Dentinogenesis and the dentin-pulp complex

In total, an adult has 32 teeth, and they do everything from chew on gum, to rip apart a delicious burger.

Each tooth contains both enamel and dentin.

So, the physical properties of enamel and dentin are complementary, there’s the extremely hard and unyielding enamel on the outside and the dentin foundation below which is somewhat flexible and absorbs stress from chewing.

By volume, the majority of a tooth is made of dentin, which is a hard, whitish-yellow mineralized tissue.

In fact, a good example of how dentin looks is ivory in elephant tusks, which are solid dentin all the way through.

So, if we cut a human tooth in half, we see a number of layers.

From the outside to the inside, the tooth crown is covered with translucent-ish enamel.

Next, there’s the dentinoenamel junction, which is where enamel and dentin meet.

Right under that is a thin layer of mantle dentin.

Next follows a layer of primary dentin.

As we move towards the middle, primary dentin becomes somewhat softer, and is called secondary dentin.

Mantle dentin and primary dentin are made before we’re born, whereas secondary dentin grows continuously to support the primary dentin.

The secondary dentin grows from the soft, unmineralized dentin, called predentin, which is just above the pulp.

Additionally, in some places, there’s tertiary dentin which is a sort of band-aid that helps fix up injured teeth.

It is also important to know that the tooth is covered by enamel only on its crown, the root is covered by a slightly softer substance called cementum.

The process of creating dentin is called dentinogenesis.

For primary teeth, it mostly happens around the 14th week of fetal development, and for permanent teeth, it mostly happens when an infant is about 3 months old.

After that, the process continues to happen, but at a very slow pace.

Dentinogenesis is performed by odontoblasts, and it’s done from the outside inwards.

So the process begins with creation of the mantle dentin, which is the outermost layer of dentin, right under the enamel.

First, odontoblasts secrete a soft organic matrix made out of proteins, which is called predentin.

Fibroblasts assist by creating a mesh of thick collagen fibers, called Korff’s fibers, which become the framework for dentinogenesis.

Think of these as rebar, which help form a very strong wall.

Slowly, the odontoblasts and fibroblasts move towards the center of the tooth.

Odontoblasts do this by growing a protrusion from themselves, which stays in place, embedded within the matrix, while the rest of the cell moves toward the center of the tooth - towards the pulp.

These hair like protrusions are called odontogenic processes, and they secrete calcium hydroxyapatite, which forms crystals and mineralizes the predentin.