Scientists in Britain have applied for permission to create human-animal hybrid embryos by injecting human DNA into cows' eggs for stem cell research.

The scientists said the hybrid human-bovine embryos could prove useful in pursuing treatments to prevent Parkinson's disease and Alzheimer's, as well as spinal cord injuries, diabetes and arthritis.

But the proposal has appalled critics of stem cell research, who slam the idea of creating so-called "chimera" embryos as an unethical and potentially dangerous practice.

Researchers from Newcastle University and King's College London submitted the application to the Human Fertilization and Embryology Authority (HFEA), an independent regulatory body that oversees embryo research and fertility treatment in Britain.

Instead of using human eggs, the researchers will remove the nuclei from cows' eggs and replace them with cells from the patients to create cloned stem cell lines that contain the same genetic mutation that results in these neurological disorders.

"We feel that the development of disease-specific human embryonic stem cell lines from individuals suffering from genetic forms of neurodegenerative disorders will stimulate both basic research and the development of new treatments for devastating brain diseases," Dr. Stephen Minger, of the stem cell biology laboratory at King's College London, said Monday in a release.

Minger cited the short supply of human eggs left over from in vitro fertilization, which have been used for stem cell research but have to be acquired through surgery on the women.

Embryos '99.9 per cent human'

If the application were approved, the hybrid embryo would be 99.9 per cent human. The only bovine element would be found in DNA outside the nucleus of the cell.

Moira McQueen, the director of the Canadian Catholic Bioethics Institute and a professor of theology at the University of Toronto, said the request is an "unnecessary" step beyond the use of embryos in practices such as in vitro fertilization.

"It adds another layer of complexity and I think an unnecessary one," McQueen, who has written and co-authored several articles on bioethics, told CBC.ca on Tuesday. "Now, we're talking about something where people are just experimenting almost to see what happens.

"This is just another action that, to me, seems unreasonable and pointless. I'm not exactly sure why they're doing it."

The scientists said the hybrid embryos would not be allowed to develop for more than a few days. But McQueen said the embryos' destruction "presents another problem from the Catholic perspective, since destroying it is equally wrong."

Worldwide debate

The request comes as Australia's Senate narrowly voted Tuesday to lift the country's ban on cloning human embryos for stem cell research.

Opponents, including Australia's Health Minister Tony Abbott, warned that therapeutic cloning is open to abuse and could create animal-human hybrids.

Stem cell research has also been a central issue leading up to Tuesday's U.S. mid-term elections, following controversy over actor Michael J. Fox appearing in a campaign ad for a pro-stem cell research candidate in Missouri.

Conservative radio commentator Rush Limbaugh mocked Fox, who suffers from Parkinson's, and suggested the actor was embellishing symptoms of his disease in the ad. Limbaugh later apologized.

With files from the Associated Press