Dentin is one of the four major elements that make up the teeth; the other elements are cementum, enamel, and pulp. Above the gumline, dentin is the layer of the tooth directly under the enamel. Below the gumline, at the root of the tooth, dentin lies directly beneath the cementum. Both above and below the gumline, dentin completely encompasses the tooth’s pulp. Though slightly softer than enamel, dentin is hard and yellowish in color, a hue that may be visible through the enamel in some cases. Made primarily of calcium and phosphate ions, along with inorganic matter and water, dentin is formed throughout the lifespan, and its formation, called dentinogenesis, can also be triggered by outside stimuli like tooth decay. Unlike enamel, dentin is a living tissue that grows and can repair itself. This means that it is also receptive to pain and sensation.
Dentin makes up the majority of the mass of a tooth, providing support to the tooth’s enamel and protecting the tooth’s pulp from infection and other damage. Dentin contains cellular material that makes it integral to the vitality of the tooth; while it is mineralized, it is by no means dead. The properties of dentin vary depending on the location in the tooth, with each type of dentin serving a highly specialized function.
Dental pulp is connected to the outer shell of dental enamel or cementum by microscopic tunnels, called dentinal tubules, that run throughout the dentin. These dentinal tubules facilitate the dentin regeneration process, as cellular activity in the layer of pulp can travel through the tubules to generate new dentin, which is then mineralized, or hardened. These tubules are part of what makes dentin a living tissue, and because of these dentinal tubules and the nerves they contain, dentin is porous and also susceptible to pain. Dentin is primarily made up of inorganic mineral material, like calcium phosphate, and also contains organic materials like collagen and protein. It is more susceptible to decay than enamel, but its flexibility also provides good support for enamel, which is more brittle.
The outermost layer of dentin, immediately beneath the enamel, is the mantle dentin layer. Rich with collagen and dentinal tubules, this layer is particularly susceptible to decay. In the root of the tooth, there are two layers of root dentin. Under these layers of dentin is a layer of predentin, which is the source of lifelong dentinogenesis. This predentin layer is not yet mineralized, or hardened, and is made up primarily of collagen and proteins. Beneath these layers is the tooth’s pulp. Primary dentin is the first layer of dentin to form, while secondary dentin forms once the tooth’s root has completely developed. Tertiary dentin is created as a biological response to trauma or other stimulus, such as a cavity or erosion of the tooth. As humans age, the layer of secondary dentin continues to grow, causing the pulp chamber inside the tooth to shrink. Because young people have a thinner layer of dentin, dentists must exercise extreme caution when preparing children’s teeth for cavity restorations, so that the pulp inside the teeth is not exposed. In adults, if a dental cavity has passed through the enamel and into the dentin, the cavity can no longer be reversed and must be treated with a restoration.