Just when you think it's safe to go back to the dentist, out comes another tooth. This is the unfortunate reality for many Americans, with only 34% of those aged 40-64 possessing all of their own teeth. But a new study may bring us a step closer to human tooth regeneration, by shedding light on how sharks regrow their pearly whites.
Sharks have the ability to continuously regenerate their teeth.
Sharks can have up to 3,000 teeth at any one time, spread over multiple rows. Unlike our teeth, sharks' teeth are embedded in the gums rather than the jaw.
Researchers have long known that sharks have the ability to continuously regrow their teeth; they lose at least 30,000 teeth over a lifetime, but each one lost can be individually regrown over a period of days or months.
The genetic mechanisms underlying this process, however, have been unclear.
Now, study leader Dr. Gareth Fraser, from the Department of Animal and Plant Sciences at the University of Sheffield in the UK, and colleagues have identified a network of genes that are responsible for tooth development and lifelong tooth regeneration in sharks.
Because humans possess the same genes, the team says the discovery could help develop new treatments for human tooth loss.
They publish their findings in the journal Developmental Biology.
Humans possess the same 'regenerative program' as sharks
To reach their findings, Dr. Fraser assessed gene expression in the early formation of shark teeth by analyzing catshark embryos.
The team identified expression patterns from several genes that led to the formation of the dental lamina - a set of epithelial cells - in the catsharks. The dental lamina was found to drive tooth development and continuous tooth regeneration in the sharks.
Interestingly, humans also possess the same genes that drive formation of dental lamina; the cells are responsible for the growth of baby and adult teeth. Once a person's adult teeth are fully formed, however, dental lamina is lost.
In the US, untreated tooth decay and dental caries are the primary causes of tooth loss, or edentulism, in adults.
According to the Centers for Disease Control and Prevention (CDC), in 2011-2012, around 52% of American adults aged 20-64 had lost at least one of their permanent teeth due to dental disease.
These findings, however, indicate that it may one day be possible to trigger tooth regeneration in humans.
"What it means is because we have the same genes to make teeth, we also have a regenerative program," Dr. Fraser told The Daily Mail.
"The point is at some level it's not so far-fetched we can use and re-utilize what nature has provided," he added.
"At some point during adolescence, we lose the cells, they break down. There is a possibility we can re-invigorate them with future dental therapies."
Genes likely responsible for all vertebrate teeth, including humans
Dr. Fraser and colleagues say their findings indicate that sharks have held on to their tooth-making genes since the beginning of their evolutionary history around 450 million years ago, and that the genes are likely responsible for the development of all vertebrate teeth, from sharks to mammals - including humans.
In sharks, these tooth-making genes are likely to have evolved over time to trigger a regeneration process when a tooth is lost - a way of maintaining their hunting abilities. In humans, however, they say the ability to use these tooth-making genes has faded out over time.
Commenting on the findings, Dr. Fraser says:
"We know that sharks are fearsome predators and one of the main reasons they are so successful at hunting prey is because of their rows of backward pointing, razor-sharp teeth that regenerate rapidly throughout their lifetime, and so are replaced before decay.
The Jaws films taught us that it's not always safe to go into the water, but this study shows that perhaps we need to in order to develop therapies that might help humans with tooth loss."