To be clear, for those that believe life begins at conception, "embryo development was stopped" means a life was ended.
"It opens up a new area of research", says Dietrich Egli, a Columbia University biologist who studies stem cells and was not involved in the study. "Many embryos arrest in culture, or fail to continue developing after implantation; this research will significantly help treatment for infertile couples, by helping us to identify the factors that are essential for ensuring that human embryos can develop into healthy babies".
Niakan's team chose to use it to stop a key gene from producing a protein called OCT4, which normally becomes active in the first few days of human embryo development.
The researchers said their experiments, using a technology that is the subject of fierce global debate because of fears that it could be used to create babies to order, will deepen understanding of the biology of early human development.
A handful of "special" cells in very immature embryos showed high Oct4 activity, the scientists found.
"Other research methods, including studies in mice, suggested a later and more focused role for OCT4, so our results highlight the need for human embryo research", says study author Norah Fogarty, in the Francis Crick Institute press release. To learn about its role in human embryonic development, the researchers used CRISPR-Cas9 to inactivate POU5F1, the gene encoding OCT4, in about 40 embryos.
This is not the first time researchers have edited human genomes in embryos.
The central question we are addressing is what are the molecular mechanisms that regulate embryonic stem cell pluripotency and how is it disengaged during cellular differentiation?
Dr Kathy Niakan led the research at the Francis Crick Institute in London.
"That's not predicted anywhere in the literature", Niakan says. The researchers plan further work with CRISPR to find out exactly what genes OCT4 controls in the different cell types. This summer, researchers at Oregon Health and Science University fertilized eggs using sperm carrying a genetic defect, then deployed CRISPR to correct it, as The Scientist reported, following on 2015 work using CRISPR in embryos by a research team in China.
Dr Ludovic Vallier, study co-author from the Wellcome Trust Sanger Institute and the Wellcome - MRC Cambridge Stem Cell Institute, says: "This study represents an important step in understanding human embryonic development". "We would have never gained this insight had we not really studied the function of this gene in human embryos". And in 2016, the British government approved editing of human embryos for research purposes.
Gene editing in the United States is only permitted if there are no other options and only for inheritable diseases, according to a committee formed by the National Academy of Sciences and the National Academy of Medicine. That has critics like Darnovsky anxious. "We don't want to add ideas that some people are biologically better and some people are biologically inferior to others".
The team spent over a year optimising their techniques using mouse embryos and human embryonic stem cells before starting work on human embryos.
Marcy Darnovsky, the head of the Center for Genetics and Society, a genetics watchdog group, told NPR, "The concerns are that we would be opening the door to fertility clinics vying to offer gene-editing to make future children taller or stronger or whatever they wanted to market".