CHAPTER 14
Oral Cavity
269
Figure 14-10A.
Bell stage
,
weeks 12–14
. H&E,
3
72
The
bell stage
is a continuation of the cap stage. The tooth germ undergoes further morphogenesis, proliferation, and differentiation.
At this stage, the enamel organ detaches from the dental lamina and loses connection with the oral epithelium. The dental lamina
gradually degenerates. However, the enamel organ continues to differentiate and forms four cell layers: the
inner enamel epithelium
,
stratum intermedium
,
stellate reticulum
, and
outer enamel epithelium
(Fig. 14-10B). Some cell types may also be found in the later
cap stage. The dental papilla differentiates and forms the
outer cells of the dental papilla
and the
inner cells of the dental papilla
near
the base of the enamel organ. As the tooth germ continues to grow, the
outer cells
of the dental papilla will differentiate into future
odontoblasts
; the
inner cells
of the dental papilla will differentiate into future dental pulp tissues.
T. Yang
Outer enamel
epithelium
Outer
enamel
epithelium
Stellate
reticulum
Dental
sac
Inner enamel
epithelium
Inner enamel
epithelium
Stratum intermedium
Outer cells
of dental
papilla
Cervical
loop
Bone
Bone
Bone
Outer enamel
Outer enamel
epithelium
epithelium
Outer enamel
epithelium
Stellate
Stellate
reticulum
reticulum
Stellate
reticulum
Stratum
intermedium
Enamel
Enamel
organ
organ
Enamel
organ
Outer cells of
Outer cells of
dental papilla
dental papilla
Outer cells of
dental papilla
Dental
Dental
papilla
papilla
Dental
papilla
Inner cells of
Inner cells of
dental papilla
dental papilla
Inner cells of
dental papilla
Bone
Bone
Bone
Inner enamel
Inner enamel
epithelium
epithelium
Inner enamel
epithelium
A
1
2
3
Stratum
Stratum
intermedium
intermedium
Stratum
intermedium
Inner enamel
Inner enamel
epithelium
epithelium
Inner enamel
Inner enamel
epithelium
epithelium
Inner enamel
epithelium
Inner enamel
epithelium
Stellate
Stellate
reticulum
reticulum
Stellate
reticulum
Outer
Outer
enamel
enamel
epithelium
epithelium
Outer
enamel
epithelium
Outer
Outer
enamel
enamel
epithelium
epithelium
Outer
enamel
epithelium
Outer enamel
Outer enamel
epithelium
epithelium
Outer enamel
epithelium
Dental
sac
Bone
Bone
Bone
Cervical
loop
Dental
papilla
Enamel
organ
Mesenchymal
tissue
Inner enamel
epithelium
Outer cells of
dental papilla
Stellate
reticulum
1
2
3
B
Figure 14-10B.
Bell stage
,
cell layers
. H&E,
3
37; insets
3
472
During the differentiation process of the enamel organ at the
bell
stage
, four cell layers are distinguishable. (1) The
inner enamel
epithelium
is a row of columnar cells that contain rich RNA and
are much taller in the tip region where initial enamel formation
will take place at the apposition stage. The inner enamel epithe-
lium differentiates and later becomes ameloblasts. (2) The
stra-
tum intermedium
is composed of two to three layers of squamous
or cuboidal cells and is located between the stellate reticulum and
inner enamel epithelium. Cells in this layer are rich in
alkaline
phosphatase
, which assists in the inner enamel epithelium pro-
duction of enamel. (3) The
stellate reticulum
is located within
the enamel organ. It may also be present in the cap stage, but is
more developed in the bell stage. The cells of the stellate reticu-
lum are star shaped and have many cellular processes intercon-
nected with one another to form a network within the enamel
organ. The extracellular spaces of the stellate reticulum are large
and ± lled with a fl
uidlike, fl
uffy material. These cells contain
glycosaminoglycans
and
alkaline phosphatase
and have
desmo-
somes
and
gap junctions
between the cells. The stellate reticulum
plays a role in maintaining tooth shape and protecting underly-
ing dental tissue. (4) The
outer enamel epithelium
is composed of
cuboidal cells and is the outermost layer of the enamel organ. This
cell layer separates the enamel organ from the nearby mesenchy-
mal tissues. It serves as a protective barrier and also helps main-
tain the shape of the enamel organ. The inner enamel epithelium
and outer enamel epithelium meet to form the cervical loop.
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