HFEA approves in principle cytoplasmic hybrid research

11 September 2007

On the 5th of September the Human Fertilisation and Embryology Authority (HFEA) approved in principle the use of animal eggs in the creation of cytoplasmic hybrid embryos for stem cell research. Contrary to press headlines implying blanket support for true animal-human hybrids and chimeras, the decision pertained solely to research involving hybrid embryos cloned by cytoplasmic nuclear replacement (CNR) – the insertion of human nuclear material into an enucleated cow or rabbit egg. Unlike true hybrids, which obtain 50% of their genetic material from each parent, more than 99% of the DNA in cytoplasmic hybrids comes from the donated human nucleus, the remaining tiny fraction being extra-nuclear animal DNA.

Despite the presence of mitochondrial animal DNA, a cytoplasmic hybrid, which contains a complete human genome, is considered to be a ‘live human embryo’ within the remit of the Authority under section 1 of the Human Fertilisation and Embryology Act. All embryo research requires a licence, and last week’s decision was prompted by two 2006 applications which sought to resolve difficulties associated with obtaining human eggs for research by using animal eggs. The HFEA decision did not address the outstanding licensing applications, but decisions are expected in November.

Following a comprehensive consultation on the subject, the HFEA decided that there was no fundamental reason to prevent cytoplasmic hybrid research which is, ‘in principle, necessary and desirable in both scientific and ethical terms’.  It acknowledged that public opinion is finely divided and that people are generally opposed to this research unless it is tightly regulated and it is likely to lead to scientific or medical advancements. Those opposed to human embryo research argue that it is no less unethical to create partly human embryos solely to harvest their stem cells, while others express concern about the potential for gestation of grotesque inter-species hybrids. The British Medical Association and major science funders, the Medical Research Council and the Wellcome Trust pressed for a favourable ruling.


The HFEA underscores that applications must be approached with caution and careful scrutiny.  Individual research teams undertaking projects involving the creation of cytoplasmic hybrid embryos are required to demonstrate, to the satisfaction of an HFEA licence committee, that their proposed research is necessary, desirable, and meets the HFEA standards for embryo research.  Importantly, embryos must be destroyed within 14 days, and are under no circumstances to be transferred to a woman.

The Authority rejected the notion of making a decision on broader hybrid and chimera research without an adequate evidence base, but will continue to monitor the potential for this wider research and any emerging evidence through its ‘horizon scanning’ programme.

For more details, see HFEA meeting documentation, and further information about the consultation.

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