Teeth, hard, bony structures in the mouths of humans and animals used
primarily to chew food, but also for gnawing, digging, fighting, and
catching and killing prey. Teeth are the body’s hardest, most durable
organ—long after bones and flesh have dissolved, archaeologists find
well-preserved teeth from humans and other animals that lived thousands
of years ago.
Humans use teeth to tear, grind, and chew food in the first step of
digestion, enabling enzymes and lubricants released in the mouth to
further break down food. Teeth also play a role in human speech—the
teeth, lips, and tongue are used to form words by controlling airflow
through the mouth. Additionally, teeth provide structural support to
muscles in the face and form the human smile.
Like humans, most animals use their teeth to chew food, although many
animals have evolved teeth that perform other specialized tasks. For
example, many carnivorous (meat-eating) animals, such as tigers, have
developed long, sharp teeth for clamping down on and killing prey.
Beavers have chisel-like front teeth that they use to cut down large
trees for building dams.
II. HUMAN TEETH
Human teeth are made of four distinct types of tissue: enamel, dentin,
pulp, and cementum. Enamel, the clear outer layer of the tooth above
the gum line, is the hardest substance in the human body. In human
teeth, the enamel layer is about 0.16 cm (about 0.06 in) thick and
protects the inner layers of the teeth from harmful bacteria and
changes in temperature from hot or cold food. Directly beneath the
enamel is dentin, a hard, mineral material that is similar to human
bone, only stronger. Dentin surrounds and protects the pulp, or core of
the tooth. Pulp contains blood vessels, which carry oxygen and
nutrients to the tooth, and nerves, which transmit pain and temperature
sensations to the brain. The outer layer of the tooth that lies below
the gum line is cementum, a bonelike substance that anchors the tooth
to the jawbone.
The visible portion of the tooth is called the crown. Projections on
the top of each crown, used primarily for chewing and grinding, are
called cusps. The portion of the tooth that lies beneath the gum line
is the root. The periodontal ligament anchors the tooth in place with
small elastic fibers that connect the cementum in the root to a special
socket in the jawbone called the alveolus.
A. Types of Human Teeth
Adult humans typically have 32 teeth—16 in the upper jaw and 16 in the
lower jaw—that fit together and work in concert to chew food. Teeth on
the right side of each jaw are usually identical to the teeth on the
left side and matching teeth on opposite sides are referred to as sets,
or pairs. Humans are heterodonts—that is, they have teeth of different
sizes and shapes that serve different functions, such as tearing and
grinding. In contrast, the homodont teeth found in many animals are all
the same size and shape, and perform the same function.
Humans have four types of teeth, each with a specific size, shape, and
function. Adult humans have eight incisors, located at the front of the
mouth—four in the upper jaw and four in the lower jaw. Incisors have a
sharp edge that is used to cut food. On either side of the incisors are
the canines, named for their resemblance to the pointy fangs of dogs.
The upper canines are sometimes called eyeteeth. There are two canines
in each jaw, and their primary role is to tear food. Behind the canines
are the bicuspids, or premolars, flat teeth with pronounced cusps that
grind and mash food. There are two sets, or four bicuspids, in each
jaw. Behind the bicuspids are the molars, where the most vigorous
chewing occurs. There are twelve molars—three sets in each jaw—referred
to as the first, second, and third molars. Third molars are often
called wisdom teeth; they developed thousands of years ago when human
diets consisted of mostly raw and unprocessed foods that required the
extra chewing and grinding power of a third set of molars. Today wisdom
teeth are not needed for chewing and, because they can crowd other
teeth, are often removed.
B. Tooth Development
Humans are diphyodont—that is, they develop two sets of teeth during
their lives. The first set of teeth are the deciduous teeth, 20 small
teeth also known as baby teeth or milk teeth. Deciduous teeth start
developing about two months after conception and typically begin to
erupt above the gumline when a baby is six or seven months old.
Occasionally a baby may be born with one or more deciduous teeth at
birth, known as natal teeth. By the time a child is six years old, a
second set of 32 larger teeth, called permanent teeth, start to erupt,
or push out of the gums, eventually replacing the deciduous teeth.
Human tooth development occurs in stages. The hard tissue of the
deciduous teeth, or the dentin, forms while the fetus is in the womb.
After the child is born, tooth enamel develops in stages. Front tooth
enamel, for example, is usually complete around one month after birth,
while the enamel on the second molars is not completely developed until
a child is about a year and a half old. When the enamel is fully
developed the tooth erupts. Front teeth usually erupt when a child is
from 6 to 12 months of age, second molars between 13 and 19 months old,
and canines usually erupt at 19 months or older. The final stage of
tooth development is root completion, a slow process that continues
until the child is more than three years old.
Around the age of six, the roots of deciduous teeth slowly dissolve as
the developing permanent teeth start to push them out. Deciduous teeth
eventually fall out and are replaced by the erupting permanent teeth.
This begins a transitional phase of tooth development that takes place
over the next 15 years. As baby teeth are pushed out by permanent
teeth, the entire mouth and jaw transform from their childhood shape to
a more pronounced, adultlike structure. From age six to age nine, a
child’s permanent incisors, canines, and first molars erupt. The
bicuspids erupt from age 10 to age 12, and the second molars come in by
age 13. The third molars, or wisdom teeth, usually erupt by the age of
When human teeth grow to a certain size, the root essentially closes
and the teeth stop growing. Closed-rooted teeth have narrow root
openings that are only big enough for the periodontal ligament, blood
vessels, and a nerve.
