POSTED 24 OCT 2002
a lot of things distinguishing human from rodent. Those ever-growing teeth
and that sneaky way of digging through foundations. The ability to design
web pages, play golf, or even dream up such a ridiculous game...
In case you haven't looked under the hood lately, these talents are credited to that quintessentially human section of brain, the cerebral cortex. This thin layer of neurons drapes over the top of your mental motor, and indeed there is so much of it that the cerebral cortex needs to fold into ridges and valleys.
The cortex is where you think, read and plan. Without your cortex, you would be about as smart as a brick. Well, maybe a mouse, whose cerebral cortex is so small it skips the ridges-and-valleys bit.
Getting to the point yet?
The mice received a gene that made extra quantities of beta-catenin (a protein that helps primitive brain cells "choose" what types of cells to divide into). Since the new beta-catenin is also unusually stable, the result was a huge increase in beta-catenin levels during embryonic development.
This mattered. By the time the mice were ready to be born, their brains were two or three times normal size. And since the enlargement was primarily in the cerebral cortex, it represented a dramatic shrinking of the evolutionary -- not to mention intellectual -- gap between mice and, well, men.
The brain sells
How would extra beta-catenin expand the cerebral cortex? Not by increasing the rate of cell division or slowing the rate of cell death. Instead, the protein seems to change the outcome, not the rate, of cell division.
When they divide, neural precursor cells may form identical daughter cells, or brain cells (either neurons or support cells called glia). Because precursor cells ultimately do form brain cells, you get more brain cells if the precursors initially divide into precursor cells.
A larger cortex is a sign of higher development, and indeed Chenn expects his studies to help elucidate the evolutionary development of the large-brained primates. It may also explain the development of some cancers whose cells, Chenn says, resemble neural precursors.
Rodents with real brains!
Still, an IQ test for the mice is in the cards, he says, if they can be born alive. "It's questionable," he warns, "because the brains are quite distorted."
And while a herd of brilliant mice applying to graduate school might be profitable to institutions of higher education, Chenn adds that "It's probably not going to be so simple, just having more brain is not a precursor to having more intellect.... It probably will depend on more than one gene, and on how the neurons connect to each other. I'd guess they will not be more intelligent."
-- David Tenenbaum
Regulation of Cerebral Cortical Size by Control of Cell Cycle Exit in Neural Precursors, Anjen Chenn and Christopher A. Walsh Science, July 19, 2002, pp. 365-369.