitting and becoming twins. Under the US National Academy of Sciences guidelines, for example, embryos can be raised outside the human body until either 14 days have passed or the primitive streak has begun to appear, whichever comes first.
But Insoo and his co-authors say the primitive streak is one of many possible limits, not a "bright line" where human life begins. The Catholic Church, for example, opposes both creating embryos through in vitro fertilization and destroying them in any state. On the other hand, it’s far earlier than most abortion laws set the limit — usually a matter of months.
Today’s commentary doesn’t weigh in on what a new rule might look like, and the authors admit the possibility of a backlash. Policymakers and the public could see the change as redrawing moral boundaries "when the limits become inconvenient for science," the authors warn, raising skepticism of any embryonic research. In order to prevent this, Insoo and his co-authors are pushing researchers and advisory boards to proactively talk about the importance of human embryo research, and to involve as many people as possible in reworking the rules. "Any decisions to revisit the 14-day rule should involve a wide swath of stakeholders, not a single entity," Insoo told The Verge. "The time limit for embryo research should be drawn where it will best promote valuable science while respecting as many different moral views as possible."