If you’re an adult human, odds are you already know a thing or two about tooth regeneration. Around age six, most of us begin to lose baby teeth in a process called eruption, exchanging our delicate, first set for more burly, permanent teeth. The phenomenon calls to mind the critters that continuously regrow their chompers — for example, sandbar sharks, which sprout tens of thousands of serrated teeth over time; and rabbits, whose incisors grow continuously as they’re worn down by roughage. If fish, bunnies, and kiddos do it in their sleep, then why don’t adults naturally expel their aging molars with shiny, new replacements? And on that note, just how close is science to making such a feat a reality? Please, I feel a toothache coming on…
Off the bat, why don’t we do this already? To better understand what we’re up against in this toothy quest, Dr. Ophir Klein—a professor of orofacial sciences and pediatrics at the University of California, San Francisco—offered Popular Science a brief history lesson.
Long ago, before celebrity veneers, bleach kits, or even dental floss, “animals diverged into invertebrates and vertebrates,” explained Klein. At the time, hundreds of millions of years ago, “the earliest vertebrates [were] sort of reptile-like creatures,” and “mammals came out of that, as did dinosaurs and birds and amphibians.”
[ Related: Why do humans have toenails? Because we’re evolutionary ‘weirdos.’ ]
As fate would have it, Klein explained, “teeth became an integral part of the vertebrate mouth,” but it’s “not exactly clear where they originated,” he added—“whether they started inside the mouth or whether they started as scales, like fish have, that migrated from outside to inside.” Okay, gross! We know these early teeth were simple, and they might’ve been somewhat like the teeth we see in fish today. “If you open a salmon’s mouth, all the teeth are the same and they’re continuously replacing,” explained Klein. “That’s a stem-cell driven process.”
Teeth got more complicated with the emergence of mammals, and eventually, humans. “Rather than having all the teeth within a species being the same, which is called homodont dentition, we have heterodont dentition,” said Klein. With the development of roots, “we have molars and premolars and canines and incisors,” each with specific jobs to do. Although plenty of mammals evolved teeth and tusks that grow continuously, a defensive strategy against wear and tear, humans did not. When our adult teeth arrive, the hard, outer part (enamel) “is permanent and we don’t have the cells anymore to make that.” In other words, somewhere in the evolutionary process, we lost some special progenitor cells necessary to continuously replace teeth.
So, maybe our perma-teeth represent a sort of ancestral tradeoff, in which we exchanged replicability for complexity. In any case, just how close are experts to undermining (or augmenting) this evolutionary development?
An “intermediate step” towards regrowing permanent human teeth could involve a mash-up of synthetic materials and stem cells.
“We’re pretty good at making artificial enamel,” said Klein. “We could, perhaps, use the stem cells that exist inside the tooth to regrow the living part of the tooth and then make a crown—just like we do now for a root canal—to bioengineer a new tooth.” Later on, Klein added, “if we can learn how other animals are able to regrow their teeth from stem cells, we could actually really grow a full, new tooth in vitro.” This isn’t something Klein thinks we’ll see in the next five years, but he said he “would not be shocked if it happened during the next couple of decades, just because things are moving so fast.”
It might sound like sci-fi on first blush, but there’s a whole lot to chew on here.
“The tooth, by itself, is one challenge—getting it to become part of bone, integrating into your jaw, is another aspect of regeneration,” explained Dr. Salvador Nares, a professor in the periodontics department at the University of Illinois, Chicago’s College of Dentistry. Nares spoke to Popular Science on a call along with Dr. Afsar Naqvi, an associate professor in the same department.
“Ultimately, the vision would be to seed, if you will, some sort of capsule or something within the gum tissue, and then let it grow out into a tooth,” said Nares. “However, there are challenges with that, because you have to have a certain morphology [that’s] accurate and complete.” The tooth must be durable and the right shape, plus it has to stay in place and work well with the rest of the body. On top of all that, “it has to stop growing, because if it keeps growing, then that’s called cancer,” said Nares. As for where things stand today, the professor pointed to research on stem cells in erupted baby teeth, which “certain laboratories have been able to utilize to actually manufacture parts of teeth.”
[ Related: Why do we have earlobes? They make no evolutionary sense. ]
Fields such as scaffolds (structures in which we can seed cells), and bio-printing, have likewise progressed, as have gene-editing techniques a la CRISPR.
Naqvi explained, “Gene editing could be one very promising avenue of avoiding the rejection of the organoids, using patients’ own cell types, guiding them into the desired cell type, and using it for the purpose of repair and regeneration.”
There’s also research towards an antibody drug that might, theoretically, spur human tooth growth some day. However, Naqvi—who was not involved in the research—raised concerns about the possible treatment, which would target a gene (USAG-1) that “is not specific to the dental tissues.”
“This gene is expressed in different tissues, including kidneys, where it is expressed at a very high level.” Naqvi added, “What if, beyond tooth [growth], it affects our bone growth in a positive, negative, or whatever manner? It has to be controlled.”
Popular Science emailed lead author Dr. Katsu Takahashi and Kyoto University Hospital for comment on their USAG-1 research, but neither responded.
Anyhow, given the scope of inquiry into tooth regeneration, surely something will work someday, right? Twice-annual cleanings be damned?
“If you were to ask this question five, ten years ago, you’d probably get a different answer in terms of how far we are in the whole process,” said Nares. “But with AI being able to rapidly make calculations and see patterns and things that we don’t see, we would envision that this is going to accelerate discovery and bring this notion of growing teeth or growing other tissues to bear.” Still, Nares cautioned that he doesn’t expect to see it all realized in the next decade; “I think we’re still quite a ways off,” he said, citing safety concerns, trials, regulatory rules, and generally “a lot of work to be done.”
[ Related: What does oil pulling do to your teeth? We asked dentists. ]
In the meantime, Nares spoke of the adult teeth we do have with reverence. “The natural dentition that we were born with is one-of-a-kind,” he explained.
To function through adulthood, permanent teeth need ongoing care, including brushing, flossing, and cleanings. Plus, dental health isn’t limited to teeth and gums; the state of your mouth is an indicator of overall health. Researchers have linked gum disease to Alzheimer’s, diabetes, and other conditions.
“I would caution readers not to give up on their oral hygiene,” Nares said. “All the microbes that cause cavities and more so gum disease, which loosen teeth and [cause them to fall] out, disseminate into other parts of the body and can really create effects away from the mouth.”
He added, “So, definitely keep the mouth clean.”
This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us.
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