270
UNIT 3
Organ Systems
Dental
Dental
papilla
papilla
Dental
Dental
papilla
papilla
Dental
Dental
papilla
papilla
Dental
papilla
Preameloblasts
Preameloblasts
Preameloblasts
Dentinal
Dentinal
matrix
matrix
Dentinal
matrix
Odontoblasts
Odontoblasts
Odontoblasts
Stratum
Stratum
intermedium
intermedium
Stratum
intermedium
Preameloblasts
Preameloblasts
Preameloblasts
Dentinal
Dentinal
matrix
matrix
Dentinal
matrix
Odontoblasts
Odontoblasts
Odontoblasts
Cervical
Cervical
loop
loop
Cervical
loop
Stellate
Stellate
reticulum
reticulum
Stellate
reticulum
Enamel
Enamel
organ
organ
Enamel
organ
Odontoblasts
Odontoblasts
Odontoblasts
Preameloblasts
Preameloblasts
Preameloblasts
Dentinal
Dentinal
matrix
matrix
Dentinal
matrix
Outer enamel
Outer enamel
epithelium
epithelium
Outer enamel
epithelium
Inner enamel
Inner enamel
epithelium
epithelium
Inner enamel
epithelium
Dental
Dental
sac
sac
Dental
sac
A
Figure 14-11A.
Apposition (crown) stage
,
dentinogenesis
. H&E,
3
76; insets
3
315
During the
apposition (crown) stage
, induction
occurs between the
ectodermal
(enamel organ)
and
mesenchymal
(dental papilla) tissues. The
inner enamel epithelium from the enamel organ
has become
preameloblasts
, which induce
outer cells of the dental papilla (Fig. 14-11B)
to
differentiate into
odontoblasts
. When
odontoblasts become mature and active, they
appear columnar in shape and begin to secrete
dentinal matrix (
predentin
), which is the ±
rst
hard tissue formed during tooth development.
The formation of dentin is called
dentinogen-
esis
. The crown of the tooth develops much
earlier than the root of the tooth. The crown
dentin is ±
rst produced as
predentin
by odon-
toblasts. The predentin soon becomes calci±
ed
and is called
dentin
(Fig. 14-11B). The cervical
loop is formed by the inner enamel epithelium
and the outer enamel epithelium. Along with
the dental sac, the cervical loop will contribute
to develop future root structures.
Ameloblasts
Ameloblasts
Ameloblasts
E
n
a
m
e
l s
l
s
p
a
c
e
Ename
l space
Dentin
D
e
n
t
i
n
Dentin
DEJ
Enamel matrix
Enamel matrix
(uncalcified)
(uncalcified)
Enamel matrix
(uncalcified)
Enamel matrix
Enamel matrix
(uncalcified)
(uncalcified)
Enamel matrix
(uncalcified)
Ehe en space
Ehe en space
Enamel space
Ameloblasts
Ameloblasts
Ameloblasts
Odontoblasts
Odontoblasts
Odontoblasts
Predentin
Predentin
Predentin
Dentin
Dentin
Dentin
Alveolar
Alveolar
bone
bone
Alveolar
bone
Epithelial
Epithelial
diaphragm
diaphragm
Epithelial
diaphragm
Tomes
Tomes
processes
processes
Tomes
processes
Dental
Dental
papilla
papilla
Dental
papilla
Dental
Dental
papilla
papilla
Dental
papilla
Odontoblasts
Odontoblasts
Odontoblasts
Enamel matrix
Enamel matrix
(uncalcified)
(uncalcified)
Enamel matrix
(uncalcified)
Enamel space
Enamel space
Enamel space
Dentin
Dentin
Dentin
Ameloblasts
Ameloblasts
Ameloblasts
Dentin
Dentin
Dentin
E
E
n
a
m
e
l
m
m
a
trix
t
r
i
x
Enamel
matrix
B
Figure 14-11B.
Apposition (crown) stage
,
amelogenesis
. H&E,
3
19; insets (
right
)
3
104;
inset (
left
)
3
354
The formation of crown dentin induces preamelo-
blasts to differentiate and become
ameloblasts
.
Active ameloblasts are columnar cells, and their
nuclei are located toward the stratum interme-
dium. They actively secrete enamel matrix with
assistance from the stratum intermedium. The
newly secreted enamel matrix, in contact with the
dentin matrix, forms the
DEJ
. At the same time,
the ameloblasts and the odontoblasts retreat
from the DEJ as deposition of the matrix pro-
ceeds. Enamel formation moves outward (toward
the enamel organ), and dentin formation moves
inward (toward the dental pulp). The process of
enamel formation is called
amelogenesis
. Dur-
ing amelogenesis, conical processes, known as
Tomes processes
, are developed at the secretory
(apical) surface of the ameloblasts. In the apposi-
tion stage, the two types of hard tissues (dentin
and enamel) begin to form at the tooth crown.
These two types of hard tissues are formed in a
regular rhythm.
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