CHAPTER 14
Oral Cavity
271
Stratum
Stratum
intermedium
intermedium
Stratum
intermedium
Odontoblasts
Odontoblasts
Odontoblasts
Ameloblasts
Ameloblasts
Ameloblasts
Ameloblasts
Ameloblasts
Ameloblasts
Enamel space
DEJ
Reduced
enamel
organ
Dental
Dental
sac
sac
Dental
sac
Primary
apical
foramen
Odontoblasts
Dental
papilla
Crown dentin
Crown dentin
Crown dentin
Inner enamel
epithelium
Outer enamel
epithelium
Begining of root sheath
Begining of root sheath
Begining of root sheath
A
Figure 14-12A.
Tooth root development
. H&E,
3
29; (
upper
)
inset
3
160; (
lower
) inset
3
90
Root development
begins after the formation of the crown has been
completed. It involves three structures: the
epithelial root sheath
,
the dental sac (follicle)
, and the
dental papilla
. Dentin formation
proceeds from the crown to the root. The formation of the root
begins at the
epithelial root sheath (Hertwig epithelial root sheath)
,
which develops from the cervical loop (Fig. 14-11A). The inner and
outer enamel epithelia of the
cervical loop
extend to form the root
sheath. The epithelial root sheath grows around the dental papilla.
It bends at a 45-degree angle and is called the
epithelial diaphragm
(Fig. 14-11B). The epithelial diaphragm gradually encloses the dental
papilla with the exception of the
apical foramen
. The root sheath is
important in forming the shape of the root. It induces the outer cell
layer of the dental papilla to differentiate into odontoblasts, which
produce the root dentin. When the root dentin has formed, the mes-
enchymal cells from the dental sac come in contact with the surface
of the root dentin and induce these cells to differentiate into cemen-
toblasts, which produce cementum. Fibroblasts, which differentiate
from the dental sac, begin to form the PDL.
Enamel
space
Root
Root
dentin
dentin
Root
dentin
Tooth
Tooth
root
root
Tooth
root
Cementum
Cementum
Cementum
DEJ
CEJ
CEJ
CEJ
Crown
dentin
Initial
Initial
gingiva
gingiva
Initial
Initial
junction
junction
epithelium
epithelium
Initial
junction
epithelium
Dental
Dental
Pulp
Pulp
Dental
Pulp
Initial
gingiva
B
Figure 14-12B.
Tooth eruption
. H&E,
3
14; inset
3
5
As root length increases, additional space is needed for root growth, and
the tooth gradually moves and erupts into the oral cavity.
Tooth erup-
tion
and root growth, therefore, occur at the same rate. The formation
of the tooth root includes
root dentinogenesis
(formation of root den-
tin),
cementogenesis
(formation of cementum),
pulp formation
, forma-
tion of the
PDL
, and
alveolar bone development
. The crown of the tooth
passes through the bony crypt, dental epithelium, and oral epithelium to
emerge into the oral cavity. As the tooth moves vertically, the overlying
bone gradually becomes absorbed by osteoclasts. The
dental epithelium
(outer enamel epithelium, stellate reticulum, stratum intermedium, and
ameloblasts) is compressed into a thin layer called the
reduced enamel
epithelium
(
REE
). The oral epithelium fuses with the REE and gradu-
ally degenerates, enabling the tooth crown to erupt into the oral cavity.
The
initial junction epithelium
is created during tooth movement and
eruption; later it becomes the
junction epithelium
(Fig. 14-1), forming
a seal between the gingiva and enamel. In a regular H&E stain, highly
calci± ed enamel disappears because of the decalci± cation process during
tissue preparation.
CLINICAL CORRELATION
Figure 14-12C.
Dilaceration
.
Dilaceration
is a developmental anomaly of the tooth in
which there is an
abrupt bend
between the crown and the
root of a tooth.
Trauma
to the
primary predecessor tooth
and
developmental
disturbances
to the
primary tooth germ
are the
possible causes of this condition. Dilaceration can happen in
any tooth, but is more common in
mandibular third molars
,
maxillary second
premolars
, and
mandibular second molars
.
Visual examination can ± nd a crown dilaceration, but the x-ray
is the most appropriate diagnostic tool to diagnose a root dilac-
eration. Dilacerations may be associated with some
syndromes,
such as
Smith-Magenis
syndrome
, a chromosomal disorder
that produces a set of characteristic physical, behavioral, and
developmental features. Dilacerations can complicate cavity
preparation, root canal preparation, and other treatments.
Abrupt bend
Tooth crown
Tooth root
C
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