There are few fields of research that have the potential to evoke such strong feeling as experimentation on human embryos. The use of human embryos for research is tightly controlled but recent advances have led to calls from some scientists for a loosening of current regulations.

Currently, experimentation on human embryos is governed by an international consensus which states embryos should not be allowed to develop in culture for more than 14 days following fertilisation, the so called “14 day rule”. Until recently limitations in our ability to grow embryos outside the womb have meant that the 14-day rule was not in danger of being broken; it was rare that embryos could be sustained in vitro for more than seven days. However, several recent studies have described new methods for embryo culture that have enabled researchers to culture them for as long as 12-13 days (1,2). Now some scientists are arguing for an extension to the 14-day rule, arguing that studying the later stages of embryonic development would increase our knowledge of human development and could ultimately lead to advances in treatments for early forms of miscarriage.

The 14-day limit was first proposed in 1979 by the Ethics Advisory Board of the US Department of Health, Education, and Welfare and in the UK the 1984 Warnock report agreed with this proposal, settling on a 14 day limit that remains in place to this day. Currently at least twelve countries have enshrined the 14-day limit in law, and in five others national scientific guidelines set the limit at 14 days of development.

One rational for this limitation is that 14 days is the first point at which an embryo can be said to have an individual identity. Before this point embryos can fuse and split, forming monozygotic twins. Another is that it is at this time that a structure known as the primitive streak is formed. This is a band of cells which mark the beginnings of a head-to-toe axis, and is therefore also the beginnings of the transition from a ball of cells towards something that will begin to resemble a foetus. The fact that the primitive streak can be easily identified also makes this time point convenient for ascertaining the extent to which an embryo kept in culture has developed, something which might not necessarily precisely correlate with the number of days the embryo has been kept in the dish.

But some scientists see the 14-day rule as somewhat of an arbitrary line in the sand. After all, the rule was never intended to make a moral distinction between the stage at which an embryo can be considered too human to experiment on. What it did do was to create a convenient boundary to allow space for vital research on human reproductive development whilst also making a concession to the legitimate concerns about experimentation on fertilised human embryos at late stages of development.

But an extension of the 14-day limit will require an international consensus. When, and based on what criteria, should the cut-off be set?  For example, should the cut-off be set based on the point at which the embryo develops a nervous system and can therefore be said to have the potential to feel pain?

Such questions are moral rather than scientific, and such choices are dependent on a number of considerations. For example, a consideration of the potential benefits to mankind of such research versus the desire to appease the moral or religious qualms of people that hold that such experimentation is wrong. Such a trade off is typified by the ongoing debate surrounding the use of animals in research.

But ultimately it will be up to the scientific community to make their case for the benefit for the extension of the 14-day rule and to suggest a suitable replacement. It will then be the role of policy makers, the media and society in general to decide for themselves if they deem such an extension morally acceptable.

  1. Deglincerti,A. et al.Nature http://dx.doi. org/10.1038/nature17948 (2016).
  2. Shahbazi, M. N. et al. Nature Cell Biol. http://dx.doi. org/10.1038/ncb3347 (2016).

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