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expert reaction to two papers studying human embryo development

Mouse models of embryo development have told us a great deal about the early stages of life, but until now attempts to model these stages using human embryos have been unable to take us beyond the first few days of development, past the stage where the embryo implants itself into the womb. Now, in parallel papers in Nature and Nature Cell Biology, two international teams report the development of a technique that allows scientists to culture human embryos further than ever before, up to day 13 of development, the limit allowed by law. These comments accompanied a briefing.


Professor Azim Surani, Director of Germline and Epigenomics Research, The Gurdon Institute, said:

“The two studies are important and show that it is possible to culture human embryos through to early postimplantation period. These will be useful for studies on early stages of extra embryonic development, including aspects of human embryo implantation. They reach maximum development that is equivalent to Carnegie Stage 5c (~day 11-12 post fertilisation). According to the Brivanlou paper, the embryos collapsed at this point and became disorganised and could not be interpreted with respect to in vivo  embryos. This however also marks the currently allowed limit for in vitro culture of human embryos; 14 days.

“There are two challenges looking ahead. First, the regulatory framework would need to be looked at so that the embryos can be cultured beyond the 14 day limit. Second, there might be technical challenges to getting the embryos go through gastrulation, that is when single layered epiblast develops into trilaminar structure consisting of ectoderm, mesoderm and endoderm; Carnegie stage 7 (~15-17), when primordial germ cells, precursors of sperm and eggs are also established (my own particular interest). To my knowledge, the equivalent stages of mouse development from blastocysts in vitro have not so far been reported.

“In my opinion, there has been a case to allow culture beyond 14 days even before these papers appeared. ”



Prof. Harry Moore, Professor of Reproductive Biology, University of Sheffield, said:

“These two papers are important milestones as the investigators have found a way of consistently culturing human embryos further than previously reached in the lab.

“Both studies clearly demonstrate the incredible intrinsic ability of the embryo to organize itself as it starts to create the body plan (even in the absence of the mother). The results also point to some important differences between human embryos and those of the main model species, the mouse. This shows how important it is to study human embryos directly.

“Up to now, this stage of human embryo development has been very much a ‘black box’ and we have had to rely mainly on histological samples obtained in the 1930 and 40s when cell preservation techniques were much more rudimentary. Having these early stages of embryo development in the lab where we can start to use all the latest technology of genetic analysis is a major step forward. I would anticipate that over the next few years, further improvements in 3-dimensional culture techniques will mimic the embryo implantation process ever more accurately.

“Being able to carry out this sort of research is crucial for understanding how the early stem cells of the embryo develop.

“This research provides the detailed information we need to derive effective stem cell therapies to treat degenerative diseases, as well as improving the treatment of conditions of pregnancy such as recurrent miscarriage and pre-eclampsia.”


Prof. Daniel Brison, Honorary Professor of Clinical Embryology and Stem Cell Biology and Scientific Director of the Department of Reproductive Medicine, University of Manchester, said:

“These two papers are highly significant as the culture system developed by the Cambridge group has allowed the study of early human embryo development in the laboratory much later than previously, to stages beyond the time of implantation into the womb. This has shown that the early human embryo is remarkably capable of self organising, even in an artificial laboratory system with no input from the mother. Some of the details appear to be different to animal models such as mouse, further strengthening the case for doing this research with human embryos. The culture system used may be capable of sustaining human embryo development in the laboratory beyond the current legal limit of 14 days after fertilisation. This limit was chosen more than 20 years ago as it was thought to represent the first point when individuality is assigned and twinning no longer possible, and carries strong support in the UK from patients and researchers.  However, given the potential benefits of new research in infertility, improving assisted conception methods, and in early miscarriage and disorders of pregnancy, there may be a case in the future to reconsider this.”


Prof. Allan Pacey, Professor of Andrology, University of Sheffield, said:

“These are two very elegant papers which have the potential to revolutionise our understanding of the early events of human embryo development and, in time, some of the reasons behind aspects of human disease and disability. The papers are also an example of what can be achieved in a country with strong primary legislation designed to regulate such research in a well thought out ethical framework.

“The framework in which this work has been carried out was first set out in the Warnock report published in 1984, which concluded that it was ethical to conduct research on human embryos until day 14 of their development (when the rudimentary nervous system begins to be developed). Parliament agreed with this recommendation when the 1991 Human Fertilisation and Embryology Act was passed. But, until now, this has been a theoretical legal restriction as no one, until today, had the technical means to keep embryos alive in the laboratory much beyond day 7.

“What the scientists have done in these work is use a very clever system to keep human embryos alive in the laboratory beyond day 7 by utilising methods of culture first tested with mouse embryos. This has allowed them to undertake an almost hour by hour series of observations of human embryo development to see how key parts of them are developed and organised within the first two weeks. This has already provided new information in these papers, but in my opinion it is the potential that the approach offers to future researchers which is the most exciting.

“There will no doubt be people who will be opposed to this kind of research and may disagree fundamentally with the idea that legitimate and ethical research on human embryos can take place for up to 14 days in the laboratory. Whilst I respect the strength of their views and their conviction, this is a framework which was agreed over 30 years ago and I see no reason to change or revisit that decision. It will not open the door to couples being able to grow babies in the laboratory; this is not the dawn of a Brave New World scenario. But it does open up exciting opportunities to understand the nature of human disease and disability and for that reason the scientists involved should be congratulated.”


