The Physics of Folds and Grooves

Guest Post by Kayt Sukel

Gel-printed brain. Image courtesy of T. Tallinen, J-Y Chung and L. Mahadevan

Gel-printed brain forming folds after it was placed in liquid solvent. Image and video below courtesy of T. Tallinen, J-Y Chung, and L. Mahadevan

The human brain has a remarkably distinct shape. With its folds and valleys (gyri and sulci, respectively), there is no bodily organ like it. How and why does it have this accordion-like structure? That’s remained an open, and highly debated, question among neuroscientists. Some argued there must be some biological factors within neurons and supporting cells that program them to grow this way. Others hypothesized that the brain’s gulfs and valleys are due to simple physics—that is, the mechanical compression involved with developing inside the enclosed case of the skull. Now, work done at Harvard University and Finland’s University of Jyväskylä suggests the latter argument may be the correct one.

“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,” says Lakshminarayanan Mahadevan, a physicist and applied mathematician at Harvard. “This puts the cortex under compression leading to a mechanical instability that causes it to crease locally, a process called gyrification.”

This, Mahadevan thought, is why we see the telltale grooves and folds we do in the healthy brain. To test the idea, Mahadevan and colleagues programmed a 3-D printer to create a model of the developing brain. 3-D printing uses successive layers of material to build a three-dimensional object; the researchers created a brain based on neuroimaging data of the fetal human brain, using a variety of soft gel layers. The researchers used different gels that would swell and grow in different ways when placed into liquid solvent. When they put the 3-D model of the brain into that solvent, they saw folds and grooves forming as the gels grew—showing a pattern of development very similar to that seen in fetal brain development.

Mahadevan says gyrification is a simple evolutionary innovation that allows the expansive cortex to be packed into the small volume of the skull. This model may help us better understand the physics of brain development—and how and where the gyrification process may go awry in certain disorders and diseases. The research was reported in Nature Physics today (DOI: 10.1038/nphys3632 ).

In biology, function often follows form,” he says. “So perhaps our work might be able to shed light on aspects of dysfunction by highlighting how different cortical growth patterns lead to abnormal form and thus function.”

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