Innovation in Imaging (Summer Science Series Pt. 1)
We’ve told you how centuries of innovation in biological imaging brought us to serial block face imaging, the technique we used to obtain the EyeWire dataset. Microscopes today continue to stretch the boundaries of resolving power but each successive breakthrough to higher resolution is more difficult to achieve than the last. This is where the out-of-the-box thinking of Karl Deisseroth and Edward Boyden comes in. Their projects increase resolving power without getting more microscopic. Deisseroth and his team created CLARITY, the technology that makes brain tissue transparent. Boyden and his team created expansion microscopy, which, instead of magnifying tissue, enlarges the tissue itself. Both of these technologies involve changing the properties of brain tissue through injection.
CLARITY is not the first tool that attempts to make brain tissue clear, but it is the most functional and practical. Past technologies struggle to create truly transparent brains while keeping the brain tissue structurally intact and maintaining the molecular composition of the tissue. CLARITY does this by replacing the lipid based cell membranes with hydrogel, a clear polymer substance. Proteins and nucleic acids are left preserved.
What are the benefits of a see-through brain? Well, it looks cool. But wait! There’s more. With the removal of cell membranes, more space is created in between the cells. Now, not only can the visible light spectrum penetrate through the surface of the brain, so can macromolecules. These macromolecules can be used for data collection techniques such as neural pathway staining, and DNA sequence location.
CLARITY is not the only new technology that utilizes the injection of a polymer to increase resolving power. In Ed Boyden’s expansion microscopy, molecules of sodium polyacrylate are infused into the brain. These are the same salt molecules used in super absorbent baby diapers! Using other chemicals, the molecules join together to create uniform, super absorbent mesh. Then, just add water! Brain tissue swells 4.5 times it’s original size. In order to make use of this huge brain, researchers preserve and track biomolecules through the enlargement process. They label the biomolecules with fluorescent antibodies, and they bind these trackers to the uniform mesh to maintain relative spatial relationships between the target biomolecules.
Imagine playing EyeWire with lower resolution images. Accuracy in the game hinges on the high resolution cubes, and your ability to interpret images at the best resolution we can offer you. Centuries of worldwide struggle to increase the resolving power in light microscopes have allowed scientists to distinguish the lines between smaller and smaller components of biological tissue. Though the microscopes we use for EyeWire (electron microscopes) image at a higher resolution than light microscopes, they don’t do so in color, and color is valuable. So the world continues to look for a way to use light microscopy to resolve beyond the conventional restriction of 200 nm, half the wavelength of light. And Boyden’s lab creatively worked with that restriction by taking the specimen, and making it larger. Conventional confocal microscopy techniques can now be used in conjunction with expansion techniques to distinguish molecules at high nanometer resolution.
Top right: super resolution fluorescence microscopy
Bottom: enlarged SRF image
Lysosome membranes, Credit: Eric Betzig, Stefan W. Hell and William E. Moerner, The Royal Swedish Academy of Sciences