by National Institutes of Health

Facial nerve injuries are often treated with tissue taken from a patient’s body, called autografts. Alternatively, researchers are developing engineered conduits which could potentially be more accessible. Credit: NIBIB. Created with BioRender.com

A gesture as simple as a smile can often convey what words cannot. This is part of why nonverbal communication is so central to human interaction. It is also why facial nerve disorders and injuries can be devastating.

These conditions are typically treated with nerve tissue taken from elsewhere in a patient’s body, known as autografts. This technique for repairing injured nerves presents issues for patients, such as damage to the donor site and the odds of functional recovery being nearly a coin toss. Synthetic alternatives have been explored in the past but have yet to live up to the performance of autografts.

Bioengineers at the University of Pittsburgh may have developed a new solution with the help of some of nature’s best engineers—stem cells. Leveraging these cells’ ability to create a restorative environment, the team produced implantable conduits to act as bridges, providing directional, mechanical, and biochemical guidance for injured nerves to regenerate across large gaps.

Experiments in the facial nerves of rats showed that the technology matched autografts. These results were published in the Journal of Neural Engineering.

“We leaned into the idea that the cells know what they’re doing, and they know how to make tissue,” said oral and craniofacial sciences and bioengineering professor Fatima Syed-Picard, Ph.D., the senior author of the study. “These engineered tissues ended up being more biomimetic than many other synthetically derived scaffolds used in tissue engineering.”

Getting neurons in line

For nerves to be repaired, the long projections that extend from neurons, called axons, need to both regrow and reconnect to the appropriate tissue. With autografts, the former is slow, and the latter is no guarantee, as many patients experience unwanted muscle activity due to regrown nerves connecting to the wrong tissue.

The authors of the study created their nerve conduits by rolling up cell-made sheets of extracellular matrix. Credit: Journal of Neural Engineering (2024). DOI: 10.1088/1741-2552/ad749d

Researchers have wielded specific cell populations to accelerate growth, such as neural support cells and stem cells, which produce biomolecules that aid neural tissue regeneration. To orient growing tissue so that axons reach the proper targets, researchers have designed synthetic tissue scaffolds with features, such as grooves, that act as guiderails to regenerating neurons.

“It’s difficult to embed and distribute cells evenly in synthetic scaffolds without harming them. Another concern is trying to get these scaffolds to match the structural complexity of innate tissue,” said first author Michelle Drewry, Ph.D., who conducted this research while a graduate student at the University of Pittsburgh.

Many cell types in the body frequently make or remodel the biomolecular scaffolding surrounding them, known as extracellular matrix (ECM). So, instead of making tissue scaffolds from scratch themselves, the researchers thought it might be better to let cells make their own.

The authors of the study tested this hypothesis with dental pulp stem cells (DPSCs), a hardy and readily available cell population that produce proteins known to encourage nerve growth. After extracting these cells from adult wisdom teeth provided by the University of Pittsburgh School of Dental Medicine, the researchers put them to work.

They wanted to give DPSCs the freedom to create ECM but, at the same time, nudge them into making an environment conducive to supporting aligned axons. To accomplish this, the researchers fabricated rubber molds with rows of 10 micrometer-wide grooves and then covered them with DPSCs. After several days, the DPSCs secreted aligned ECM around themselves, forming thin biological sheets. The authors then peeled the sheets from the rubber templates and rolled them up into cylindrical conduits.

The researchers used this approach to make a type of bandage in a previous study, which successfully regenerated the axons of a crushed nerve. With their new work, they sought to clear a higher hurdle of using the conduit to bridge a 5-millimeter gap in the facial nerve of rats—a defect so large that the nerve would not be able to heal on its own.

Specifically, they implanted their aligned conduits into gaps made in the buccal branch of the facial nerve. For comparison, the team also implanted autografts into another group of rats.

To confirm that their nerve conduits contained aligned grooves to guide nerve regeneration, the researchers used scanning electron microscopy. Credit: Journal of Neural Engineering (2024). DOI: 10.1088/1741-2552/ad749d

“The buccal branch is the part of the facial nerve that helps with smiling. It’s a big part of your quality of life because it’s a large piece of how you communicate with other people and how you’re seen in the world. Injury to that nerve can have a life-changing effect,” Drewry said.

Crossing the bridge

Twelve weeks after implantation, the authors evaluated how well axons had regenerated, primarily through histology. They found that their cell-made conduits contained regenerated axons across their full length. And, in general, the density and number of axons were similar to what they found in autografts.

Indicators of developing axons were prevalent in the conduits, suggesting that regeneration may have been more robust with additional time, Drewry noted.

But did all this regenerated tissue translate to improved function? To find out, the authors electrically stimulated the nerves on one end and measured the animals’ whisker movement on the other side. The tests showed that the motions of rats implanted with conduits were on par with those treated with autografts.

Syed-Picard’s lab aims to better understand the roles the ECM and cells play in healing and then use that information to improve their technology. For example, in addition to encouraging regrowth directly, the conduits may also be helping by dampening inflammation, Syed-Picard explained.

More information: Michelle D Drewry et al, Enhancing facial nerve regeneration with scaffold-free conduits engineered using dental pulp stem cells and their endogenous, aligned extracellular matrix, Journal of Neural Engineering (2024). DOI: 10.1088/1741-2552/ad749d

Journal information:Journal of Neural Engineering

Provided by National Institutes of Health

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