Figuring out how the brain works is enough to make your head spin. But now we seem to have a handle on how it gets its folded shape.
The surface layer of the brain, or cortex, is also referred to as our grey matter. Mammals with larger brains have a more folded cortex, and the human brain is the most wrinkled of all, cramming as much grey matter into our skulls as possible.
L. Mahadevan at Harvard University and his colleagues physically modelled how the brain develops in the embryo, using a layer of gel to stand in for the grey matter. This gel adhered to the top of a solid hemisphere of gel representing the white matter beneath.
In the embryo, grey matter grows as neurons are created or others migrate to the cortex from the brain's centre. By adding a solvent to make the grey matter gel expand, the team mimicked how the cortex might grow in the developing brain. They didn't model what effect, if any, the skull would have had.
Hills and valleys
The team varied factors such as the stiffness of the gels and the depth of the upper layer to find a combination that led to similarly shaped wrinkles as those of the human brain, with smooth "hills" and sharply cusped "valleys".
There are several theories about how the brain's folds form. These include the possibility that more neurons migrate to the hills, making them rise above the valleys, or that the valleys are pulled down by the axons – fibres that connect neurons to each other – linking highly interconnected parts of the brain together.
Mahadevan's model shows that, as long as the cortex is attached to the white matter beneath, the only thing needed is expansion of the cortex, and it will physically buckle. "Once you have that, everything else follows," says Mahadevan. "It's an extremely simple mechanism."
Knowing how the cortex folds will help us understand disorders where people are born with brains whose surface is smooth and unwrinkled or with too many folds, says Mahadevan.
Journal reference: PNAS, DOI: 10.1073/pnas.1406015111