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New method renders whole organs transparent

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By Anette Breindl
Science Editor

Wouldn't the NSA just love to have this – if they don't already, that is: a see-through organism.

That's what Caltech researchers describe in the July 31, 2014, issue of Cell.

The technique can be used to make whole bodies transparent, rather than taking out individual organs, thus severing their connections with the rest of the body. It will enable a much richer understanding of the connections within the brain, from the brain to the body, and from the body back to the brain – an understanding that will enable scientific as well as practical progress.

Combined with advances in microscopic techniques, the new method will allow researchers to look at transparent blocks of tissues that are up to 8 millimeters thick, yielding a wealth of information on long-range anatomical connections within and between organs. Currently, such connections are reconstructed from 2-dimenional images of slices that are usually 40 microns thick. Brains, for example, are fixed, thinly sliced, stained, imaged and put back together with using bioinformatics.

The process, senior author Viviana Gradinaru told BioWorld Today, is "excruciating. And it is actually quite error-prone."

Gradinaru and her team have developed thee separate methods. In their paper, they described those methods as "PACT (passive clarity technique), a protocol for passive tissue clearing and immunostaining of intact organs; RIMS (refractive index matching solution), a refractive index matching media for imaging thick tissue; and PARS (perfusion-assisted agent release in situ), a method for whole-body clearing and immunolabeling."

The multistep protocols consist of first fixing tissue, then washing out the fixature and infusing monomers that combine to make a hydrogel mesh.

Lipids, of course, play important roles in the body – notably, the method removes cell membranes. But due to the fixation, the proteins and nucleic acids that are embedded in those membranes stay put and can be visualized.

Once that mesh is in place, they used detergent to flush out lipids – which scatter light and are what makes tissues opaque rather than transparent – while retaining proteins and nucleic acids. Specific molecules of interest can then be labeled, for example with antibodies or dyes.

The potential applications of the new technique are vast, and go beyond what any one single laboratory would be able to do. Gradinaru plans to teach other labs how to use the techniques as part of her work as faculty director of the Beckman Institute Optogenetics Neuroscience Initiative and CLARITY (BIONIC) Center.

Gradinaru's own lab studies the peripheral nervous system, and she plans to use the technique for visualizing long-range axons.

One of her practical interests is the therapeutic use of electrical stimulation, and one practical use of the technique could be to identify nodes within the brain, as well as points of connection between the nervous system and other organs where such stimulation could be effective.

And some applications of the technique, she hopes, are yet to be devised – by people who will replace the current tedious imaging methods with her team's new ways and so "free up time for creative thinking."