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People
have two sets of teeth, the baby (also called primary teeth)
and permanent teeth. Baby teeth can begin erupting at six months
of age, and will continue until about 33 months. Baby teeth
can begin shedding from six years of age. Children have 20 primary
teeth, and will develop 32 permanent teeth by age 13.
Teeth consist of enamel, dentin, cementum, and pulp tissue.
Enamel is calcified tissue that is the hardest substance in
the body. It covers the outer layer of the tooth in a protective
shell. It contains no nerve endings and is not sensitive to
a dentist's drill. (Tooth anatomy image)
Dentin, the next layer of the tooth, makes up about 75 percent
of a normal adult tooth. It has about the same density as bone,
but contains special cells, called odontoblasts, that are sensitive
to temperature and touch. These threadlike cells extend throughout
the dentin in the tooth. Their job is to regenerate new dentin
from the nutrients delivered by the blood cells. When the dentin
is damaged (when it loses a portion of the protective enamel,
for example) the odontoblasts relay responses to the nerves,
which usually register feelings from mild discomfort to severe
pain.
The center of the tooth and its soft tissues are known as the
pulp, which extends to the bottom of the root. Pulp is comprised
of blood vessels that carry oxygen and nutrients back and forth
from the heart. These help keep the tooth in top working condition
and protect it from infection. Extremely sensitive nerves also
line the inside of the tooth. These nerves respond to multiple
stimuli such as heat, cold and pressure.
Cementum covers the entire surface of the root, and is very
thin, about the thickness of a fingernail. The main purpose
of the cementum is to connect the tooth's root to the bone of
the jaw. By forming a bond, it acts like a shock absorber for
the tooth.
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