U.S. scientists have produced laboratory mice which have one fourth of their brains made up of human brain cells. They now want to create mice with brains almost entirely made up of human cells, but some are worried about the ethics of creating such a “frankenmouse.”

“We are not recreating a human brain. We’re really just trying to understand how these stem cells can function, and how they can be used in the treatment of specific diseases,” says Ann Tsukamoto of StemCells Inc. in California.

But Irving Weissman, a Stanford University professor working on the project, says there should be an ethical review before more research is done. “You would want to ask the ethicist what percentage of the brain would be human cells before you start worrying, and if you start worrying, what would youstart worrying about,” he says.

These mice are being developed to test treatments for human brain diseases such as Parkinsons, Alzheimer’s and stroke. Analysis of the mice showed human neurons performing a full range of specialized functions in the main regions of the brain. Ann Tsukamoto says, “What we need to establish next is what functional connections they are making. Are there human-to-human or human-to-mouse connections?”

Weissman says the next step would be to create a strain of mice whose own brain cells die as the human cells take over. But he feels that, “a lot of people would have misgivings. They would fear we had created a miniature human brain where there should not be one.” However, researchers say that even a mouse whose head was entirely filled with human cells would have the brain size and overall anatomy of a mouse, and would still behave like a mouse, rather than a man.

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