There are multiple structures in our mouths that occur in repetitive patterns, such as teeth and taste buds. Now, researchers working with little fish called cichlids have discovered how those two structures share a similar origin, despite being so different in adults. Hard teeth and soft taste buds alike come from the same kind of precursor tissue in the jaws of embryonic fish. And the same genes and pathways involved in their development in fish could potentially play similar roles in mice as well. The findings, published in Proceedings of the National Academy of Sciences, might explain how fish (unlike humans) grow replacement teeth throughout their lives. Most mammals get just two sets to last a lifetime.
Researchers have spent decades trying to understand how the epithelium (the outer tissue layer) is fated to form so many different structures. But the genetic and developmental underpinnings of tooth and taste bud patterning and density have remained elusive. What determines the fate of these cells?
To investigate, Georgia Tech's Ryan Bloomquist and colleagues examined how different structures arise from the same epithelial tissues in the embryos of multiple Lake Malawi cichlid species. While our taste buds are on the tongue and the soft part of the palate, the teeth and taste buds of many aquatic animals are located in rows next to each other. The plankton-eating cichlids (Cynotilapia afra) have only a few taste buds and few, widely spaced teeth. They rely on their vision to find food, which they typically swallow whole. Meanwhile, the jaws of algae-eating ones (Pseudotropheus elongates) are packed with hundreds, even thousands of teeth, and a lot of taste buds too. That’s because these fish have to snip and scrape off algae from rocky surfaces, and having more taste buds helps them distinguish between what’s food and what’s not.
After breeding the two different species, the researchers saw a lot variation in teeth and taste bud density in the 382 hybrid offspring. So they examined the differences among the second generation to better understand the underlying genetics. "We were able to map the regions of the genome that control a positive correlation between the densities of each of these structures," study co-author Todd Streelman of Georgia Tech says in a statement.
Then the team manipulated the development of teeth and taste buds in the jaws of embryos several days post-fertilization. When they boosted the growth of taste buds, in one case, this came at the expense of teeth. "There appear to be developmental switches that will shift the fate of the common epithelial cells to either dental or sensory structures," Streelman explains. The two look similar during the early stages of development, until later on, when the tooth starts to form enamel and dentine.
An additional examination of dental differentiation in mice revealed that the structures responsible for growing new teeth appear to stay active longer than previously assumed. With more research, we may discover that tooth regeneration is possible in people.
Image in text: Juvenile Lake Malawi cichlids. Rob Felt/Georgia Tech