Why is tooth enamel so hard? Tooth enamel is so hard because it is made of about 96% minerals, which is more than anything else in the human body.
Enamel is the thin layer that sits on top of our teeth. A tooth is made up of layers, and the job of enamel is to protect the softer inner layers. The very center of the tooth is called the pulp and it contains nerves, blood vessels, and connective tissue. If you have any tooth pain, or the pain you feel when you have a tooth extracted, comes from this area. Then we have the cementum. It covers the root of the tooth and its job is to cement the tooth firmly into the jaw. Hence its name. It uses periodontal ligaments to anchor the tooth. Then there is dentin. Dentin is a yellowish layer just underneath the enamel. It makes up most of the tooth you can see and supports the enamel. It contains a lot of microscopic tubes that allow hot and cold sensations to travel to the nerve. And last, on the very surface, is the enamel The very hard protective cover that shields the tooth from bacteria, decay, and damage.
The enamel on the tooth is produced when the tooth develops and pushes out through the gum. As humans, we have two sets of teeth, milk and adult, and each time a new tooth grows in, it gets covered in enamel. The enamel is produced by cells called ameloblasts. These cells interact with the cells that produce dentin and are triggered to start producing enamel. They secrete a protein matrix to lay the enamel down on. Once they have done this, they start depositing hydroxyapatite crystals on this matrix. The ameloblast cells move slowly outwards as they lay down the enamel. When this is done, the ameloblasts secrete enzymes that eat up the protein matrix and the dead proteins and water are pumped out, leaving the enamel. When the tooth moves up through the gum, the enamel layer gets carried up and away from the cells that made it, which means, because all of the ameloblasts have left the enamel, it is not living, and there is no way for it to fix itself or replenish. The enamel we have on our teeth is all we will ever have, so we need to take care of it.
So, why is enamel so hard? It is hard because it is mostly mineral, and also because of the type of mineral and the way that mineral is arranged. Enamel is made of a mineral called hydroxyapatite, which is a crystalline material made up of calcium, phosphate, and hydroxyl ions. The fact that it is crystalline is important. As a hint, diamond is also crystalline. The atoms inside the mineral are arranged in a highly ordered, repeating three dimensional structure. The negatively charged phosphate is very strongly attracted to the positively charged calcium and it takes a lot of energy to break that structure. On top of this, the hydroxyapatite crystals are formed into very long and densely packed together structures called enamel rods and they are arranged in different directions to distribute force.
The mineral apatite is incredibly hard. There is a 1 to 10 scale that measures the hardness of materials called the Mohs scale. At 1 is talc. It is so soft that you can crumble it with your hands. And at 10 is diamond. Apatite comes in at 5 on this scale.
Enamel is very strong, but it is also brittle. Bone is only about 70% mineral, so it is not as hard as enamel, but it has a lot of fibrous collagen which makes it flexible. We need bones that can bend a little so they can put up with the impact stresses we put them through. The enamel in our teeth is not flexible, but the dentin it sits on it. This allows the enamel to compress and move slightly so that it doesn’t shatter.
Because the cells that produce the enamel are no longer on the surface of the teeth, once it is damaged or worn away, there is nothing we can do. A lot of animals get around this by just making more teeth, or having teeth that never stop growing. However, a group of scientists are working on a method to help our teeth regrow their enamel. The scientists have come up with a way to coat the teeth in an artificial protein matrix. It mimics the matrix that ameloblasts use when enamel originally develops. The coating encourages new apatite crystals to grow in alignment with the crystals already present in the tooth, recreating some of enamel’s highly organized structure. It has been tested on teeth out of a person, and animals, but it is only just about to start human testing. It could be a gamechanger. And this is what I learned today.
Sources
https://www.kin.es/en/que-le-da-dureza-al-esmalte-de-los-dientes
https://en.wikipedia.org/wiki/Tooth_enamel
https://www.bridgforddental.co.uk/latest-news/why-havent-we-found-a-way-to-restore-enamel