This post is the second installment in a two part series. Check out last week’s post here.
Thanks to modern technology, the field of cellular neuroscience has become illuminated with brightly colored images – tissue samples, cells, and individual molecules have been stained, photographed, colorized, and even reconstructed in three dimensions. A Google Image search quickly provides thousands of examples, and a walk through the research wing in IU’s Multidisciplinary Sciences Building II is no different. The images that IU neuroscientists have collected are proudly displayed on posters and signs that line the hallways.
However, the field of neuroscience hasn’t always been so bright. Before the late 1800s, scientists could look at samples of brain tissue through the lens of a microscope, but there was no way to pick out individual brain cells, or neurons, from the background–the detailed structure of individual neurons was essentially invisible. That all changed when Camillo Golgi, an Italian biologist, developed a technique that he called “the black reaction.” He found a way to stain entire neurons black against a brown background. For the first time, neuroscientists were able to see individual neurons. His technique, now called Golgi histology, was quickly adopted by Spanish scientist Santiago Ramon y Cajal, who created the first maps of how neurons are organized in the brain.
You might be surprised to learn that Golgi histology is still in use today. By leafing through virtually any neuroscience textbook, you’ll find images of Golgi-stained neurons and, many times, copies of Cajal’s original illustrations. Microscope slides containing samples of Golgi-stained brain tissue can be found in research labs across the globe and, as you might have guessed, here at IU as well.
Myself and other members of Dr. Cara Wellman’s Neurobiology of Stress Lab use Golgi histology to study the rat brain. Using a process derived from Golgi’s original methods, we soak whole brains in solutions that contain microscopic metallic crystals. After soaking for a few weeks in solution, neurons become filled with metal. At this point, we cut the brains into thin sections and place the sections onto microscope slides. We run the slides through a series of solutions that are similar to those used to develop black and white photographs: they cause the metallic crystals inside neurons to turn black. Any metal that is not contained within neurons is washed away, leaving clear images of entire neurons. Under a microscope, it is easy to see large neuronal cell bodies, long branch-like projections which extend from each cell body, called dendrites, and microscopic protrusions on the surface of each dendrite, called dendritic spines.
While these images look similar to what Golgi and Cajal saw over 200 years ago, today’s neuroscientists study neurons a bit differently. Thanks to modern microscopy tools, I can photograph neurons that I find in my tissue samples rather than draw them by hand (and, yes, I proudly display them in the halls of MSBII). I also use advanced methods to measure and analyze the shape and size of Golgi-stained neurons. Using neuron tracing software, I can superimpose a computer screen onto the image I see under the microscope. I then select neurons that are completely stained and click along their dendrites to reconstruct them in three dimensions. I save these reconstructions as digital files, and can automatically measure the length of a neuron’s dendrites, the number of dendritic spines on each dendrite, and many other features of the neuron’s overall structure.
These methods are helpful for studying how dendrites change over time or after a given experience. For example, my colleagues in the Wellman lab study how stressful experience affects dendritic structure. They have found that neurons in the medial prefrontal cortex, a brain region that controls emotional reactions, shrink in response to stress. I have found that neurons in the amygdala, another brain region that is important for emotional behavior, are affected by early life stress. Meanwhile, others in the lab are studying how neurons recover following stressful experiences. Better understanding the effects of stress on neurons could be an important key to understanding how stress-induced psychological disorders arise and, importantly, how they may be treated. As we gain insights into the effects of stress on neurons, we will have Camillo Golgi and Santiago Ramon y Cajal to thank.
Want to read more about how this technology is used in neuroscience research? Check out my previous post!