In the mouth, the bone holding the bottom row of teeth is the mandible, and the bone
holding the top row of teeth is the maxilla.
The mandible and maxilla - like most bones in the human body - have a core of less dense
cancellous bone, wrapped in an outer layer of more dense alveolar bone.
The part of the mandible and maxilla that are in the mouth are covered by the gums.
And the teeth rest in bony sockets within the mandible and maxilla and are surrounded
by the gums.
Usually, at around six months of age, infants will have their first pair of deciduous teeth,
also called milk teeth or baby teeth, erupt through the gums and into the mouth.
New deciduous teeth continue to erupt every one to two months until the baby is around
two years old and has a full set of deciduous teeth.
Since the mouth is symmetrically divided down the middle, let’s look at just one side
of it.
The teeth that pair up vertically have the same names.
Starting at the front of the mouth, in the center, there are the central incisors and
then the lateral incisors.
These teeth are shaped like chisels and are good at biting off small bits of food.
Next, are the canines, which got their name from being the same teeth that are extra long
and sharp in dogs.
They’re also called the cuspids, which comes from the word cusp, meaning point.
They’re good for puncturing holes, tearing things, and are an essential part of any vampire
costume.
Both incisors and canines typically have one root each.
Then, there are the first molars and the second molars.
Molars usually have four to five cusps, and are great for crushing and grinding food.
That makes 20 deciduous teeth in total.
As the permanent teeth grow in the bone below the deciduous teeth throughout childhood,
the roots of the deciduous teeth begin to get absorbed into the gums.
This loosens them and allows them to fall out, making room for the permanent teeth to
take their places.
These permanent teeth are also called the adult teeth or succedaneous teeth, which means
to succeed or follow after.
Children usually begin to lose teeth starting around age six and the process finishes around
age 12.
As the permanent teeth move in, a few more types of teeth begin to show up.
Starting from the center of the mouth again, the permanent teeth include the central and
lateral incisors, and then the canines, just like before.
But then, the teeth that replace the deciduous molars are called the first and second premolars
or bicuspids.
As the name bicuspid suggests, they have two cusps each, and each has one root.
Just like the molars, these teeth are great for crushing and grinding food.
As the jaw bones grow throughout adolescence, they make room for a few new teeth toward
the back of the mouth.
The first permanent molars usually appear around age six and the second permanent molars
around age 12.
The third permanent molars, or wisdom teeth, usually don’t erupt until around the ages
of 17 to 25, but sometimes never erupt – remaining impacted in the jaw – or simply aren’t
present at all.
While the upper molars typically have three roots, the bottom molars only have two.
All together, that makes 32 permanent teeth.
Now, let's build a model of a tooth and its surrounding structures.
Each bony socket, is lined on the inside by a periodontal ligament.
Protecting the alveolus on the outside is a layer of soft, supportive tissue called
the gingiva, or gums, that sits on top of the bone and surrounds the teeth.
The gums meet the tooth at the cementoenamel junction - where the cementum and enamel come
together.
The cemento-enamel junction is called the cervical area of the tooth.
Cervical means “neck”, so you can think of this area as the “neck” of the tooth.
The gums surround it like a shirt collar.
The tooth itself can be roughly divided into a few parts.
The first part is the root, and it sits within the bone.
Teeth can either have one or multiple roots, and this can vary from person to person.
The root is covered by cementum, which is a bone-like substance that the periodontal
ligament’s fibers attach to.
Next, there’s the neck, which is the transition between the root portion covered by bone and
the crown.
The crown is the visible part of the tooth that protrudes from the gingiva, and it’s
covered in enamel, which has such a high mineral content that it’s the hardest substance
in the human body.
When the teeth are developing, enamel is made before the tooth erupts into the mouth by
a group of cells called the ameloblasts that die once the tooth erupts - meaning that the
teeth lose the ability to make more enamel forever.
About 1% of enamel’s weight is from an organic matrix of proteins, and about 96% of it is
mineralized by a substance similar to hydroxyapatite, which is the substance that gives bone much
of its hardness.
As a comparison, dentin is around 70% mineralized, and cementum is around 65%.
The portion of gingiva that sticks up and is not anchored to the tooth is sometimes
called the free gingiva, and the space between the free gingiva and the crown is called the
gingival crevice or gingival sulcus.
This space has a tiny bit of gingival crevicular fluid which contains various immune proteins
and cells like neutrophils, complement proteins, and antibodies.
This fluid helps protect against overgrowth of pathogenic bacteria.
You see, in a healthy mouth, there’s a balance of commensal bacteria that compete with each
other, and they are all kept in check by immune factors like the gingival crevicular fluid
in the mouth.
Now, let’s fill the tooth in from the inside out.
Blood vessels and nerves come from the jaw bones, and enter the center of the root through
a narrow passage, called the apical foramen.
From there, they travel through the root canal and enter the soft center of the tooth, called
the pulp, where they provide nutrition and sensation.
The outer wall of the pulp contains odontoblasts which are cells that secrete dentin.
The odontoblasts have long processes, or arms, that lay within tiny tubules in the dentin
- like an octopus sticking its arms through a block of cement.
These arms allow them to monitor and repair the dentin as needed throughout life, unlike
enamel.
These tubules are important for sensory nerves as well, which use them to travel through
the dentin from the pulp to the dentin-enamel junction.
The nerves that provide the sensations of touch, pain, and changes in temperature to
the teeth are all branches of the trigeminal nerve, also known as cranial nerve five.
Its second main branch is the maxillary nerve which innervates the upper row of teeth with
many smaller branches, most notably, the superior alveolar nerves.
The trigeminal nerve’s third main branch is called the mandibular nerve which innervates
the bottom row of teeth with multiple branches as well, most notably, the inferior alveolar
nerve.
In terms of blood supply, oxygenated blood leaves the heart and a portion travels through
the external carotid artery which gives off a branch called the maxillary artery.
Its small branches supply both rows of teeth.
Deoxygenated blood leaves the pulp and ends up in the maxillary vein which travels back
towards the heart.
All right, as a quick recap...The deciduous teeth start to erupt around six months of
age, and this continues to happen every one to two months until the baby is around two
years old.
While there are 20 deciduous teeth, there are 32 permanent teeth in total.
The shape of each tooth gives it a specialized function for chewing food.
The root of each tooth is covered in cementum, the crown is covered in enamel, and dentin
covers the pulp which contains blood vessels and nerves.
Branches of the trigeminal nerve provide sensation to the teeth, while the maxillary artery provides
oxygenated blood.