BIRTH OF AN EMBRYO: The haploid nuclei from an egg and a sperm in the zygote following fertilization© PETIT FORMAT/SCIENCE SOURCE
Last May, to much fanfare, an international group of researchers published two papers describing a new in vitro system that had maintained human embryos in culture for 13 days.1,2 The experiments could have continued beyond two weeks, if not for the “14-day rule”—a widely recognized limit to how long scientists are permitted to maintain human embryos for research purposes. Bioethicists first proposed the rule, which was subsequently enshrined in the laws of several countries and as a guideline in the U.S., in 1979. Three and a half decades elapsed before the technology existed to keep embryos alive outside of a womb past the implantation stage, which typically occurs about a week after egg and sperm cells fuse. Now, the rule was finally coming into play.
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