As supporters of human-embryo research, we are troubled by the recommendations from the International Society for Stem Cell Research (ISSCR) to allow some in vitro studies of human embryo-like entities beyond the 14-day limit (see go.nature.com/3gfkkw8 and Nature 593, 479; 2021).
There are 4 compelling reasons for the 14-day limit. Its clarity leaves little room for misinterpretation. It corresponds to important biological events, including the beginning of ectoderm/neural progenitors. In marking the end of the possibilities of twinning or chimaerism, it is the start of a unique biological identity. There is no later relevant nexus of events.
The guidelines recommend instead case-by-case approval for integrated embryos that are based on stem cells. This would permit research up to the “minimum time necessary” for scientific questions deemed “highly meritorious” through a “rigorous review process”.
We caution that these utilitarian objectives are limitless. Furthermore, the questions deemed “highly meritorious” can be addressed using in vivo murine or non-human primate models.
The ISSCR must offer more-compelling arguments for abandoning the 14-day limit.
Nature 594, 333 (2021)
The authors declare no competing interests.