The main current debate over the Human Fertilisation and Embryology Bill (HFE Bill) has concerned a particular category of hybrid embryos: so-called cytoplasmic hybrids. The main reason for this is that in 2006 two groups of scientists submitted proposals to the Human Fertilisation and Embryology Authority (HFEA) to start creating such embryos, and one group of scientists, based in Newcastle are now doing this.
In addition to cytoplasmic hybrid embryos, there are a number of other theoretical possibilities for creating hybrid between humans and animals, which create mixing of human and animal material at different levels, ranging from the sub-chromosomal level (genetic modification) to the mixing of different types of cells in the embryo (chimeras). Apart from the cytoplasmic hybrids, there are no current proposals by scientists to create any other type of hybrid embryo, and in most cases no scientific groups have even been able to come up with potential uses for these embryos in research. Nonetheless, once the science lobby had forced the government to retract its initial proposal to ban hybrid embryos, it has succeeded in inserting the possibility of creating these other types of embryos in the future, into the legislation.
This brief guide is intended to explain the differences between the different types of embryo manipulation.
1 Cytoplasmic hybrid embryos
The creation of this type of embryo uses the same technology used in creating cloned animals. The DNA is removed from an animal egg by sucking out its nucleus with a pipette. This egg is then fused with a human skin cell, so that it now contains human nuclear DNA. The embryo is then stimulated to start growing and divide. This type of embryo has human DNA in its nucleus, but the vast bulk of the cell comes from the animal egg, including animal mitochondria, which are subcellular bodies containing DNA, which are responsible for creating energy in the cell. As the embryo grows, proteins and other molecules in the egg cytoplasm reprogram the human DNA back to an embryonic state, so that the human DNA then begins to create direct the production of human proteins, which gradually take over from the animal proteins in the embryo. However, this process is very unlikely to be complete until a stage in embryo development later than that at which stem cells are hoped to be extracted from the embryos. Therefore these embryos are subcellular hybrids, and are not as has often been stated, 'basically' human.
2 'True hybrids'
True hybrids are created by mixing sperm and egg from different species. This can occur naturally between closely related species, the most well-known example being mules, which are hybrids between horses and donkeys. Although this can occur naturally, it is very rare, because the main point of their being different species in nature is to prevent interbreeding. It is thus very unlikely that hybrid embryos using human sperm and animal eggs could produce a living animal although it might be possible between humans and chimpanzees.
3 Genetically modified embryos
Another way of creating hybrid at the genetic level is through genetic modification, in which a few genes are inserted into the chromosomes of an embryo. There are already many thousands of genetically modified (GM) mice and rats and farm animals that have been created using this technique. The genes that are inserted may come from any other species. For example, a human embryo could be genetically modified with DNA from bacteria, plants, animals or humans. In the HFE Bill, only human embryos genetically modified with animal genes are included in the category of 'human admixed embryos', which is the terminology the government is now using for human animal hybrid embryos (see below). However, the Bill also legalises the creation of GM human embryos with genes from any species (HGA legal briefing on the HFE Bill)
Once the foreign DNA has been inserted into the chromosomes of the embryo, the embryo can copy them every time the cells divide, so that they are present in every cell of the embryo, and unlike hybrids, there is no reason why these embryos cannot, in principle, develop into live animals (i.e. human babies). This is the only category of hybrid embryo where there are clear plans by scientists and Government to implant such embryos in order to create live animals (ie a GM human baby), and so the issues that they raise go beyond those raised by those raised by embryo research (See Stop GM embryos page).
A chimera is created by mixing cells of different species, for example by injecting human cells into an animal embryo, or vice versa. The name chimera comes from Greek mythology, in which there were many types of mixed species creatures, normally seen as dangerous monsters. Scientists have been creating different types of chimeras for more than ten years, although these are usually created by mixing cells at a stage much later than the embryo eg by putting human cells into animal foetuses or animals. Although the HFE Bill would require such embryos to be destroyed after fourteen days, in the USA some scientists are hoping to do experiments which would involve implanting such embryos into an animals womb, and possibly even allowing the chimeraic animal to be born. This raises many issues which we cannot deal with properly here.
The terminology, 'human admixed embryos'
During the passage of the HFE Bill through the House of Lords, the Government has changed the terminology to describe hybrid embryos, 'human admixed embryos'. The aim of this terminology is to emphasise the humanness of the embryos, since the legislation deals only with human embryos - animal material is dealt with in other legislation. It is also clear that the government and supporters of the creation of hybrid embryos want to allay public concerns, by emphasising that the animal element is only very minor. As may be seen from the above description however, this new terminology is far from scientifically accurate. True hybrid embryos are absolutely 50% human, 50% animal genetically and cytoplasmic hybrid embryos, although largely human genetically will be predominantly animal in composition and will only become humanized at later stages of their development. The mixture of human and animal in chimera is is unspecified and might be anything from 1% human/99% animal to 1% animal/99% human. The only types of hybrid embryo in which it is reasonable to call human are genetically modified human embryos, which will only have a few animal genes, and as we know from experience with genetically modified mice for example, that the addition of a few genes still results in something that is very clearly a mouse.
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