In 1992 Snyder announced in print that his lab had removed stemlike cells from mouse brains and had grown them in a culture. Snyder then teamed up with Dr. Jeff Macklis, a colleague at Harvard Medical School who had engineered a strain of mouse whose neurons died off in a tiny region of the cortex where cells were not known to regenerate. Snyder injected the stem cells into the mice. Like heat-seeking missiles, the cells rapidly sought out the injured part of the cortex and transformed themselves into healthy neurons. "That's the beauty of stem cells," says Snyder. "You don't have to find the injury--the stem cells do it for you. They instinctively home in on the damage even from great distances." In another experiment, Snyder used stem cells to cure mice of a disease that resembled multiple sclerosis. And in his latest, unpublished work, Snyder introduced massive brain injuries in mice--including strokes to the cortex--and cured them with stem cells.

"Where was this all leading?" Snyder says he asked himself many times. "In 20 years would I have done nothing more than create a thriving colony of healthy, smart mice that are free of brain disease? You can't take it for granted that every medical advance in mice will also benefit people." But the evidence started mounting. Over the past three years, researchers have discovered that brain cells regenerate in primate-like tree shrews, marmoset monkeys and rhesus monkeys, all of which are closer to us on the evolutionary scale than are mice (except in Kansas). The real payoff came late last year, when Fred Gage at the Salk Institute and his colleagues in Sweden reported that nerve cells are regenerated in the human hippocampus (a portion of the brain related to memory and learning).

Gage's finding--coupled with Snyder's report that same month of stem cells in the fetal human brain--has stood neuroscience on its head, so to speak. As has the latest finding, announced last month by researchers at Princeton, that adult macaque monkeys are constantly growing new cells in the highest and most complex area of the brain, the cerebral cortex. Snyder is now flush with confidence that neuroscience will ultimately cure many, if not all, diseases of the human brain. "By the year 2020 I hope we will have an active way of treating damaged brains. If we can further understand brain-cell regeneration and harness the process intelligently, then re-creating the brain, or at least parts of the brain, may lie within our grasp. Obviously there are lots of hurdles to overcome. But if we can capture and bottle the brain's now recognized plasticity, we can cure all sorts of things, maybe even damaged psyches."

The idea of implanting brain stem cells, while not as dramatic as swapping whole brains, also raises intriguing philosophical questions. "Sometimes at seminars when I talk about my work," says Snyder, "somebody will ask me whether the introduction of these stem cells will alter memory." Do the newly generated cells distort or erase old memories? Or will the transplanted stem cells bring with them memories of their upbringing in a Petri dish?

"All this is meta-neuroscience," says Snyder, laughing. "But I tend to think that the cells will take their cue from the host that houses them" rather than remembering their past lives like so many cellular Shirley MacLaines. So, in the case of brain-cell implants, it would seem, it is better to be the recipient than the donor.

Paul Hoffmann is president of Encyclopaedia Britannica and author, most recently, of "The Man Who Loved Only Numbers"

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