A Stanford University researcher has gotten a preliminary go-ahead to create a mouse with a significant number of human brain cells — as long as the creature behaves like a mouse, not a human.

A university ethics committee studied a provocative project that transplants human neurons into the brains of mice where, surprisingly, they settle in and feel right at home. The research team, led by Stanford biologist Irving Weissman, has no immediate plans to build a mouse with an entirely human brain. But it remains a theoretical possibility.

The federal government does not regulate the creation of such chimeras, named after the mythical Greek creature that is part-man, part-animal.

So Stanford asked where it should draw the line. It is the first university in the nation to tackle the philosophical question: When does a chimera stop being an animal and start becoming a person, suggesting that research should end? The report foreshadows the release of guidelines on stem-cell research, including chimeras, by the National Academy of Sciences this spring.

“We concluded that if we see any signs of human brain structures . . . or if the mouse shows human-like behaviors, like improved memory or problem-solving, it’s time to stop,” said law and genetics Professor Henry T. Greely, director of the Center for Law and the Biosciences and leader of the committee.

“We think if he takes appropriate caution — including stopping at each step along the way, to see what’s happening — the research is ethical,” he said.

The group has not yet published its final report but did issue recommendations to Weissman, who solicited the opinion about two years ago.

Weissman says he doesn’t want to build a smarter mouse. Instead, he is creating a furry test tube to learn more about devastating human diseases such as brain cancer, stroke, schizophrenia, Alzheimer’s and Lou Gehrig’s disease — impairments not easily studied in people.

The work, conducted at the university and the biotech company Stem Cells Inc. of Palo Alto, started several years ago and is gaining respect. In some cases, researchers are injecting sick or cancerous human cells into healthy mice. In others, they’re injecting healthy human cells into sick mice. What causes brain disease? How does it kill? And how can it be cured? These experiments might offer a way to find out.

In the future, it might be possible to replace injured regions of a mouse brain with healthy human cells, derived from fetuses. Scientists would watch to see if the human cells could talk to mouse cells, giving instructions to walk, for example. No one expects mice to take up ballet or break-dancing. But what is learned could have profound implications for disabled humans.

Even further in the future is the prospect of replacing an entire brain. The experiment would be done in a strain of mice with a rare disease that causes their brains to die before birth. Without their own brains, perhaps they would rely on transplanted human brain cells to take over.

Critics warn against crossing this scientific frontier. While other forms of animal-human fusion have been achieved — such as implanting the pancreas of a dog or the heart valve of a pig — the human brain is the seat of the soul in human consciousness, something uniquely human, they said. Others oppose the prospect, however remote, of creating animals with mental abilities like our own.

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