Stem cells take root

Aparna is a freelance science writer pursuing a PhD in bioinformatics and genomics at Harvard University. She uses her multidisciplinary training to find both the cutting-edge science and the human...

As they take on sugary snacks and corrosive acids, teeth can undergo a lot of damage. In particular, when wily bacteria sneak past the protective outer layer of the tooth, they can penetrate the dental pulp that lives at its core. These unwanted residents trigger inflammation — more recognizable to us as searing pain and swelling.

The teeth aren’t defenseless, though. They can fight off minor infections and repair some damage. “Dental pulp has an ability to repair itself and regenerate, but it's very limited,” said Sriram Ravindran, a bioengineer at the University of Illinois Chicago. “They're not all-powerful cells.”

This ability to regenerate comes from stem cells buried in the dental pulp. These cells can turn into many of the different cell types required for healthy dental pulp, but they don’t always have enough juice on their own to restore tissue damaged by an infection or other injury. When the infection progresses too far, dental intervention is required. This might involve a procedure such as a root canal to clear out the bacteria and fill the tooth, or a tooth extraction.

“[With a root canal,] you get rid of the soft tissue of the dental pulp tissue, and you replace that with inorganic material,” said Ana Angelova Volponi, a dentist and regenerative biologist at King's College London. “But in that moment, the tooth loses any kind of vitality.”

Rather than removing teeth or filling them up with polymers, scientists are looking for ways to channel teeth’s regenerative power into a biological solution for tooth loss. With the help of cocktails of biological molecules, carefully engineered scaffolds, and biologically inspired delivery systems, they hope to expand the clinician’s toolkit to fix not only teeth, but other degenerating parts of the body too.

Stem cells aren’t only in the dental pulp; they’re also in the gums, in the alveolar bone that anchors the teeth, and even in the thin fibers that keep the tooth securely positioned in the bone. Stem cells from different parts of the mouth can have slightly different properties: some may divide more quickly, while others are more likely to regenerate certain tissues (1).

At first glance, dental stem cells look remarkably similar to stem cells extracted from the bone marrow. They are largely classified as mesenchymal stem cells, and although they can’t regenerate every type of cell, they still have many possible paths.

“The extracellular environment that stem cells reside in determines what happens to those cells,” Ravindran said. Inside the tooth, they’re on a path to becoming various dental pulp cells. But the right combination of molecules and genetic signals can change this path, turning them into anything from bone to fat.

One way that scientists imagine repairing a damaged tooth is by delivering stem cells into the tooth and giving them the right signals to regenerate the damaged cells. The tricky part is knowing exactly which factors will lead to this outcome; after all, growing a glob of fat in a tooth socket would not be helpful.

Angelova Volponi measured the expression levels of different genes when stem cells develop normally within the tooth or when they’re pummeled with developmental signaling molecules in artificial lab settings. This approach could identify key molecules that make a stem cell turn into, say, a cell that produces dentin, one of the hard outer layers of the tooth. She is also thinking about how to genetically engineer stem cells to amp up their wound healing capabilities.

Ravindran, on the other hand, has taken a “black-box” approach. He focuses on exosomes, tiny packages of RNA and protein molecules released by cells and absorbed by other cells. “These are what you might call mail carriers for cell-to-cell communication,” Ravindran said. Exosomes from particular cells — bone cells, for example — can instruct stem cells to turn into the same type of cells. With this strategy, Ravindran doesn’t have to manually determine each factor involved in differentiation; the cells have conveniently already packaged them up into exosomes.

In his research, Ravindran has shown that he can grow cells in the lab with desired dentin-producing properties — a key staple of tooth regeneration (2). When he and his team collect exosomes from these cells, embed them in a collagen membrane, and add them to the root canals of damaged teeth, the exosomes prompt the regeneration of dental pulp.

Ravindran thinks that this method could help researchers develop better treatments for tooth decay than root canal surgeries. Instead of just clearing out the infection, he imagines adding a moldable scaffold or injectable polymer loaded with stem cell exosomes. “The exosomes may have the property to reduce the inflammation, help the cells survive, and also maintain the vitality of the teeth,” he said.

Delivering a bounty of exosomes, stem cells, or other molecules to spur tooth growth requires sophisticated materials. Not only does the material have to target a specific location, it may also have to control the substances’ release.

