2. Text and pictures in this presentation are taken
from Oral Histology text books: Ten Cates and
James Avery
3. Objectives
At the end of this chapter the student should be able
to describe the origin of the formative cells of teeth,
the stages of tooth formation and the process of
mineralization of enamel and dentin.
Sub topics
Topic 1. Formative cells of dental tissues
Topic 2. Stages of tooth formation
Topic 3. Dentinogenesis
Topic 4. Amelogenesis
6. 1 Formative cells of dental tissues
Neural crest cells arise from the neural folds that develop
on the neural plate
They are also termed as ectomesenchyme or
neuroectoderm
These cells form all of the connective tissues of the face
including the dental structures
During the 6th week of embryonic life the ectoderm
covering the oral cavity is composed of epithelial layer, two
to three cells thick
In the future alveolar area the epithelium proliferates
and forms dental lamina
New terms - (1)
8. The dental lamina then proliferates to form rounded or
ovoid structures that protrude in the mesenchyme, called
tooth buds or tooth germs
The maxillary and mandibular dental lamina eventually
gives rise to 20 such tooth buds ( primary dentition)
between the 6th and the 8th pre natal week
Tooth buds of the permanent dentition develop lingual to
the deciduous buds (except for permanent molars)
beginning from 5months I.U life
The lingual extension of the dental lamina that gives rise
to the permanent tooth buds is called the successional
lamina
A second lamina also develops along with the dental
lamina called the vestibular lamina which later forms the
oral vestibule
New terms - (1)
12. 2 Stages of tooth formation
Most organ systems like digestive system, cardiovascular
system, urinary system etc are functionally complete
within 9 months (at birth)
Tooth formation is a continuous process that also
continues long after birth
According to the shape of the epithelium different stages
of tooth formation can be classified: lamina, bud, cap and
bell stages
13. 1. The dental lamina
This stage is characterized by thickening of the epithelium
and there are no distinguishable tooth sites
2. Bud stage
This stage is marked by rounded growth of epithelial
cells of the dental lamina
It is the stage of initial proliferation of epithelial cells
and adjacent mesenchymal cells
Proliferation of epithelial cells result in formation of
bud-shaped structure called enamel organ
In this stage the mesenchymal cells surrounding the
bud form an ectomesenchymal condensation
14.
15. 3. Cap stage
Gradually the enamel organ gains a concave surface
towards the mesenchyme, then, it is considered to be in
cap stage
In this stage the dental mesenchyme that partially
surrounds the enamel organ is called the dental papilla
or embryonic dental pulp
Cells that lie outside the enamel organ ( and those
adjacent to the papilla) divide and grow around the
enamel organ to form the dental follicle or dental sac
These three structures constitute the tooth germ and
give rise to the tooth and its supporting structures
The epithelial component of enamel organ forms the
enamel, dental papilla forms the dentine and pulp, the
dental follicle forms the cementum, periodontal
ligament and alveolar bone
18. 4. Bell stage
Bell stage is also called the stage of differentiation
because of the following;
a. the shape of the future tooth crown is outlined –
morphodifferentiation
b. differentiation of various cells of enamel organ and
dental papilla – cytodifferntiation
c. differentiation of different tissues –
histodifferentiation
Enamel organ in bell stage consists of four different
type of cells
1. Outer enamel epithelium
2. Inner enamel epithelium
3. Stratum intermedium
4. Stellate reticulum
21. 1. Outer enamel epithelium - the cells that cover the
convex surface of the enamel organ
• These cells function to bring nutrition and oxygen to
ameloblasts and other enamel organ cells
2. Inner enamel epithelium - the cells that line the
concavity of the bell-shaped enamel organ.
• This is the layer that is closest to the papilla.
• The inner enamel epithelium cells elongate and
differentiate into ameloblasts. Ameloblasts are enamel-
forming cells
3. Stratum intermedium – the layer adjacent to the inner
enamel epithelium
• It is formed from a layer of spindle shaped cells
• They function with the ameloblasts in the
mineralization of enamel
22.
