A team of scientists from the Salk Institute in the United States created a stir last week with the announcement that they had created hybrid human-pig foetuses.
The intention is not to create new and bizarre creatures. Chimeras are incredibly useful for understanding how animals grow and develop. They might one day be used to grow life-saving organs that can be transplanted into humans.
The actual proportion of human cells in the chimeras was quite low and their presence appeared to interfere with development. Even so, the study represents a first step in a new avenue of stem cell research which has great promise. But it also raises serious ethical concerns.
A chimera is an organism containing cells from two or more individuals and they do occur in nature, albeit rarely.
Marmoset monkeys often display chimerism in their blood and other tissues as a result of transfer of cells between twins while still in the womb. Following a successful bone marrow transplantation to treat leukaemia, patients have cells in their bone marrow from the donor as well as themselves.
Chimeras can be generated artificially in the laboratory through combining the cells from early embryos of the same or different species. The creation of chimeric mice has been essential for research in developmental biology, genetics, physiology and pathology.