What are Ectopic Teeth or "Shark Teeth?
What do human teeth and shark teeth have in common? Lots. As a former oceanographer turned orthodontist, this subject could not have been more interesting to write about! I spent the first four years of graduate school studying Marine Biology at the Scripps Institution of Oceanography in San Diego. This blog goes full circle from teeth back to fish.
To understand the connection, we first need to go back in time to the origins of teeth themselves. Teeth are derived from the same tissues that make skin, hair and nails. These are called the ectoderm, or outer layer. When people have inherited diseases that affect teeth, they very often have disorders that affect hair and nails as well. In early fish without teeth, the ectoderm made outer plates of armor that protected the fish from prey. These plates were literally the fish’s “armor” and were made out of a very hard substance. The same biological machinery that makes these plates also makes the structures of your teeth. It’s no coincidence that the hardest substance in your body is the enamel of your teeth. This is our “armor” against foreign invaders called bacteria. As evolution developed more creative innovations, these pieces of armor developed into more specialized chewing structures like teeth.
Plates of armor on the earliest fish. These structures were made in the same way as your teeth!
These plates formed in the same way your teeth form, with a very hard outer layer called enamel, and an inner layer called dentin. Over evolution, these plates developed into scales on fish. Special forms of scales, made out of enamel, bone and dentin became teeth. The origins of teeth are still preserved in sharks today. Shark teeth form from modified scales near the tongue, and move outward along the jaw until they are eventually dislodged. They form continuously in rows.
A Megadont jaw from the CA Academy of the Sciences. These guys were up to three times the size of a great white! Notice the rows of continuously forming massive teeth.
Human teeth today retain some of these forms. Enamel, our armor, is the hardest substance in the body. Our teeth also form in rows, with the adult teeth budding off from the succedaneous teeth or baby teeth. Unlike sharks, which have continuously forming rows of teeth, humans have only two sets of teeth. These consist of 20 baby teeth and 32 adult teeth. For this reason, if you’re born missing a baby tooth, you will always be missing the same adult tooth. Sometimes the succedaneous tooth does not dissolve away in time for the adult tooth to erupt into place. This results in an ectopic tooth, or double set of teeth. Not unlike the multiple rows of shark teeth. Cool!
In some people, a genetic mutation in the tooth forming machinery can result in continuously forming teeth, with tooth formation gone wild, similar to a shark. This mutation is called cleidocranial dysplasia. Below is a picture of someone with this condition. In fact, if you watch the TV show Stranger Things, the character Dustin has this condition in real life! Sometimes you develop with lots of extra teeth, other times you are missing them, or they take longer to erupt. That’s why he goes by the nickname ‘toothless’! Fortunately it can be treated through careful collaboration between orthodontists, surgeons and pediatric dentists.
A patient with multiple rows of extra teeth due to a disorder called Cleidocranial dysplasia. Dustin in Stranger Things has this condition in real life!