Every tissue in the human body contains a network of microscopic fibers. Muscle fibers direct mechanical forces, intestinal fibers are involved in gut mobility, and brain fibers transmit signals and form the communication network to drive cognition. Together, these fibers shape how organs function and help maintain their structure.
Likewise, almost all diseases involve some form of degeneration or disruption of these fiber networks. In the brain, this translates to disturbances in neural connectivity that are found in all neurological disorders.
Despite their biological importance, these microscopic fibers have been difficult to study, as scientists have struggled to visualize their orientations within tissues.
Now, a team led by Marios Georgiadis , PhD, instructor of neuroimaging, has developed a simple, low-cost approach that makes those hidden structures visible in remarkable detail.
Described in a Nov. 5 article in Nature Communications, the method, called computational scattered light imaging (ComSLI), can show the orientation and organization of tissue fibers at micrometer resolution on any histology slide, prepared with any staining protocol and stored in any condition — no matter how old the slide is.
Michael Zeineh , MD, PhD, professor of radiology, is a co-senior author, along with Miriam Menzel, PhD, a former visiting scholar in Zeineh's lab.
"The information about tissue structures has always been there, hidden in plain sight," Georgiadis said. "ComSLI simply gives us a way to see that information and map it out."
Lighting up microstructure maps
Traditional methods to image tissue fibers have various trade-offs: MRI can map large-scale networks but does not reveal microscopic details at the cellular scale. Histology-based methods require specialized stains, expensive equipment and meticulously preserved samples. These methods also fail to offer clear images when tissue fibers cross one another.
ComSLI takes advantage of a simple principle of physics: Light scatters differently depending on the orientation of microscopic structures it passes through. By rotating the direction of the light and recording how the scattering changes, the direction of the fibers within each microscopic pixel can be reconstructed across the entire image.
The setup requires only a rotating LED light source and a microscope camera, so the approach is accessible and cost-effective compared with other specialized microscopy equipment. Software algorithms then analyze the subtle patterns in scattered light to produce color-coded maps, called microstructure-informed fiber orientation distributions, that indicate the orientation and density of fibers within the sample.
Unlike other methods, ComSLI works regardless of how the sample was prepared. It works on formalin-fixed, paraffin-embedded sections, the most common format in clinical pathology and hospitals, as well as on fresh-frozen, stained and unstained samples.
Researchers can even re-analyze slides originally made for other purposes, including those that have been stored for decades, allowing researchers to extract structural information from tissue without additional processing.
"This is a tool that any lab can use," Zeineh said. "You don't need specialized preparation or expensive equipment. What excites me most is that this approach opens the door for anyone, from small research labs to pathology labs, to uncover new insights from slides they already have."
Tracing neural microstructure
Detailed mapping of the neuronal pathways in the brain has long been a goal in neuroscience. Using ComSLI, Georgiadis and colleagues were able to visualize entire formalin-fixed, paraffin-embedded human brain sections, as well as standard-sized slides, to reveal distinct microscopic details in each section of the brain.
The team also explored how these fiber appearances change when brain tissue is affected by neurological disorders, including multiple sclerosis, leukoencephalopathy and Alzheimer's disease.
For instance, the team focused on the hippocampus, a deep-brain structure essential for forming and retrieving memories and one of the first regions affected in many neurodegenerative diseases. Using ComSLI, they compared brain tissue samples from a patient with Alzheimer's disease and from a healthy patient. In the Alzheimer's sample, the researchers saw striking microstructural deterioration. The dense fiber crossings that normally connect different parts of the hippocampus were greatly reduced, and one of the main routes for carrying memory-related signals into the hippocampus — the perforant pathway — was barely detectable. In contrast, the healthy hippocampus showed a rich weaving of interconnected fibers spanning across the entire area. These visual maps of subtle degeneration patterns make it possible to map how the brain's memory circuits break down in disease.
Knowing that ComSLI can be applied to any slide, regardless of preparation or storage, the team pushed the bounds even further, imaging a brain section prepared in 1904. The technique was still able to reveal intricate fiber pathways, opening the door to studying historical samples and tracing lineages of disease.
Beyond the brain
While originally developed for neuroimaging, ComSLI proved effective for other tissues. The researchers demonstrated its use on muscle, bone and vascular samples, each showing distinct fiber patterns related to their physiological roles.
In tongue muscle, the method revealed layered fiber orientations corresponding to movement and flexibility. In bone, it traced collagen fibers that follow the lines of mechanical stress. In arteries, it uncovered alternating layers of collagen and elastin fibers, aligned to provide both strength and elasticity.
The ability to map fiber orientation in various species, organs and decades-old specimens could reshape how scientists approach questions of structure and function. It also turns millions of archived slides worldwide into potential sources of new data.
"Although we just presented the method, there are already multiple requests for scanning samples and replicating the ComSLI setup — so many labs and clinics would like to have micron-resolution fiber orientation and micro-connectivity on their histology sections," Georgiadis said. "Another exciting plan is to go back to well-characterized brain archives or brain sections of famous people, and recover this micro-connectivity information, revealing 'secrets' that have been considered long lost. This is the beauty of ComSLI."
