The animal eggs don't reprogram human DNA in the right way to generate stem cells, researchers report.
"Instead of turning on the right genes, it turns out the animal eggs actually turn them off," said senior study author Dr. Robert Lanza of Advanced Cell Technology in Worcester, Mass.
Another scientist disputed that conclusion.
The idea of using animal-human "hybrid" embryos drew fire last year in Britain as authorities pondered whether to let scientists try it. Opponents objected to mixing human and animal material and worried that such research could lead to genetically modified babies.
Hybrid embryos have been made elsewhere, but there's no widely accepted report of getting stem cells from them. Animal eggs are attractive because human ones are hard to get for research.
Scientists prize embryonic stem cells because they can develop into virtually any cell of the body. By inserting a person's DNA into an egg and growing an early embryo, scientists hope to extract stem cells that are a genetic match to that person. That would help in disease research and enable scientists to develop transplant tissue that avoids the risk of rejection.
For the new work, Lanza and colleagues put human DNA into human, cow and rabbit eggs and grew them into early embryos. In embryos from human eggs, they found that patterns of gene activity resembled those in ordinary human embryos. But with the human-animal hybrid embryos, the patterns were much different.
The work was funded by Lanza's company and several institutions collaborating on the research. It was published online Monday by the journal Cloning and Stem Cells.
"The idea that this is the nail in the coffin for hybrids is grossly overstated," declared Stephen Minger of King's College London, who has permission from British authorities to pursue making hybrid embryos.
He cited a 2008 study that reported key genes are in fact turned on in embryos made with cow eggs, although the efficiency is low. That study concluded that cow eggs could be useful if techniques to use them were further developed.
Minger suggested Lanza's team looked too early in embryonic development to see the activation.
Lanza, however, said he found the 2008 study to be unconvincing.
Minger's group is one of three that gained permission from British authorities to try using animal eggs. He said he thought stem cells from hybrid embryos should not be used for making transplant tissue for people.