23. 4. Stellate reticulum (star-shaped) – those cells that fill
the remainder of the enamel organ
Cervical loop
The area of the enamel organ where the inner and outer
enamel epithelial cells join is called the cervical loop
It is an area of active cell proliferation and lies in a
region that will become the cervix of the tooth
After the crown formation the cells of cervical loop give
rise to the epithelial root sheath and epithelial
diaphragm
25. Odontoblasts
During the bell stage the cells in the periphery of the
dental papilla differentiate into odontoblasts
Odontoblasts form the dentine and the process of
dentine formation is called dentinogenesis
They are mesenchymal matrix-producing cells
During dentinogenesis, the dental papilla becomes
surrounded by dentine and it is then termed the dental
pulp
During this stage the dental lamina begin to degenerate
and disappears. This leaves the tooth bud independent
of the oral epithelium
26. 3 Dentinogenesis
During bell stage odontoblast differentiation begins with
cells near the basal lamina (layer of cells separating the
enamel organ and the dental papilla) which, transform into
preodontoblasts
Following multiplication, the preodontoblasts elongate
and become young differentiating odontoblasts
Odontoblasts further elongate resulting in the formation
of apical/ odontoblastic processes
Odontoblasts then secrete matrix protein at the apical
end of the cell and along its process
The secreted matrix is collagenous and not mineralized
hence it is called predentin
28. As the matrix is being secreted the odontoblasts move
away from the basal lamina towards the centre of the
future pulp
As the odontoblasts retreat the ends of the processes
maintain their positions while there is lengthening of the
process at DEJ
The DEJ will lie at the junction between the inner
enamel epithelium and the basal lamina
The matrix that forms around the elongated cell process
eventually mineralizes and the odontoblastic will lie within
a dentine tubule
Dentinogenesis takes place in two phases: first the
formation of organic collagen matrix and second the
deposition of hydroxyapatite (calcium phosphate) crystals
30. The average crystals attains a size of 100 nm in length
and 3 nm in width
As each day passes predentin is formed along the pulpal
boundary, the adjacent predentin that was formed during
the previous day mineralizes and becomes dentine
During the period of crown development approximately
4µm of dentine is laid down in every 24 hours
Incremental deposition and mineralization of dentine
begins at the tips of the pulp horns at the DEJ
Dentinogenesis continues until the entire crown is
complete and long after the tooth begins to erupt
34. 4 Amelogenesis
The inner enamel epithelium cells differentiate into
preameloblasts which later differentiate into ameloblasts
to start amelogenesis
Prior to secretion of enamel matrix the preameloblasts
begin the process of elimination of the basal lamina which
lies between them and the preodontoblasts
The ameloblasts will only become functional after the
first layer of dentine is formed
The first area of the crown to be completely formed is the
cusp tip and the last is the cervical region
Crowns of teeth increase in size by incremental
deposition of enamel matrix
35. Considering the role of ameloblasts, amelogenesis can be
divided into three main functional stages; presecretory,
secretory and the maturation stage
Presecretory stage
During this stage the ameloblasts change polarity, develop
an extensive protein synthetic apparatus, and prepare to
secrete the organic matrix of enamel
Recent research has shown that secretion of enamel
proteins starts even before the basal lamina is lost
36. Secretory stage
During this stage the ameloblasts secrete the matrix
proteins
They develop cytoplasmic extension on the apical end
called Tomes process
The acquisition of Tomes process signals the beginning of
the secretory stage
Protein in the ameloblasts are packed in secretory
granules
The contents of secretory granules are released against
the newly formed mantle dentine which, immediately
becomes partially mineralized to form the initial layer of
enamel. This layer does not contain enamel rods
37. As the first increment of enamel is formed, ameloblasts
move away from the dentine surface
When enamel formation begins Tomes process comprises
only a proximal portion, after the initial layer is formed it
develops a distal portion as an outgrowth of the proximal
process
Therefore secretion of enamel proteins is established from
two sites, the proximal portion and the distal portion of the
Tomes process
Secretion from the first site (proximal portion) results in
the formation of enamel partitions that surround a pit in
which resides the distal portion
These partitions form the interrod enamel
Secretion from the second site fills the pit which, after
mineralization forms the enamel rod
40. Enamel formed from both sites is of identical composition
but differ only in orientation of crystals
The distal Tomes process retreats leaving a narrow space
that is filled with organic material forming the rod sheath
Eventually the ameloblasts become smaller like they
were while secreting the initial enamel layer . Because rods
form in relation to the distal portion of the Tomes process
the final few increments of enamel do not contain any rods
Thus enamel is composed of a rod-containing layer
sandwiched between thin rodless initial and final layers
Initial enamel
and final enamel
44. Maturation stage
When secretion of the full thickness of enamel is
complete, ameloblasts enter the maturation stage
During this stage the ameloblasts undergo considerable
morphological changes. They shorten and transform into
cuboidal shaped cells. The adjacent stratum intermedium
and stellate reticulum reorganize to form a papillary layer
Finally when the enamel is fully mature the ameloblasts
and the papillary layer regress and form the reduced
enamel epithelium which, thereafter performs protective
function
During the maturation stage physiochemical changes
take place in the width and thickness of the pre-existing
crystals seeded during the formative stage of amelogenesis
46. Next the ameloblasts remove water and organic material
from the enamel thus forming mature enamel
Ameloblasts in this stage undergo ‘modulation’ ,
alternating between ruffle-ended apical surfaces to smooth
ended apical surfaces
Eventually they undergo programmed cell death -
apoptosis
