X-ray vision achieved by rendering brain samples transparent

Japanese scientists have cracked the problem of ‘optical clearing’ — making biological samples transparent and preserving them without damaging their structure

Researchers in Japan have devised a method for rendering brain tissue transparent, to help biologists gather information about the three-dimensional structure of the brain and processes affecting it, such as the formation of protein ‘plaques’ associated with Alzheimer’s disease. Known as optical clearing, the technique has ben a challenge in the past because of the difficulty of changing the optical qualities of the tissue without damaging it.

“While Superman’s x-ray vision is only the stuff of comics, our method, called ScaleS, is a real and practical way to see through brain and body tissue,” said lead researcher Atsushi Miyawaki of the RIKEN Brain Science Institute near Tokyo.

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3-D visualization of protein plaques (green) and blood vessels (red) in a region of cerebral cortex from a 20-month-old Alzheimer’s disease model mouse, optically cleared using the ScaleS technique

Miyawaki’s team has been working on the project since 2011, when they discovered that an aqueous solution of urea was promising for brain tissue optical clearing, but they ran into the tissue damage problem. They have now refined the technique by using a solution which also contains the sugar alcohol sorbitol, as they explain in Nature Neuroscience.

“By combining sorbitol in the right proportion with urea, we could create transparent brains with minimal tissue damage, that can handle both florescent and immunohistochemical labeling techniques, and is even effective in older animals,” Miyawaki said.

The ScaleS solution also works as a preservative, keeping samples usable for over a year. It can also be combined with other sample preparation techniques: the team used it with a technique for labelling structures that trigger a response from the immune system and another for fluorescent chemical compounds to produce high-resolution images of Alzheimer’s plaques, which are not detectable in standard 2D microscopy.

This experiment demonstrated for the first time that the plaques are not isolated, but grow around cells called micoglia — mobile cels which surround and protect neurons.

“Clearing tissue with ScaleS followed by 3D microscopy has clear advantages over 2D stereology or immunohistochemistry,” Miyawaki stated. “Our technique will be useful not only for visualising plaques in Alzheimer’s disease, but also for examining normal neural circuits and pinpointing structural changes that characterise other brain diseases.”