These materials can be made of a wide range of natural and synthetic polymers (3). Ravindran and his team, for example, are testing hydrogels made from the algae-derived polymer alginate in combination with collagen membranes. They tether exosomes to the membrane, and when they place the membrane on the damaged tooth, the exosomes either sink into the tooth naturally or are chemically detached from the membrane.

Other groups have developed 3D-printed scaffolds designed to match the shape of the damaged area (4). A team of scientists at Columbia University made scaffolds shaped like human and rat teeth out of biodegradable polymers. The scaffolds had tiny channels to deliver molecules that would steer the stem cells toward bone- and tooth-supporting cell types. After nine weeks implanted in a rat, the desired cells began to form at the base of the scaffold.

Angelova Volponi imagines a future where this approach could become highly personalized to an individual’s damage. “You have a bone defect that can be scanned with a CT scan, and then that will be taken to produce a personalized, 3D-printed scaffold, which really fits the defect that you have,” she said. If there are still viable cells left in the damaged site, this scaffold may just hold molecules that can steer the existing stem cells toward wound healing and regeneration; but if the damage is severe, the scaffold could be loaded with stem cells to start the process.

The ultimate goal for many regenerative dentists is to grow a whole human tooth for implant. However, that possibility is still far away, Ravindran said.

Teeth are complex organs, with many different components making up even the smallest incisor: dental pulp, enamel, dentin, and more. During human development, mesenchymal stem cells interact with other types of cells to generate the inner and outer layers of a tooth; this process is still poorly understood, and replicating it artificially is no small task.

Another big question is how to make different teeth look different. What makes the front teeth flat while the molars look like stumps? “At this point, we don’t know a lot about why different teeth have different numbers of roots or a certain shape,” Ravindran said. To better understand this, Angelova Volponi said that it is important to study gene expression patterns that drive the formation of diverse tooth shapes.

The idea of building a whole tooth from stem cells was what originally got Angelova Volponi interested in regenerative dentistry. Over the 15 years she has spent in this field, she has seen the technology improve drastically. In a project that she led, she combined stem cell-like mesenchymal cells from mouse molars with gum cells from a human and grew a tooth-like structure — with roots and all (5).

“That excitement of seeing a tooth germ in a dish, after you have combined a blob of cells, and you have managed to get it developed into a full tooth,” she recalled. “I still absolutely cherish that moment, and I'm still very excited whenever we see that happening in a dish.”

But these lab-grown teeth are still far from being ready for human patients. One challenge that continues to stump the scientists is how to get the teeth to grow faster. The teeth still seem to follow their normal biological clocks, Angelova Volponi said, so they grow too slowly to make them usable on demand in the clinic.

While some people may see a tooth extraction as torturous, Angelova Volponi sees it as a gold-mining expedition.

“On every extraction of a tooth, you potentially have a tissue that can be used to derive cells with stem cell properties very close to the bone marrow… which will usually be discarded,” Angelova Volponi said. Given the difficulty of accessing stem cells for regenerative medicine, scientists are finding ways to use this valuable fount of regeneration even in organs far from the mouth.

On every extraction of a tooth, you potentially have a tissue that can be used to derive cells with stem cell properties very close to the bone marrow… which will usually be discarded.- Ana Angelova Volponi, King's College London

Extracting stem cells from the bone marrow can be an invasive process. Teeth, on the other hand, are so easy to access that removing them is a routine procedure; the first round of baby teeth even fall out on their own.

“They’re very accessible,” Angelova Volponi said. “That makes [them] really attractive as a source to be studied and potentially translated into the clinic.”

Researchers have already used tooth-derived stem cells to regenerate everything from insulin-producing pancreatic beta cells to bladder muscle cells (6). In one study, scientists carefully crafted a cocktail of molecules that made dental pulp stem cells transform into cells that resembled those in the cornea (7). These new cells packed themselves into the three-dimensional structure of the cornea when grown on a bed of nanofibers. They even behaved like corneal cells when injected into rats’ eyes, suggesting that these tooth-derived cells might be useful for developing eye therapies.

Harnessing the regenerative power of human biology is already an area of active research for organs outside the mouth. Now, Angelova Volponi said, dentistry needs to catch up.

“It’s a shift in the way we see future treatments,” she said. “They will be based on biological repair and regeneration, rather than replacing the tissues with inorganic materials.”

Aparna is a freelance science writer pursuing a PhD in bioinformatics and genomics at Harvard University. She uses her multidisciplinary training to find both the cutting-edge science and the human...

June 2023 Issue