A chimeric organism is one which contains genes from a foreign species. The genetic basis for desired traits are identified and then recombined into another organism, adding novel characteristics in a useful way. Today there are a number of techniques to accomplish this mixing of genomes. Chimeric organisms are extremely useful in genetic and medical research, and are quite widespread. Commercialized chimeras are also increasingly common. Some of these could have tremendous value. To given just example, one chimeric organism - a combination of the common cold and the polio virus - has shown great promise in curing brain cancer. Similar combinations with HIV show promise for other diseases. This is very mature technology and the required expertise can be found throughout the world.
Although it may not seem particularly wise to combine lethal pathogens with the common cold, a sufferer of some terminal condition which might be cured by a chimera might see the situation a bit differently. In any event, it is certainly the case that the scientific community takes great precautions with this research. Everyone is aware of the danger and these researchers have the best of intentions.
But, as you may have noticed on occasion, not everyone in the world has the best of intentions. This is certainly the case for those who are in the professions of biological warfare or biological terrorism. (Although, even here, many of those involved believe they have the best of intentions. Such is the human condition).
In zoology, a chimera is an animal which has (at least) two different populations of cells, which are genetically distinct and which originated in different zygotes (fertilised eggs). Chimeras are named after the mythological creature Chimera.
Chimerism may occur naturally during pregnancy, when two non-identical twins combine in the womb, at a very early stage of development, to form a single organism. Such an organism is called a tetragametic chimera as it is formed from four gametes—two eggs and two sperm. As the organism develops, the resulting chimera can come to possess organs that have different sets of chromosomes. For example, the chimera may have a liver composed of cells with one set of chromosones and have a kidney composed of cells with a second set of chromosomes. This has occurred in humans, though it is considered extremely rare, but since it can only be detected through DNA testing, which in itself is rare, it may be more common than currently believed. As of 2003, there were about 30 human cases in the literature, according to New Scientist.
In biological research, chimeras are artificially produced by mixing cells from two different organisms. This 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. A chicken with a quail's brain has been produced by grafting portions of a quail embryo into a chicken embryo.
In August 2003, researchers at the Shanghai Second Medical University in China reported that they had successfully fused human skin cells and 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.
Chimeras should not be confused with hybrids, which are organisms formed from two gametes (each from a different species) which formed a single zygote. All cells in a hybrid originate from this single zygote. For example, a mule is a hybrid created from the sperm of a donkey and the egg of a horse.
Chimeras should also not be confused with mosaics, which are organisms with genetically different cell types, but which again originate from a single zygote.
Now scientists create a sheep that's 15% human
By CLAUDIA JOSEPH - More by this author » Last updated at 21:26pm on 24th March 2007
Scientists have created the world's first human-sheep chimera - which has the body of a sheep and half-human organs.
The sheep have 15 per cent human cells and 85 per cent animal cells - and their evolution brings the prospect of animal organs being transplanted into humans one step closer.
Professor Esmail Zanjani, of the University of Nevada, has spent seven years and £5million perfecting the technique, which involves injecting adult human cells into a sheep's foetus.
Chimera: sheep have 15 per cent human cells and 85 per cent animal cells
He has already created a sheep liver which has a large proportion of human cells and eventually hopes to precisely match a sheep to a transplant patient, using their own stem cells to create their own flock of sheep.
The process would involve extracting stem cells from the donor's bone marrow and injecting them into the peritoneum of a sheep's foetus. When the lamb is born, two months later, it would have a liver, heart, lungs and brain that are partly human and available for transplant.
"We would take a couple of ounces of bone marrow cells from the patient,' said Prof Zanjani, whose work is highlighted in a Channel 4 programme tomorrow.
"We would isolate the stem cells from them, inject them into the peritoneum of these animals and then these cells would get distributed throughout the metabolic system into the circulatory system of all the organs in the body. The two ounces of stem cell or bone marrow cell we get would provide enough stem cells to do about ten foetuses. So you don't just have one organ for transplant purposes, you have many available in case the first one fails."
At present 7,168 patients are waiting for an organ transplant in Britain alone, and two thirds of them are expected to die before an organ becomes available.
Scientists at King's College, London, and the North East Stem Cell Institute in Newcastle have now applied to the HFEA, the Government's fertility watchdog, for permission to start work on the chimeras.
But the development is likely to revive criticisms about scientists playing God, with the possibility of silent viruses, which are harmless in animals, being introduced into the human race.
Dr Patrick Dixon, an international lecturer on biological trends, warned: "Many silent viruses could create a biological nightmare in humans. Mutant animal viruses are a real threat, as we have seen with HIV."
Animal rights activists fear that if the cells get mixed together, they could end up with cellular fusion, creating a hybrid which would have the features and characteristics of both man and sheep. But Prof Zanjani said: "Transplanting the cells into foetal sheep at this early stage does not result in fusion at all."
Animal Farm is on Channel 4 at 9pm tomorrow