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 stresses 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.
This mineralization is like concrete that is settles around the rebar to make the mantle
dentin layer very strong but also flexible.
The end result is mantle dentin, which has numerous tiny canals, which are the dentin
tubules, each containing an odontoblastic process.
The next layer of dentin is called the primary dentin and it’s the thickest layer.
Once odontoblasts have finished making the mantle dentin layer they get bigger and continue
building the primary dentin.
Meanwhile the fibroblasts head towards the pulp to catch a rest.
So compared to mantle dentin, the primary dentin contains less collagen and more mineral
content.
Odontoblasts continue moving towards the center, so the primary dentin forms from the outside
in.
Next up is the thin layer of secondary dentin.
Secondary dentin resembles the primary dentin in structure, but the main difference is that
it’s formed after root formation is complete, and occurs at a slow rate, throughout our
lifetime.
Its purpose is to maintain the tooth strength for as long as possible.
After the secondary dentin, there’s a thin layer called the predentin which borders the
pulp.
The predentin is a layer of organic matrix that hasn’t yet mineralized.
And as it mineralizes, it becomes secondary dentin.
Unlike enamel, dentin keeps growing, regenerating, and repairing itself throughout our lifetime.
So when a tooth is injured, odontoblasts residing in the pulp move out and head to the site
of injury.
Once they get there, they start rapidly producing what’s called tertiary dentin and mineralizing
it immediately.
Tertiary dentin is used for damage control, in order to strengthen the damaged area.
Think of the injury as a crack in a dam - it needs an injection of concrete immediately.
That’s why tertiary dentin is found at sites like dental caries, or dental attrition, caused
by excessive wear of the teeth.
All right, as a quick recap.
Dentin is a mineralized structure that makes the bulk of our teeth.
It is made through a process of dentinogenesis, by odontoblasts.
Odontoblasts secrete the matrix, which they mineralize by depositing calcium hydroxyapatite.
The outermost layer is mantle dentin layer, which is rich in minerals and collagen.
It is followed by primary and secondary dentin layers.
The innermost layer is soft, unmineralized predentin.
Tertiary dentin can form as a response to injury.