Chapter 23
The Digestive System
861
23
Bacterial metabolism of the trapped sugars produces acids,
which dissolve the calcium salts of the teeth. Once the salts are
leached out, enzymes released by the bacteria readily digest the
remaining organic matrix of the tooth. Frequent brushing and
daily flossing help prevent caries by removing plaque.
More serious than tooth decay is the effect of unremoved
plaque on the gums. As dental plaque accumulates, it calcifies,
forming
calculus
(kal
9
ku-lus; “stone”) or tartar. Tese stony-
hard deposits disrupt the seal between gingivae and teeth, deep-
ening the sulcus and putting the gums at risk for infection by
pathogenic anaerobic bacteria. In the early stages of such an
infection, called
gingivitis
(jin
0
jĭ-vi
9
tis), the gums are red, sore,
swollen, and may bleed.
Gingivitis is reversible if the calculus is removed, but if it is ne-
glected the bacteria eventually form pockets of infection which
become inflamed. Neutrophils and immune system cells attack
not only the intruders but also body tissues, carving deep pockets
around the teeth, destroying the periodontal ligament, and acti-
vating osteoclasts which dissolve the bone. Tis serious condition,
periodontal disease
or
periodontitis
, affects up to 95% of all peo-
ple over age 35 and accounts for 80–90% of tooth loss in adults.
±ooth loss from periodontitis is not inevitable. Even ad-
vanced periodontitis can be treated by scraping the teeth,
cleaning the infected pockets, cutting the gums to shrink the
pockets, and following up with anti-inflammatory and antibi-
otic therapy. Tese treatments alleviate the bacterial infestations
and encourage the surrounding tissues to reattach to the teeth
and bone. Clinical treatment is followed up by a home regimen
of consistent brushing, flossing, and hydrogen peroxide rinses.
Periodontal disease may jeopardize more than just teeth. Some
contend that it increases the risk of heart disease and stroke in at
least two ways: (1) the chronic inflammation promotes athero-
sclerotic plaque, and (2) bacteria entering the blood from infected
gums stimulate the formation of clots that clog coronary and ce-
rebral arteries. Risk factors for periodontal disease include smok-
ing, diabetes mellitus, and oral (tongue or lip) piercing.
Check Your Understanding
16.
Seven-year-old Tina ran to her daddy to show him her lower
central incisor which she had wiggled until it “fell out.” Is
this a primary or permanent tooth? What name is given to
teeth that (according to Tina) fall out?
17.
Which tooth substance is harder than bone? Which tooth
region includes nervous tissue and blood vessels?
18.
Which teeth are the “grinders”?
For answers, see Appendix H.
The Pharynx
From the mouth, food passes posteriorly into the
oropharynx
and then the
laryngopharynx
(see Figure 23.7a), both common
passageways for food, fluids, and air. (Te nasopharynx has no
digestive role.)
Te histology of the pharyngeal wall resembles that of the
oral cavity. Te mucosa contains a friction-resistant stratified
ing lower molars have two. Te root pattern of the third molar
varies, but a fused single root is most common.
A constricted tooth region called the
neck
connects the
crown and root.
Cement
, a calcified connective tissue, covers
the outer surface of the root and attaches the tooth to the thin
periodontal ligament
(per
0
e-o-don
9
tal; “around the tooth”).
Tis ligament anchors the tooth in the bony socket (alveolus)
of the jaw, forming a fibrous joint called a
gomphosis
. Where the
gingiva borders on a tooth, it dips downward to form a shallow
groove called the
gingival sulcus
.
In youth, the gingiva adheres tenaciously to the enamel cov-
ering the crown. But as the gums recede with age, the gingiva
adheres to the more sensitive cement covering the superior re-
gion of the root. As a result, teeth
appear
to get longer in old
age—hence the expression “long in the tooth.”
Dentin
, a protein-rich bonelike material, underlies the
enamel cap and forms the bulk of a tooth. More resilient than
enamel, dentin acts as a shock absorber during biting and
chewing. Dentin surrounds a central
pulp cavity
containing a
number of so² tissue structures (connective tissue, blood ves-
sels, and nerve fibers) collectively called
pulp
. Pulp supplies nu-
trients to the tooth tissues and provides tooth sensation. Where
the pulp cavity extends into the root, it becomes the
root canal
.
At the proximal end of each root canal is an
apical foramen
that allows blood vessels, nerves, and other structures to enter
the pulp cavity.
Te teeth are served by the superior and inferior alveolar
nerves, branches of the trigeminal nerve (see ±able 13.2, p. 496).
Te superior and inferior alveolar arteries, branches of the max-
illary artery (see Figure 19.22b, p. 727), supply blood.
Dentin contains unique radial striations called
dentinal tu-
bules
(Figure 23.11). Each tubule contains an elongated process
of an
odontoblast
(o-don
9
to-blast; “tooth former”), the cell type
that secretes and maintains the dentin. Te odontoblasts line
the pulp cavity just deep to the dentin. Dentin forms through-
out adult life and gradually encroaches on the pulp cavity. New
dentin can also be laid down fairly rapidly to compensate for
tooth damage or decay.
Enamel, dentin, and cement are all calcified and resemble
bone (to differing extents), but they differ from bone because
they are avascular. Enamel differs from cement and dentin be-
cause it lacks collagen and is almost entirely mineral.
Homeostatic Imbalance
23.6
A blow to the jaw can result in death of a tooth’s nerve. If local
swelling pinches off the blood supply to the tooth, the nerve dies
and the tooth darkens. ±ypically, bacteria infect the pulp some-
time later and must be removed by
root canal therapy
. A²er the
cavity is sterilized and filled with inert material, the tooth is
capped (covered with an artificial crown).
Tooth and Gum Disease
Dental caries
(kār
9
ēz; “rottenness”), or
cavities
, result from
bacterial action that gradually demineralizes enamel and un-
derlying dentin. Decay begins when
dental plaque
(a film of
sugar, bacteria, and other mouth debris) adheres to the teeth.
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