Tooth histology (enamel, dentin, cementum, and pulp): Dental assisting

Tooth histology, or the study of the microscopic composition, structure, and function of teeth, focuses on four dental tissues including enamel, cementum, dentin, and pulp. As a dental assistant, knowledge of these tissues is foundational to your understanding of the normal function of teeth and how teeth respond to wear and decay; and is essential for providing safe and effective patient care.

Okay, let's begin with an overview of the tooth tissues. The outermost layer is the enamel. It covers the anatomical crown, which is the portion of the tooth that extends from the very top of the tooth to the beginning of the root, or the portion of the tooth that’s imbedded in the bony alveolar process of the jaw. As a side note, the anatomical crown differs from the clinical crown, which is the part of the tooth we can see above the gums. As far as appearance goes, enamel is translucent, and ranges in color from yellow to grayish white. It’s the hardest substance in the body, and provides a strong surface for crushing, grinding, and chewing food.

Now, if we move down the tooth where the crown meets the root, we find an anatomical border called the cementoenamel junction, or CEJ. At this point, enamel ends, and the next type of tissue, the cementum, begins. Cementum is a hard, mineralized connective tissue, similar to the composition of the bone that covers the root of the tooth. It’s easy to distinguish the cementum from the enamel due to its darker hue of yellow and lack of luster.

Next, as we move inward toward the center of the tooth, we encounter dentin, which makes up the main portion of the tooth structure. The dentin is covered by enamel on the crown and by cementum on the root. It’s a strong, mineralized tissue that’s harder than bone. The tissue is somewhat transparent and has a light-yellow color that tends to darken with age.

Finally, the innermost tissue is the pulp, which extends almost the entire length of the tooth from crown to root. Unlike enamel, cementum, and dentin which are hard tissues, pulp is a soft tissue made of blood vessels, nerves, and connective tissue.

Okay, next let’s take a closer look at the structure of enamel and dentin. As a highly mineralized and extremely hard substance, enamel can withstand forces of up to 100,000 pounds per square inch! At the same time, enamel is also brittle, so it can chip or crack from biting down on hard objects like ice or popcorn kernels; chronic teeth grinding; or from physical trauma like sports injuries. Enamel can also be weakened by acids produced by bacteria, so poor oral hygiene is another important cause of damaged enamel. Notably, once enamel is damaged, it can’t be repaired, though it can be strengthened through remineralization.

Enamel serves as the protective covering for dentin in the crown. It’s mostly made of inorganic matter, especially calcium-rich hydroxyapatite crystals, along with a small amount of organic matter. Microscopically, it's made up of millions of densely packed calcified enamel prisms, also known as enamel rods, which extend from the surface of the tooth to the dentinoenamel junction, or DEJ for short, which is the boundary between the enamel and the underlying dentin. The rods are grouped in rows that sit perpendicular to the long axis of the tooth. An easy way to picture how the enamel rods are situated on the tooth is to imagine you’re holding a bunch of plastic straws in your hand and placing them upright on a flat table, so the ends of all the straws are flush against the tabletop. The straws are like enamel rods, and the table is like the surface of the tooth.

On the flipside, dentin is a hard, mineralized tissue. It’s slightly softer and more elastic than enamel since it has more organic matter than inorganic minerals, so it provides a cushion against the forces of biting and chewing. The dentin is penetrated throughout by microscopic fluid-filled tubes called dentinal tubules that radiate outward from the pulp beneath it. These tubes have two important functions: first, they support the vitality of the dentin by delivering oxygen, fluid, and nutrients; and second, they contain dentinal fibers that connect to nerve endings within the pulp. These fibers join the dentin directly to enamel rods and help transmit sensory information, including pressure, so you know when you’re chewing, as well as heat, cold, and pain. Another feature of dentin is its ability to repair itself through the formation of tertiary dentin, also known as reparative dentin in situations like physical trauma or excessive wear.