Illegal immigration. Neurons in a forebrain from a 20-week-old human fetus travel from telencephalon structures (yellow) to the dorsal thalamus.

New Route to Big Brains

Never mind the bipedal posture, relative lack of fur, or opposable thumbs. What really sets humans apart from other animals is our supersized brain. But how do we manage to build such big brains? A new report shows that in humans--but apparently not other species--some neurons in the developing brain take a detour that allows them to support the most overgrown and recently evolved parts of the cerebral cortex.

During fetal development, the nervous system is segregated into several regions of cells. The so-called telencephalon gives rise to the most sophisticated parts of the brain--including the frontal lobes and other so-called association areas that do the heavy lifting when it comes to problem solving, social interactions, and memory. These cells normally don't mingle with neurons from the diencephalon, which gives rise to less advanced structures such as the hypothalamus and optic nerves. No other researcher had found evidence that neurons can breach the wall between these regions, but developmental neurobiologist Pasko Rakic thought he saw hints of such a migration in brain slices he examined in 1969.

To find out whether developing neurons can indeed travel from the telencephalon to the diencephalon, Rakic and Kresimir Letinic, both of Yale University, examined fetal tissue from humans (obtained from aborted tissue donated to a brain bank), mice, and monkeys. In mice and monkeys, neurons stayed well within their respective zones, the researchers report in the September issue of Nature Neuroscience. But in humans, they found a stream of cells migrating from the telencephalon to the dorsal thalamus, a part of the diencephalon that sends information to the frontal lobes and other association areas.

"The exciting point here," says developmental neurobiologist Gord Fishell of New York University, is that the study identifies a "fundamental difference in the way human brains develop." Neuroscientist Edward Jones of the University of California, Davis, points out that in the mature brain, the frontal cortex can't do anything without input from the dorsal thalamus. The study implies, he says, that as the human cerebral cortex evolved to such massive proportions, the thalamus had to expand as well to support all the extra activity.

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