C. Disorders of Human Teeth
The three main diseases of human teeth are tooth decay, also called
dental caries; gum disease, or periodontal disease; and problems with
tooth alignment, called malocclusions. Human teeth problems are treated
or prevented by dentists, professionals who are specially trained to
Tooth decay affects approximately 90 percent of all children by the
time they are 14 years old. Tooth decay begins when bacteria are passed
from mothers or caregivers to children between their first and second
birthdays. When these bacteria are exposed to sugars commonly found in
foods, the bacteria produce harmful acids that attack tooth enamel.
Left unchecked, the acid eats holes in the enamel and forms cavities of
tooth decay. Most tooth decay forms in the deep grooves on the chewing
surfaces of the molars, called pits and fissures. Daily tooth brushing
and proper dental care help prevent and reduce tooth decay. Dentists
use preventive treatments to reduce the risk of tooth decay; clear
plastic coatings painted on the teeth, called dental sealants, and
applications of the mineral fluoride, which fortifies tooth enamel, are
two such treatments. Fluoride is also added to public water supplies in
a process called fluoridation, which benefits more than 150 million
Gum disease, or periodontal disease, is a progressive condition that
worsens with age. Gum disease occurs when bacteria eat away at gum
tissue, causing it to pull away from the teeth. This space between the
tooth and gum, called a periodontal pocket, traps even more bacteria.
Gum disease develops in two stages. Gingivitis, the early stage, causes
red, swollen gums that bleed easily. Gingivitis can be eliminated
through good oral hygiene and dental care. If not treated, gingivitis
can progress to periodontitis, when bacteria attack the bone supporting
the teeth. To treat periodontitis, dentists may have to surgically cut
out the infected portion of the gum so the bacteria can be removed.
Malocclusions—teeth that are crowded, crooked, or out of alignment—make
it more difficult to clean teeth, which can lead to other oral health
problems such as tooth decay and gum disease. Many of these disorders
start to appear between the ages of 6 and 12, when permanent teeth
begin to erupt. Generally, malocclusions result when the jaw is too
small to hold all of the teeth. Malocclusions are often genetic,
tending to run in families. In other cases, dental injury or chronic
thumb sucking may lead to poorly aligned teeth. Malocclusions are
treated by dentists specially trained to correct them, called
III. TEETH IN OTHER ANIMALS
Animal teeth have the same four tissues that make up human teeth:
enamel, dentin, cementum, and pulp. The composition and structure of
each substance may differ in each animal. In horses, for example,
enamel is found inside the tooth as well as on the outer surface,
rather than simply encasing the dentin and the pulp as it does in human
Some animals are monophyodont, developing only one set of teeth that
grow continuously throughout an animal’s lifetime. These animals have
open-rooted teeth, which have wide openings at the root that permit
dentin-forming cells to grow and multiply. Most rodents, for example,
have open-rooted teeth. The gnawing habits of these animals wear down
their teeth, otherwise the teeth would grow very long. The front teeth
of beavers, for example, can grow up to 1.2 m (4 ft) a year. Sharks and
some other fish are polyphyodont—that is, they continuously lose their
teeth and develop new ones.
Most mammals that depend on catching, chewing, and digesting food for
survival have developed teeth that meet these needs. Mammal teeth are
classified by the type of food the animal eats. Insectivores are
animals that eat only insects, such as bats, shrews, anteaters, and
armadillos. These animals have square teeth with special V-shaped edges
that efficiently grind the hard coverings of insects. Carnivores, such
as dogs, cats, hyenas, and walruses, generally have large and
well-developed teeth with long canines for clamping down on prey or
fighting. A walrus also uses its canines, which grow up to 1 m (about 3
ft) long, as hooks in climbing on ice. Herbivores—cows, sheep, deer,
and horses, for example—eat only plants and have sharp incisors for
cutting vegetation and flat teeth with complicated ridges for grinding
and mashing. Piscivores, or fish-eaters, have sharp teeth that angle
backward to catch and hold their prey. Seals and dolphins swallow food
whole without chewing it first; they are equipped with many identical,
conical-shaped teeth that are used to catch and grasp their slippery
prey before swallowing.
Fish teeth have evolved to perform different functions in each species.
For example, fish that feed on crab, shrimp, and other crustaceans have
developed strong, blunt teeth for crushing and grinding the hard outer
shells. Piranhas have serrated teeth that fit together like scissors,
enabling the small fish to cut the flesh from their prey. Many fish
have teeth on their tongues or gills. A hagfish uses the rasping teeth
on its tongue to bore holes in its victims and drain their blood.
Sawfish have long, flat beaks with a row of weaponlike tooth
projections in each jaw that can cut their prey in half.
Only some reptiles and amphibians have teeth. Salamanders have rows of
small pointed teeth, but frogs and toads do not have teeth after
infancy. Some snakes and frogs develop an egg tooth that enables a
hatching young to chip its way out of its egg. This tooth eventually
disappears. Many reptiles have teeth growing on the tongue or the
palate, and some even have a second set in the throat. Some snakes,
such as rattlesnakes, have prominent fangs for delivering injections of
venom to their victims. Crocodiles have between 30 and 40 teeth in each
jaw. These reptiles use their daunting teeth not for chewing, but to
gradually tear food into bits as they thrash violently with their prey
in the water.
Chris Martin, B.A.
Manager, Media Relations, American Dental Association.
"Teeth," Microsoft® Encarta® Online Encyclopedia 2009
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