Sensitive Teeth? Blame Ancient Fish

By Paige Anderson, CRDH

Dentinal hypersensitivity is one of the most common complaints dental clinicians hear every day. Anti-sensitivity products are big business, and no wonder! Tooth sensitivity can make it painful to eat, drink, or even breathe deeply on a cold day.

Sensitivity can be frustrating for our patients and make our job as clinicians more difficult as well. But who is to blame for all this discomfort and annoyance? According to new research from the University of Chicago, it may all come down to ancient organisms like armored fish and arthropods.

A Bit About the Researchers

Neil Shubin is the author of Your Inner Fish, a book that explores humankind's evolutionary journey from its aquatic origins. He is now the Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy at the University of Chicago. In that role, he heads up many projects exploring ancient organisms, including the new paper, published in Nature, by postdoctoral researcher Yara Haridy, which may have incidentally helped explain why dentin in the teeth is so sensitive.

Nature Loves an Echo

Much of Shubin's work has focused on structures that make repeated appearances throughout the fossil record. For example, the structure of limbs in vertebrates tends to follow what he calls a “one big bone, two smaller bones, lots of little bones” pattern. Similarly, tooth-like structures are scattered throughout the fossil record and persist today in a wide variety of animals and uses.

You may already know that sharkskin gets its armor-like, rough texture from denticles, tiny scale-like structures that are closer in morphology to teeth than scales. It turns out that denticles or denticle-like structures can be found throughout ancient and modern arthropods, as well as other vertebrates and invertebrates. The new paper from UChicago is significant because it turns our previous hypothesis about how teeth developed inside out. Literally.

Searching for the Origin of Teeth

There are two competing theories for how these related structures in teeth and exoskeletons developed. The first is that teeth arose first and adapted over time into the denticles and similar structures we find in exoskeletons. The second, which this paper provides significant new support for, suggests that sensitive external structures may have developed first to create the genetic basis that would later be used in teeth. This hypothesis also provides a potential explanation for why our teeth are so sensitive.

Why Sensitivity Was Good for Survival

Early armored fish and arthropods lived in a highly predatory world. The more sensory input these animals could access, the more likely they would be able to evade predation. Armor to help make them more durable would have been equally advantageous, but losing sensitivity to the environment would have undercut that armor's value. It turns out that, like modern horseshoe crabs, many ancient animals may have had the best of both worlds by developing an exoskeleton that helped them sense the world around them. Specifically, by developing odontodes, tiny bumps on an exoskeleton filled with dentin.

It's All About the Tubules

This sensitive armor is structured just like our own teeth, with tubules providing innervation from a pulp chamber. Like our teeth, fluid within tubules of exoskeletal structures can expand and contract to stimulate the nerve fibers inside, providing detailed feedback about changes in the environment. However, there are differences in the details. For example, dentin itself (both in teeth and external structures) is unique to vertebrates to the best of our knowledge, and the tubules in arthropod exoskeletons are structurally distinct. This finding also suggests these structures are an example of parallel evolution that may have arisen along multiple evolutionary branches.

The main focus of Haridy's research was on Anatolepis, which had previously been identified as the earliest known vertebrate. In the course of her research, she demonstrated that the structure of its tissues corresponds to the structure of an arthropod exoskeleton rather than that of an armored fish, changing our understanding of when vertebrates first appeared. The important thing is that innervated tubules developed long before the appearance of teeth, as far as we can tell, making this a likely example of evolution's tendency to repurpose what has already worked.

What Does All This Mean for Our Teeth?

Over time, the genetic playbook that gave rise to odontodes and denticles likely became the blueprint for the structure of teeth, right from their earliest appearance millions of years ago. This finding may also help to explain the increasing body of evidence that reveals the close interrelationship between our teeth and other neurological structures, as they may have originally been one of our major sensory organs.

Dentinal tubules may seem like the bane of our existence, both for patients suffering with hypersensitivity and for clinicians who want to help them live more comfortably. However, as significant as this sensitivity was for our ancient ancestors, it's just as important for our oral health. While ancient fish used this sensory capability to detect predators in their environment, we use it to prevent major oral health catastrophes. Feedback from our dentin guides us in chewing hard enough and when to hold back. It lets us know that we've found something in our food that's too hard and may not be digestible. It also alerts us to problems like caries and fractures.

Can This Information Help Our Patients?

Knowing this may not help your patients feel better about their pain, but it may help add a little context. It also helps us to understand why current anti-sensitivity products work. As you may already know, there are a variety of strategies for minimizing sensitivity. Finding the right product for each patient may still take some trial and error to determine what works best for them, but this research could potentially lead to novel approaches in the future.

This information may also help us provide more comfortable cleanings and other treatments. If nothing else, at least you can let your patients know where to point their frustration when they can't eat ice cream without wincing!

Author: Paige Anderson is a certified registered dental hygienist with eight years of clinical experience and an English degree. She blends her two areas of expertise to create resources for dental providers so they can change lives by giving their patients the highest possible standard of care.