Visualization of the inside of the human body is possible today through many different means. Magnetic resonance imaging (MRI) scans, for instance, trace the flow of blood within the body. A new study published in the journal eLife details a novel technique of imaging neurons in the brain. By coating these cells with gold and silver, then energetically exciting them, the researchers are able to view the internal, 3D structure of the brain. While unsuitable for live subjects, it could help researchers better track disease progression over time.
Staining cells with silver in order to image them in itself isn’t actually new. This technique, called the Golgi stain method, dates back to the 1880s; cells were infused with microscopic crystals of silver chromate and viewed under a microscope. This staining revealed the minute physical characteristics of neurons that had previously gone unseen by medical researchers – as a result, it revolutionized our understanding of the brain.
This new technique is essentially a vast improvement on the original. Firstly, a slice of brain tissue was taken from a grasshopper. Specifically, an abdominal ganglion was removed; this dense cluster of interconnected neurons was treated by the researchers as a “mini-brain,” representative of the general structure of the entire brain.
These cells were then coated in metal particles; this means that more light will reflect off them, improving their visibility under a microscope. However, viewed under a conventional bright-field microscope – wherein the cells are illuminated by a white light from behind – the smaller structures of the cells were still fairly difficult to perceive.
GIF credit: Ganglion neurons imaged using the new technique, in true 3D. University of Minnesota/eLife
The same ganglion was then coated in silver and gold nanoparticles, no longer than one millionth of a centimeter. Specific frequencies of light were focused on these nanoparticles, which caused them to become energetically excited. The surface electrons of these nanoparticles began to vibrate and reemit some of the energy they absorbed. This vibratory process is known as “surface plasmon resonance.”
The silver particles emitted a red hue, whereas the gold particles appeared to “glow” yellow. Using a highly sensitive device called a Laser Scanning Confocal Microscope (LSCM), the detection of this resonance-generated energy was possible, as well as its point of origin.
This allowed the researchers to build up a remarkably detailed image of the ganglion, viewable in true 3D. In addition to this, this method will have preserved the quality of the neurons for over a century, meaning that stained samples could be accurately studied for generations.
GIF credit: A single neuron imaged using the new technique. Grant M. Barthel, Karen A. Mesce, Karen J. Thompson
This method is somewhat similar to using fluorescent dyes in order to image cell structures. However, these dyes become less fluorescent with repeated use in a process called photobleaching. As this study points out, both gold and silver staining does not fade over time, and so “neither will their information.”
“With the prediction that superior resolution microscopic techniques will continue to evolve, older archived samples could be reimaged with newer technologies and with the confidence that the signal in question was preserved,” said Karen Mesce, a coauthor of the study, in a statement. “The progression or stability of a cancer or other disease could therefore be charted with accuracy over long periods of time.”
Image credit: Silver-impregnated neurons. Grant M. Barthel, Karen A. Mesce, Karen J. Thompson