Enamel (anatomy)

Enamel is the hardest substance in the human body. It covers the outermost, visible part of teeth and protects the inner portions of the tooth. Although it is very hard, it is still susceptible to breakage. Dental enamel can be become cracked or chipped and experience decay and normal wear. Damaged enamel is what leads to cavities, or decay, which exposes the softer, more sensitive interior parts.

Tooth Anatomy

Tooth enamel is solid tissue that is calcified, or hardened, by the deposit of calcium salts. It is not living tissue, and cannot regenerate or repair itself. Enamel covers the anatomical crown, or visible portion of the tooth. Inside crowns are the softer parts of teeth, including the dentin, pulp chamber, and root canal. The dentin is a hard layer—though not as hard as enamel—that provides additional protection to the nerve and blood vessel-filled pulp chamber and root canal.

At the point where the tooth disappears into the gingiva, or gums, the outermost layer of the tooth becomes cementum. This hard substance under the gums covers the tooth root and connects the tooth to the periodontal ligament, which holds the tooth in place in its socket.

Properties of Enamel

Enamel is better than any known man-made material at maintaining its form when subjected to abuse. While it will crack, it does so in specific ways that normally prevent it from cracking straight through to the dentin and other internal structures of the tooth. It also remains steadfastly attached to the dentin, resisting the tendency to separate from the underlying structure as many man-made materials would. Dentists can repair enamel with gold, ceramic, or other composite materials, but none of these will remain as solid and firmly attached to the inside parts of the tooth as enamel will.

The enamel in teeth is composed of many tiny crystals that form fibers that are one thousand times thinner than a human hair. Many of these fibers are combined to form rods, which are lined up precisely to form the shape of a tooth on the outside while extending to different depths in the dentin inside. This allows the enamel to form a uniquely shaped protective outer surface for each tooth and to cling tightly to the inside portions that it protects.

How Enamel Forms

Dental enamel is formed through a unique process that starts with living cells called ameloblasts. Ameloblasts begin their work while a mammal is still in utero. They make a specific form of protein, amelogenin, which is the most prevalent form of protein in dental enamel. Other forms of protein that are present include ameloblastin, enamelin, and enamel proteinases such as enamelysin, kallikrein-4, and designated enamel matricx serine proteinase 1.

Ameloblasts release calcium phosphate, which is the same material some organisms use to make shells and pearls. The calcium phosphate creates a crystal framework called a hydroxyapatite. Some researchers believe the amelogenin protein helps to shape the crystals that form the hydroxyapatite. These crystals ultimately become the fiber-packed rods that make up the enamel. All of this takes place below the gums as the tooth buds are forming in the embryonic and infant mammals. Once the tooth breaks through the gum, these ameloblasts die and the enamel-making process is ended.

Bone and other mineralized body materials maintain about 20 percent organic material after they are fully formed; this organic material allows regeneration and self-repair. Dental enamel, however, is less than 1 percent organic matter and cannot regenerate. This can become a problem when tooth enamel is damaged by a blow, by biting something hard, by grinding, or some other physical force. Dental enamel can also be damaged by the acids in many foods, and by bacteria that feed on small food particles in and around the teeth, resulting in decay. Dental cavities, or holes in the enamel of a tooth, result from this damage.

New Discoveries

Researchers classify enamel as an evolutionary success. Enamel formed on the teeth of dinosaurs and prehistoric sharks, providing them with the ability to eat from a wide range of food sources. This adaptation was so successful that subsequent species maintained the trait during the evolutionary process.

It has long been known that some sharks have dermal denticles, which are scales covered with enamel that form a streamlined coating over the shark and help it move through the water with great speed and efficiency. Additional research has determined that many ancient fish and land vertebrates had scales that were covered with an enamel-like substance known as ganoine. This was confirmed by the presence of the genes that form ganoine in the skin but not in the teeth. Scientists find this insight into the changing function of enamel and the way it migrated to a different body part and function to be important in studying evolution and in determining new ways to improve dental health.

In 2009, researchers at Oregon State University noticed that some mice did not grow enamel coverings on their teeth. Their investigation revealed that these mice were born without a specific gene, and determined that this gene is responsible for the process that forms enamel. This gene, Ctip2, is responsible for forming the ameloblasts that are crucial to the development of enamel. Researchers are optimistic that this discovery can lead to the development of ways to turn the enamel-formation process back on and allow for natural repair of teeth without the need for drilling and replacement with man-made materials.

Bibliography

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