CHAPTER 14
Oral Cavity
265
Teeth
Introduction and Key Concepts for Teeth
Teeth
are prominent structures in the oral cavity. They can be
divided into
maxillary
(upper) and
mandibular
(lower)
teeth
.
The root of the tooth is surrounded by the alveolar bone (alveo-
lar process or alveolar arch), which forms a socket to hold and
support each tooth. The alveolar bone of the maxillary teeth
is a part of the maxilla (upper jaw); the alveolar bone of the
mandibular teeth is a part of the mandible (lower jaw). There
are two sets of teeth:
primary
(baby) and
permanent teeth
. The
primary teeth are eventually replaced by permanent teeth. In
adults, there are 32 permanent teeth including two
incisors
, one
canine
, two
premolars
, and three
molars
on each of the four
quadrants in the maxillary and mandibular arches (Fig. 14-7).
Each tooth can be divided into three parts: the
crown
, the
cervix (neck)
, and the
root
. The
crown
of the tooth is covered
by enamel and extends above the gingiva (gum); the shape of
the crown is unique in different types of teeth and is adapted to
their functions. The
cervix
is also called the
neck
of the tooth
and is the junction between the crown and root. The region
where the enamel and cementum meet is called the
cemen-
toenamel junction
(
CEJ
). The
root
of the tooth is covered by
cementum and surrounded by the alveolar bone. A tooth has
one or more roots; the apex is the end of the root. The
api-
cal foramen
is a small opening where nerves and blood vessels
enter and exit the dental pulp (Figs. 14-1 and 14-7).
TOOTH DEVELOPMENT (ODONTOGENESIS)
stems from
two different origins: ectodermal and neural crest–derived mes-
enchymal tissue. The oral epithelium derives from ectodermal
tissue, which gives rise to the dental lamina, and later becomes
the enamel organ. The dental papilla derives from mesenchymal
tissue and gives rise to the future dental pulp. The enamel organ,
dental papilla, and dental sac (follicle) as a whole are described
as a
tooth germ
, which eventually forms a tooth. The initiation
of tooth germs occurs along the oral epithelium of the maxillary
and mandibular prominences (processes). Tooth germs undergo
initiation phases
(development of the dental lamina and initia-
tion of tooth germs),
morphogenesis
phases
(cell movement and
formation of the shape of the tooth), and
histogenesis phases
(formation of the hard tissue and development of the tooth
root). Although tooth development is a continuous process,
odontogenesis can be divided into several stages based on the
shape of the tooth germ and formation of the tooth structure.
These stages include
initiation
,
bud
,
cap
,
bell
, and
apposition
(crown) stages
. When the primitive oral cavity is forming, the
± rst pharyngeal arch gives rise to the
maxillary
and
mandibu-
lar prominences
or
processes
(developing dental arches), which
lead to the future upper and lower jaws (Fig. 14-9A).
1.
Initiation stage
: At about 6 to 7 weeks into development,
some oral epithelial cells on the surface of the maxillary
and mandibular prominences increase proliferation activ-
ity, become thicker, and invaginate into the underlying mes-
enchymal tissue to form the
primary epithelial band
(Fig.
14-9A).
2.
Bud stage
: At about 8 to 9 weeks, the primary epithelial
band gives rise to the
vestibular lamina
and
dental lamina
.
The vestibular lamina forms a cleft that becomes the vesti-
bule between the cheek and tooth; the dental lamina forms
a U-shaped structure and develops into tooth buds (enamel
organs and mesenchymal tissue), which become
primary
deciduous teeth (Fig. 14-9A). The development of perma-
nent teeth comes from the secondary tooth buds, which
sprout from the primary tooth buds.
3.
Cap stage
: At about 10 to 11 weeks, the enamel organ
appears as a cap shape and the condensed mesenchymal
cells beneath the enamel organ form the dental papilla (Fig.
14-9B).
4.
Bell stage
: At about 12 to 14 weeks, the enamel organ con-
tinues to grow into a bell shape, and cells of the enamel
organ differentiate into four distinguishable cell layers: the
outer enamel
epithelium
,
inner enamel epithelium
,
stellate
reticulum
, and
stratum intermedium
. At the same time, the
dental papilla grows and helps to form the shape of the
tooth crown. Cells in the dental papilla are differentiated
into outer and inner cell groups. The outer cells of the dental
papilla will develop into odontoblasts that produce future
dentin; the inner cells will develop into future dental pulp
tissues (Fig. 14-10A,B).
5.
Apposition (crown) stage
: At about 18 to 19 weeks, cells of
the tooth germ continue to differentiate; the inner enamel
epithelial cells have become preameloblasts which induce
the outer cells of the dental papilla to become odontoblasts
and begin to produce the dentinal matrix at the tooth crown
region. This material is called
predentin
, and after undergo-
ing calci± cation will become dentin. When dentin is formed,
it induces preameloblasts to differentiate and become active
ameloblasts, which produce enamel. Thus, the production of
both the enamel matrix and the dentin for the tooth crown
has begun (Fig. 14-11A,B). When the two hard tissues, den-
tin and enamel, have formed and the shape of the crown is
completed, the root of the tooth begins to develop. The tooth
then starts to erupt into the oral cavity in order to make
room for the tooth root to grow (Fig. 14-12A,B).
ENAMEL
,
DENTIN
,
AND DENTAL PULP
are components
of teeth.
Enamel
is the hardest tissue in the body. The basic
morphological unit of enamel is the
rod
, also called the
prism
,
which is composed of a head and a tail. The rods are arranged
in a three-dimensional complex, perpendicular to the
dentinoe-
namel junction (DEJ)
. They extend from the DEJ to the surface
of the enamel. Enamel provides a seal for the dentin and makes
a strong surface for chewing (Fig. 14-14A,B). The crown of the
tooth is covered by enamel and the root of the tooth is covered
by cementum.
Dentin
surrounds and forms the walls of the den-
tal pulp. It is composed of numerous
dentinal tubules
,
odon-
toblastic
processes
, and the
dentinal matrix
and is the second
hardest tissue of the body (Fig. 14-15A,B). The central core of
the tooth is occupied by
dental pulp
, which is made up of loose
mucous connective tissue containing blood vessels and nerve
± bers (Fig. 14-17A,B).
THE PERIODONTIUM
refers to the structures surrounding
and supporting the tooth root and includes the
cementum
, the
PDL
, and the
alveolar bone
. The
cementum
is a thin layer of hard
tissue that covers the root dentin (Fig. 14-18A,B). The
PDL
is
the dense ± brous connective tissue that attaches the cementum to
the alveolar bone (Figs. 14-18C and 14-19A). The
alveolar bone
,
also called the
alveolar process
, is part of the maxilla and man-
dible. The alveolar bone supports and protects the tooth root. It
includes the
alveolar crest
, the
alveolar bone proper
, and
support-
ing bone
(Figs. 14-18C and 14-19A).
previous page 280 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online next page 282 Dongmei Cui -  Atlas of Histology with Functional and Clinical Correlations 2011 read online Home Toggle text on/off