Researchers at MIT have reported a method they’ve developed for generating very high-resolution tissue sample images. The technique is also much cheaper than tools that provide similar results. Reporting in Nature Methods, the investigators improved upon expansion microscopy, which you can learn about in the following video, from one of the researchers.
These scientists have previously done work on tissue expansion with light microscopy, expanding tissue volumes by 100 times to provide an image resolution of 60 nanometers. The updated technique can get good resolution at 25 nanometers. This is similar to, but far cheaper than tools like stochastic optical reconstruction microscopy (STORM) because there is no need for specialized reagents or equipment.
This resolution level enables the observation of stuff like proteins that aggregate in complex patterns at the connections of nerve cells, synapses in the brain, which helps those cells communicate. It may also aid in the mapping of neural circuits, said the senior author of the work Ed Boyden, an Associate Professor of Biological Engineering and Brain and Cognitive Sciences at MIT.
"We want to be able to trace the wiring of complete brain circuits," explained Boyden. "If you could reconstruct a complete brain circuit, maybe you could make a computational model of how it generates complex phenomena like decisions and emotions. Since you can map out the biomolecules that generate electrical pulses within cells and that exchange chemicals between cells, you could potentially model the dynamics of the brain."
The researchers created what they call “iterative expansion.” Tissue gets embedded in dense, absorbent polyacrylate gel after labeling the proteins of interest to their research. While they had gotten a resolution of about 60 nanometers, there was a need for improvement, as “individual biomolecules are much smaller than that, say 5 nanometers or even smaller," Boyden explained. "The original versions of expansion microscopy were useful for many scientific questions but couldn't equal the performance of the highest-resolution imaging methods such as electron microscopy." So the researchers expanded a gel that swells a second time.
They had to create a special gel for it to be stable enough to work. "If you reduce the cross-linker density, the polymers no longer retain their organization during the expansion process," said Boyden, who is a member of the Media Lab and McGovern Institute for Brain Research at MIT. "You lose the information."
In their publication, iterative expansion was used to see tiny structures around neurons, like the neurotransmitter receptors that sit on the surface of certain neurons, called postsynaptic, inside the brain.
"My hope is that we can, in the coming years, really start to map out the organization of these scaffolding and signaling proteins at the synapse. By combining iterative expansion with temporal multiplexing, we could in principle have essentially infinite-color, nanoscale-resolution imaging over large 3-D volumes," Boyden said. "Things are getting really exciting now that these different technologies may soon connect with each other."
The researchers are now working on a third round of expansion; they suggest it could bring the resolution to around 5 nanometers. Antibodies that label the cells are roughly 10 to 20 nanometers long though, so the investigators need to find smaller tags or another modification that won’t interfere with a resolution at that level.