Mammals usually have one set of baby teeth (also known as milk teeth) and after those are lost, we have one set of adult teeth and these are not replaced if they are lost. This condition is called “monophyodont.” Reptiles and sharks, however constantly replace their teeth. This condition is called “polyphyodont.” Alligators and crocodiles are among one group of reptiles that replace their teeth throughout their lives, and because the development of these creatures has been studied to some extent, it is known that the ability of these creatures to replace their teeth on a regular basis results from a resident stem cell population. Studying that stem cell population more closely might provide clues for tooth replacement in humans.
A research team led by scientists at the Keck School of Medicine professor of pathology Cheng-Ming Chuong at the University of Southern California. Dr. Chuong and his collaborators from around the world have identified unique cellular and molecular mechanisms behind tooth renewals in American alligators.
Chuong explained, “Humans naturally have only two sets of teeth – baby teeth and adult teeth. Ultimately, we want to identify stem cells that can be used as a resource to stimulate tooth renewal in adult humans who have lost teeth. But, to do that, we must first understand how they renew in other animals and why they stop in people.”
Even though humans cannot replace their adult teeth, a tissue called the dental lamina remains, which is known to be crucial for tooth development.
Why are alligators potentially a good model system for tooth replacement in mammals? First author of this study, Ping Wu, explained it this way, “Alligator teeth are implanted in sockets of the dental bone, like human teeth. They have 80 teeth, each of which can be replaced up to 50 times over their lifetime, making them the ideal model for comparison to human teeth.”
Through the use of microscopic imaging techniques, Chuong and others found that each alligator tooth is a complex unit of three components: a functional tooth, a replacement tooth, and the dental lamina, all other which are at different developmental stages.
The tooth units are built to enable a smooth transition from dislodgement of the functional, mature tooth to replacement with a new tooth. Further imaging studies strongly suggested that the dental lamina contains a stem cell population from which new replacement teeth develop.
“Stem cells divide more slowly than other cells, said co-author Randall B. Widelitz, who serves as an associate professor of pathology at USC. Widelitz continued, “The cells in the alligator’s dental lamina behaved like we would expect stem cells to behave. In the future, we hope to isolate those cells from the dental lamina to see whether we can use them to regenerate teeth in the lab.”
The researchers also intend to learn what molecular networks are involved in repetitive renewal and hope to apply the principles to regenerative medicine in the future.
The authors also noted that novel cellular mechanisms are used during the development of the tooth unit. Also, unique molecular signaling speeds growth of replacement teeth when functional teeth are lost.