Pushing the limit

In 2006, Magdalena Zernicka-Goetz was pregnant, unexpectedly, at age 42. A genetic test of the fetal portion of the placenta showed that roughly a third of the cells carried a serious abnormality: an extra copy of chromosome 2. The University of Cambridge in the United Kingdom professor9s specialty, developmental biology, was suddenly personal. Nature could hardly have come up with a more vivid irony. Zernicka-Goetz is not only a developmental biologist; she has spent more than a decade working in mice to pinpoint when and how cells in an early embryo start to differentiate. Her child9s health depended in part on a question that had already captivated her as a scientist, she recalls: How much flexibility does the developing embryo retain? While still pregnant and uncertain about her baby9s fate, Zernicka-Goetz started mouse work that led to a paper this March revealing new details about embryos9 remarkable ability to cope with faulty cells. In May, she caught the attention of other embryologists and the wider world with another demonstration of embryos9 resilience. This time her lab showed—in concert with researchers in New York City—that human embryos created by in vitro fertilization could be kept developing in the lab for nearly 2 weeks, well past the point at which they would normally implant in the wall of the uterus. That9s almost twice as long as previously achieved, and it opens the way to studying a key phase of human development that had been inaccessible. The feat has already sparked new ethical debates about limits to embryo research. Widely accepted international guidelines dictate that human embryos should not be allowed to develop longer than 14 days in culture, but some researchers are now pushing for that limit to be reconsidered.