Prof. Martin Johnson, Professor Of Reproductive Sciences, University of Cambridge, said:

“These highly significant papers both extend, and are based on, earlier work from the laboratory of Magda Zernicke-Goetz culturing mouse conceptuses over a more extended equivalent period in vitro.  This development has enabled the researchers to demonstrate that the human conceptus can develop autonomously outside the uterine environment and that it does so in ways that differ structurally, functionally and temporally from the mouse conceptus. Perhaps the most intriguing, and unexpected, findings were (i) the deleterious effects of culturing in 5% oxygen – given that in vivo this is the oxygen level experienced by the early conceptus, and (ii) the unique molecular expression patterns shown by the emergent cell populations.

“This is a unique achievement, that potentially challenges the 14 day rule, because to stop the culture at 14 days arbitrarily prevents study of the primitive streak development which stops us asking questions about the basis of twinning, the origins of spina bifida, and how the germ cells are set aside.

“The study design is essentially robust and the results do appear to support the bold claims made, but with one caveat: what is missing from both papers are the numbers of conceptuses that were started in culture and the proportion that were successfully cultured through to each stage?”


Prof. Peter Braude, Emeritus Professor of Obstetrics and Gynaecology, King’s College London, said:

“I am delighted that my colleagues King’s College at Guy’s Hospital have been able to facilitate in the collection of very beautiful and important data presented in the Nature Cell Biology paper. These two unique concurring papers demonstrate that good science still can be achieved within the limits set by the UK parliament of not keeping an embryo in vitro beyond 14 days.

“As far as I am aware, there has been no push from any scientists in this country to have extended, the 14 day limit as advised by the Warnock Enquiry, and set in legislation by the UK parliament. The limit was a negotiated decision based on factors that seemed acceptable to those who wished to continue with the embryo research already being conducted before the Act was considered, in order to further our understanding of reproduction and our early development, and those who were concerned about the morality of creating human embryos in vitro for research alone. The limit takes into account the fact that twinning does not occur after this point and the implications of that for individuality and the soul; and that the primitive streak stage is the first formation of the three primary germ layers that will give rise to tissues, and hence prior to this time there is no question of any form of sentience.

“Yes, it is possible that additional useful information about development might be gathered from examining later stages, but much of this can, and has already been gathered using stem cells, or from tissues grown from embryos disaggregated (separated into cells) at earlier stages than 14 days.”


Prof. Robin Lovell-Badge, Group Leader, The Francis Crick Institute, said:

“These two papers provide a first glimpse of how the early human embryo develops around and just after the point when it would usually implant in the lining of the womb and become invisible to observation and impossible to study experimentally. This is a period where many essential events occur that distinguish the region of the embryo that will go on to form the fetus in contrast to others that will contribute to the placenta and yolk sac. These latter structures not only help the fetus obtain nourishment from the mother, but they help direct how it is organized and patterned.

“The work confirms that aspects of early development in humans and mice are distinct, with respect to timing of events, of gene activity, cell types and their organization. But is also reveals that some mechanisms are similar, such as the way early cavities are formed by reorganization of cell-cell contacts rather than cell death, and that these stages are independent of the presence of maternal tissues.

“The methods reported here, in combination with genetic approaches, could yield many important details about this period of human embryo development, such as the nature of the genes required and whether and how these differ from the mouse, and what specifies uniquely human cell types. This in turn may help us understand the causes of embryo failure and to develop treatments for this.

“In both papers the human embryo cultures were stopped at 13 days after fertilization, respecting the widely accepted 14-day limit to experiments on intact human embryos. The process of gastrulation, where the embryo becomes organized into a basic body plan, begins shortly after this. We know little about this critical phase in human development. So it is pertinent to ask if the 14-day limit should be extended? This question does not come from these new experiments, nor does it come from several other relevant pieces of work carried out over the last few years. Indeed scientists and ethicists have debated the limit since it was first proposed. However, this debate has taken place in the absence of methods to allow extended periods of culture and there is little point arguing strongly for something that is not possible.  The new methods are not perfect and they would probably not support the embryos for much longer. But with parallel advances in tissue engineering it is possible to envisage how they could be modified to extend the period of development.  Proposing to extend the 14-day limit might be opening a can of worms, but would it lead to Pandora’s box, or a treasure chest of valuable information ? This is not a question to be left to scientists alone.”


* ‘Self-organization of the human embryo in the absence of maternal tissues’ by Shahbazi et al. published in Nature Cell Biology on Wednesday 4th May. 

‡ ‘Self-organization of the in vitro attached human embryo’ by Deglincerti et al. published in Nature on Wednesday 4th May. 


Declared interests

Prof. Azim Surani: “I have shared interest based on our work on the specification and development of human germ cell lineage but I don’t have any conflicts of interest.”

Prof. Harry Moore: “I have no conflicts of interests.”

Prof. Daniel Brison: None received

Prof. Allan Pacey: “Chairman of the advisory committee of the UK National External Quality Assurance Schemes in Andrology, Editor in Chief of Human Fertility and Trustee of the Progress Educational Trust (all unpaid). Also, recent work for the World Health Organisation, British Broadcasting Corporation, Purple Orchid Pharma (paid consultancy with all monies going to University of Sheffield). Co-applicant on a research grant from the Medical Research Council (ref: MR/M010473/1).”

Prof. Martin Johnson: “I am no longer in paid employment, neither do I hold any personal grant funding, but I am supported in some of my travel and research expenses by a Wellcome Trust grant to Sarah Franklin. I was a member of the HFEA for 6 years, I am an advisor to PET, and a member of ESHRE”

Prof. Peter Braude: None received

Prof. Robin Lovell-Badge: Robin Lovell-Badge is a colleague of one of the authors of the paper by Shahbazi et al, but has had no involvement in the research. He is also a member of the HFEA’s Scientific and Clinical Advances Advisory Group. However, he has no conflicts of interest.

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