Why the brain is folded can be rationalized easily from an evolutionary perspective; folded brains likely evolved to fit a large cortex into a small volume with the benefit of reducing neuronal wiring length and improving cognitive function.

Less understood is how the brain folds. Several hypotheses have been proposed but none have been directly used to make testable predictions. Now, researchers have shown that while many molecular processes are important in determining cellular events, what ultimately causes the brain to fold is a simple mechanical instability associated with buckling.

The research is published in Nature Physics.

The number, size, shape and position of neuronal cells during brain growth all lead to the expansion of the gray matter, known as the cortex, relative to the underlying white matter. This puts the cortex under compression, leading to a mechanical instability that causes it to crease locally.

"This simple evolutionary innovation, with iterations and variations, allows for a large cortex to be packed into a small volume, and is likely the dominant cause behind brain folding, known as gyrification," said the author.

Researcher’s earlier work found that the growth differential between the brain's outer cortex and the soft tissue underneath explains the variations in the folding patterns across organisms in terms of just two parameters, the relative size of the brain, and the relative expansion of the cortex.

 The team made a three-dimensional, gel model of a smooth fetal brain based on MRI images. The model's surface was coated with a thin layer of elastomer gel, as an analog of the cortex. To mimic cortical expansion, the gel brain was immersed in a solvent that is absorbed by the outer layer causing it to swell relative to the deeper regions. Within minutes of being immersed in liquid solvent, the resulting compression led to the formation of folds similar in size and shape to real brains.

The extent of the similarities surprised even the researchers. "When I put the model into the solvent, I knew there should be folding but I never expected that kind of close pattern compared to human brain," said post doctoral fellow in the study.

The largest folds seen in the model gel brain are similar in shape, size and orientation to what is seen in the fetal brain, and can be replicated in multiple gel experiments. The smallest folds are not conserved, mirroring similar variations across human brains.

http://www.seas.harvard.edu/news/2016/02/new-research-replicates-folding-of-fetal-human-brain