Mice created with human brain cells
(AP)
Updated: 2005-12-13 14:33

"It's a way for us to begin to tease out the way these diseases develop," Gage said.

Human embryonic stem cells are created in the first days after conception and give rise to all the organs and tissues in the human body. Scientists hope they can someday use stem cells to replace diseased tissue. But many social conservatives, including President Bush, oppose the work because embryos are destroyed during research.

Stem cell researchers argue that mixing human and animal cells is the only way to advance the field because it's far too risky to experiment on people; so little is known about stem cells.

"The experiments have to be done, which does mean human cells into non-human cells," said Dr. Evan Snyder, a stem cell researcher at the Burnham Institute in San Diego. "You don't work out the issues on your child or your grandmother. You want to work this out in an animal first."

Snyder is injecting human embryonic stem cells into monkeys and is convinced that there's little danger.

"It's true that there is a huge amount of similarity, but the difference are huge," Snyder said. "You will never ever have a little human trapped inside a mouse or monkey's body."