This dream therapy is still years off, if it happens at all, but the first steps have already been taken by a team led by Esmail Zanjani at the University of Nevada, Reno. "Esmail has some pretty startling results," says Alan Flake of the Children's Hospital of Philadelphia.
Zanjani's team hopes the animal-human chimeras they are creating (see "A question of breeding") will one day yield new cells genetically identical to a patient's own for repairing damaged organs, and perhaps larger pieces for transplantation.
It might even be possible to transfer whole organs, since in some cases having at least a partly human organ would be better than a purely animal xenotransplant. Immune rejection of the animal portion would still be a problem, but it is not insurmountable, says Flake. "I don't think that in 10 to 15 years that's out of the question."
If perfected, the technique could overcome some of the big stumbling blocks facing researchers who want to make tissues and organs for implants. It might yield significant quantities of just about any kind of cell or tissue, for instance, with no need to fiddle about with different culture conditions or growth factors.
Instead, the host animal's own developmental program guides the injected human stem cells into their final roles. "We take advantage of the growing nature of the fetus," Zanjani says. It would also allow doctors to obtain immune-compatible cells without having to create human embryos by therapeutic cloning. Human cells could be separated from the animal ones simply by modifying existing cell-sorting machines.