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Chimeras in research
In biological research, chimeras are artificially produced by physically mixing cells from two different organisms. Chimeras are not hybrids, which form from the fusion of gametes from two species (like a donkey and a horse) that form a single zygote that will develop as much as it can (in this case into a live mule if the parents are jackass and mare, or a hinny if the parents are stallion and jenney); in comparison, chimeras are the physical mixing of cells from two independent zygotes: for example, one from the donkey and one from the horse. "Chimera" is a broad term and is often applied to many different types of mixing of cells from two different species.
Some chimeras can result in the eventual development of an adult animal composed of cells from both donors, which may be of different species — for example, in 1984 a chimeric geep was produced by combining embryos from a goat and a sheep. The "geep" has been a very important contributor to answering fundamental questions about development, and the techniques used to create it may one day help save endangered species. For example, if one tried to let a goat embryo gestate in a sheep the sheep's immune system would reject the developing goat embryo; however, if one used a geep that shared markers of immunity with both sheep and goats, the goat embryo may survive. It may be possible to extend this practice for the purpose of preventing the extinction of some endangered animal species.
Such interspecies chimeras such as the "geep" are made in the laboratory and rarely with the purpose of generating living hybrid animals. In addition to the famous geep, there are rat/mouse chimeras and a rabbit/human chimera which was destroyed within a few days for the purpose of harvesting stem cells. Intraspecies chimeras are created by transplanting embryonic cells from an animal with one trait into an embryo of an animal with a different trait. This practice is common in the field of embryology and has been a very important contributor to our current understanding of human and animal biology. For example, by mixing embryonic cells of differently coloured or otherwise genetically distinct mice (of the same species), researchers have been able to see how embryos form and which organs and tissues are related (arise from the similar cell lineages).
Hybridomas are not true chimeras as described above because they do not result from the mixture of two cell types but result from fusion of two species' cells into a single cell and artificial propagation of this cell in the laboratory. Hybridomas have been very important tools in biomedical research for decades.
In August 2003, researchers at the Shanghai Second Medical University in China reported that they had successfully fused human skin cells and dead rabbit eggs to create the first human chimeric embryos. The embryos were allowed to develop for several days in a laboratory setting, then destroyed to harvest the resulting stem cells. Because of the high therapeutic potential of human embryonic stem cells and the American moratorium on using discarded embryos from in vitro fertilization clinics as well as other concerns about using human embryos directly for research, scientists are trying to find ways to find alternative paths of research. However, increasingly realizable projects using part-human, part-animal chimeras as living factories not only for biopharmaceutical production but also for producing cells or organs (see hybridomas) for xenotransplantation raise a host of ethical and safety issues.
During November 2006, UK researchers from Newcastle University and King's College London applied to the Human Fertilisation and Embryology Authority for a three-year license to fuse human DNA with cow eggs. The proposal is to insert human DNA into a cow's egg which has had its genetic material removed and then create an embryo by the same technique that produced Dolly the Sheep. The resulting embryo would be 99.9% human; the only bovine element would be DNA outside the nucleus of the cell. This research was attempted in the United States several years before and failed to yield such an embryo. In April 2008 the researchers from Newcastle University reported that their research had been successful. The resulting embryos lived for 3 days and the largest grew to a size of 32 cells. The researchers are aiming for embryos that live for 6 days so that embryonic stem cells can be harvested.
In 2007, scientists at the University of Nevada School of Medicine created a sheep that has 15% human cells and 85% animal cells.