For the first six months after conception, a fetus's brain--like itsbottom--is smooth. But during the third trimester of pregnancy the brain's outer layer, the cortex, develops the familiar nooks and crannies that adorn adult brains. The need for this wrinkling is obvious; it's the only way to squeeze a cortex the size of a 12-inch pizza on top of each of the brain's hemispheres.
What's not so obvious is how this scrunching occurs in the first place. Many scientists assume that the explanation lies in cell division; perhaps certain cells in the cortex are programmed to proliferate faster than others, causing the brain's surface to fold. But David Van Essen, Ph.D., a professor of anatomy and neurobiology at Washington University in St. Louis, has proposed a different theory. He notes that neurons growing in a petri dish develop tension along their axons, the long appendages by which they send messages. When manY of these cells run in parallel paths through the brain, Van Essen suggests in a recent issue of Nature, the combined tension distorts the surface of the cortex, just as the shape of a rubber hand changes when you pull on it.
