A chimera or chimaera is a single organism (usually an animal) that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic. Chimeras are formed from four parent cells (two fertilized eggs or early embryos fused together). Each population of cells keeps its own character and the resulting organism is a mixture of tissues. Chimeras are typically seen in animals; there are some reports on human chimerism. Plant chimeras are discussed separately.
This condition is either inherited, or it is acquired through the infusion of allogeneic hematopoietic cells during transplantation or transfusion. In nonidentical twins, chimerism occurs by means of blood-vessel anastomoses. The likelihood of offspring being a chimera is increased if it is created via in vitro fertilization. Chimeras can often breed, but the fertility and type of offspring depends on which cell line gave rise to the ovaries or testes; varying degrees of intersexuality may result if one set of cells is genetically female and another genetically male.
The US and Western Europe have strict codes of ethics and regulations in place that expressly forbid certain subsets of experimentation using human cells, though there is a vast difference in the regulatory framework. In May 2008, a robust debate in the House of Commons of the United Kingdom on the ethics of creating chimeras with human stem cells led to the decision that embryos would be allowed to be made in laboratories given that they would be destroyed within the first 14 days were given. No such foundation has been set for chimera research regulation in the US.
April 2, 2008
We have created human-animal embryos already, say British team
A team at Newcastle University announced yesterday that it had successfully generated “admixed embryos” by adding human DNA to empty cow eggs in the first experiment of its kind in Britain.
The Commons is to debate the Human Fertilisation and Embryology Bill next month. MPs have been promised a free vote on clauses in the legislation that would permit admixed embryos. But their creation is already allowed, subject to the granting of a licence from the Human Fertilisation and Embryology Authority (HFEA).
The Newcastle group, led by Lyle Armstrong, was awarded one of the first two licences in January. The other went to a team at King’s College London, led by Professor Stephen Minger. The new Bill will formalise their legal status if it is passed by Parliament.
Admixed embryos are widely supported by scientists and patient groups as they provide an opportunity to produce powerful stem-cell models for investigating diseases such as Parkinson’s and diabetes, and for developing new drugs.
It is already illegal to culture human-animal embryos for more than 14 days, or to implant them in the womb of a woman or animal, and these prohibitions will remain in the new legislation.