3-D Maps of Blood Vessels in Brain Reveal Surprises

Blood vessels within a sensory area of a mouse’s brain loop and connect in unexpected ways, a new 3-D map has revealed. The findings could help scientists better explore what the cerebral cortex means for functional imaging of the brain and the onset of dementia.

The organization of neural cells in the sensory area of the cerebral cortex is well understood, as was a pattern of blood vessels that plunge from the surface of the brain and return from the depths, but the network in between is uncharted territory. Yet these tiny arterioles and venules deliver oxygen and nutrients to energy-hungry brain cells and carry away wastes.

David Kleinfeld and colleagues at the University of California, San Diego have now traced blood vessels in the area of the mouse brain that receives sensory signals from the whiskers by filling them with a fluorescent gel. Using an automated system developed by co-author Philbert Tsai, the team removed thin layers of tissue with a laser to capture a series of images in which they could reconstruct a 3-D map of the vessels.

Scientists at the University of California, San Diego have developed a 3-D map of blood vessels within a sensory area of a mouse’s brain. Example of data obtained throughout the full depth of cortex and extending into the white matter. Surface and penetrating arterioles are colored red, venules blue and the borders of cortical columns are denoted by a golden band. Images courtesy of Kleinfeld Lab/UC San Diego.

The project focused on a region of the cerebral cortex in which the nerve cells are so well known that they can be traced to individual whiskers. These neurons cluster in “barrels,” one per whisker, a pattern of organization seen in other sensory areas as well.

The researchers expected each whisker barrel to match up with its own blood supply, but instead found that the vessels did not line up with the functional structure of the neurons they feed.

“This was a surprise, because the blood vessels develop in tandem with neural tissue,” said Kleinfeld, a professor of physics and neurobiology at UCSD. Instead, microvessels beneath the surface loop and connect in patterns that don’t obviously correspond to the barrels.

To search for patterns, the investigators turned to a branch of mathematics called graph theory, which describes systems as interconnected nodes. Using this approach, no hidden subunits emerged, demonstrating that the mesh indeed forms a continuous network they call the “angiome.”

The vascular maps traced in this study raise a question of what we’re actually seeing in a widely used kind of brain imaging called functional MRI, which in one form measures brain activity by recording changes in oxygen levels in the blood. The idea is that activity will locally deplete oxygen. By wiggling whiskers on individual mice, they found that optical signals associated with depleted oxygen centered on the barrels, where electrical recordings confirmed neural activity, implying that brain mapping does not depend on a modular arrangement of blood vessels.

Relation of penetrating vessels to cortical columns. (a) Example data set from a flattened cortex. The location of all penetration arterioles (red squares) and all penetrating venules (blue squares) are superimposed on an axial projection of the upper 150 µm of cortex. The cortical columns are based on imaging data taken with a flattened cortex.

Taking this a step further, they calculated patterns of blood flow based on the diameters and connections of the vessels and asked how this would change if a feeder arteriole were blocked. The map allowed them to identify “perfusion domains,” which predict the volumes of lesions that result when a clot occludes a vessel. They also were able to build a physical model of how these lesions form, which may occur in cases of dementia.

The study — funded by the National Institutes of Health, including a Directors’ Pioneer Award to Kleinfeld — was published in Nature Neuroscience (doi: 10.1038/nn.3426).

For more information, visit: www.ucsd.edu