A close look at shark teeth found them full of flouride as well as a bit more flexible than previously imagined.Corbis


- The outside of shark teeth is made up of fluoride, the active component of most toothpaste.

- Although human teeth are covered in a different mineral, both shark and human teeth are equally hard.

- Sharks never get cavities, and are able to replace their teeth multiple times throughout their lives.

An in-depth look at shark teeth has found that they contain fluoride, the active ingredient of most toothpaste and dental care mouthwashes.

It helps to explain why sharks are so effective at either tearing or cutting prey. Their teeth are perfectly designed for such tasks, never suffering from cavities, according to the study, recently published in the Journal of Structural Biology,

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While shark teeth contain the mineral fluoroapatite (fluorinated calcium phosphate), the teeth of humans and other mammals contain hydroxyapatite, which is an inorganic constituent also found in bone, explained co-author Matthias Epple.

"In order to make teeth more acid resistant, toothpaste often contains fluoride," Epple, a professor of inorganic chemistry at the University of Duisburg-Essen, told Discovery News. "In the surface of human teeth after brushing, a small amount -- much less than 1 percent -- of hydroxide is exchanged by fluoride."

"In contrast," he added, "(the surface of) shark teeth contains 100 percent fluoride. In principle, sharks should not suffer from caries. As they live in water and as they change their teeth regularly, dental protection should not be a problem for sharks."

For the study, Epple and colleagues Joachim Enax, Oleg Prymak and Dierk Raabe used a multitude of high tech investigative techniques, including scanning electron micrographs, to look at the teeth of two different sharks: the shortfin mako (Isurus oxyrinchus) and the tiger shark (Galeocerdo cuvier). These sharks were chosen because of their different eating habits. Shortfin makos tear the flesh of prey, while tiger sharks cut flesh.

Despite these differences, the teeth of the both sharks were found to have a similar chemical and crystalline composition. In addition to the highly mineralized enamel exterior, the interior of shark teeth was determined to contain a soft material known as "dentin," which contains more protein and is more elastic. Human teeth also have dentin.

Mechanical measurements and tests on the micro and nano-scales determined that shark teeth are not harder than human teeth.

"This finding is surprising, because the mineral fluoroapatite is harder than the mineral hydroxyapatite, so if a tooth were to consist of the mineral alone, a shark tooth would be harder than a human tooth," Epple said. "It seems as if the human tooth makes up for the less hard mineral by the special arrangement of the enamel crystals and the protein matrix, and ends up being as hard as a shark tooth."

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Andrew Gillis, a Dalhousie University biologist, told Discovery News that the new study "nicely shows how we can see beautiful evidence of adaptation from the level of whole organs all the way down to the nano scale in the chemical nature and organization of crystals within tooth tissues. This sort of work really addresses, at the most fundamental level, how teeth have evolved to deal with the remarkable stresses that they endure in nature."

Peter Fratzl, a professor at the Max Planck Institute of Colloids and Interfaces, told Discovery News that he was interested to learn that shark teeth are covered with the mineral fluoroapatite.

"It is quite interesting to see that this repeats a very similar and very recent observation about the crayfish mandible, which also turns out to be covered with fluoroapatite although the two species, shark and crayfish, are totally unrelated, but both living under water."

Fratzl said it's possible that fluoroapatite coatings are much less water soluble than hydroxyapatite and are therefore "more stable in water and more resistant against bacterial attack."

In addition to the unique structure of their teeth, sharks also have the handy ability to replace their teeth several times during their lives. This is not due to cavities, but rather because their teeth sometimes wind up stuck in prey or are otherwise forced out.