2. Some important terms:
Intra uterine life (I.U.L)
Embryo: it develops from a
zygote after fertilization till 9th
week
Fetus: from 9th week till birth.
3. Odontogeneis
It is formation of deciduous and
permanent teeth.
It is a complex process by which
teeth form from embryonic cells,
grow, and erupt into the mouth in a
cascade manner (step by step).
Odontogenesis is continuous, there
is no clear-cut beginning or end
points between these stages.
4. Stomodeum or primitive oral cavity
Located between the brain and the pericardium
in an embryo.
It is lined by primitive ectoderm formed of two
layers: Basal layer of columnar cells separated
from the underlying primitive connective tissue
(ectomesenchyme) by a basement membrane;
and superficial layer of flattened cells.
The oral ectoderm is separated from foregut
endoderm by buccopharyngeal membrane.
In the 4th WIUL this membrane disintegrate, so
primitive oral cavity becomes continuous with
foregut.
6. Neural crest cells
They are group of cells coming from neuroectoderm they separate from lateral aspect of neural plate.
Neural crest cells migrate beneath the epithelium along the entire free margin of the jaw.
7. By then the oral mesenchyme is called:
Ectomesenchyme.
At 6th WIUL, in the region of the future alveolar
processes, the underling ectomesenchyme cells
induce the oral epithelium (which is 2-3 layers)
thickness to proliferate and invaginate into the
mesenchyme to form the primary epithelial bands.
These bands are horse-shoe shaped and correspond
in position to the future dental arches of the upper
and lower jaws. The upper situated more buccally
than the lower one.
Neural crest cells
9. 1.Dental lamina:
The primary epithelial bands are situated
in position of future dental arches of the
upper and lower jaws.
By the 7th week the primary epithelial
bands divides into two laminae, a facially
located vestibular lamina and a lingually
situated dental lamina.
The dental lamina is formed by
proliferation of the basal layer of the oral
epithelium in the underlying mesenchyme.
In certain areas the basal cells proliferate
in a rapid rate than the adjacent cells.
10. A thickening of the ectoderm at first
in the form of islands, these islands
grow forming a band called tooth
band or dental lamina.
The epithelial cells of the dental
lamina form a sheet-like structure
that extends into the alveolar
processes of the maxilla and the
mandible where deciduous teeth
form.
12. The vestibular lamina (lip furrow band):
The vestibular lamina arises from
ectodermal thickening that proliferate
into the underlying ectomesenchyme
later to dental lamina and at a labial
and buccal position to it forming much
broader process than dental lamina.
Then the central cells of this lamina
hollow out and disintegrate to form a
cleft (the oral vestibule) which is the
space between the maxilla and the
mandible on one side and lips and the
cheeks on the other side.
13.
14. 2.Bud stage:
The tooth buds of deciduous teeth
develop by cell division of dental
lamina on its labial side by the
influence of neural crest cells.
The cell division at this area is more
rapid than the adjacent cells to form
round or ovoid swellings (bud shape)
of epithelial structure called Dental
(enamel) organ
Not all tooth buds start to develop at
the same time.
The buds of the anterior deciduous
teeth appear at first then deciduous
canine and deciduous molars.
15. The enamel organ at bud stage contains two
types of cells:
1. Polygonal or rounded cells, which are
centrally situated.
2. Low columnar or cuboidal cells, which are
peripherally situated.
• Enamel organ is separated from underlying
ectomesenchyme by basement membrane.
• The condensation of ectomesenchymal cells
beneath the tooth bud is called the Dental
papillae.
• cells encapsulating the dental organ is called
Dental sac (tooth follicle).
• The dental lamina is shallow and microscopic
sections often show the buds close to the oral
epithelium.
17. Cap stage:
The tooth bud continues to grow;
but with unequal growth in the
different parts of the enamel
organ leading to a shallow
invagination on the deep surface
of the bud.
The enamel organ assumes a cap
shape in appearance and it has a
broad connection with the dental
lamina.
18. • In this stage there are a lot of changes that
happen at:
1. Enamel organ.
2. Dental papilla.
3. Dental sac.
4. New transitory structures will appear.
19. 1. Enamel organ:
A. Outer dental (enamel) epithelium
a single layer of cuboidal epithelial cells with
deeply stained rounded nucleus, and forms the
outer convex surface of the enamel organ.
B. Inner dental (enamel) epithelium
single layer of columnar epithelium with deeply
stained nucleus, arranged at the concave
surface of the enamel organ.
The basement membrane surrounds the
enamel organ to separate the outer and inner
enamel epithelium from the dental sac and
dental papilla
20. C. Stellate reticulum:
It occupies the central portion of the
enamel organ between its outer and inner
enamel epithelia.
Several layers of branching star shaped
cells. Their branches anastomose with
those of adjacent cells and with the outer
and inner enamel epithelia via
desmosomes.
These cells synthesize and secrete glucose-
aminoglycans (GAGs) into the extracellular
compartment between the epithelial cells.
(GAGs) are hydrophilic (water loving)
substances and so pull water into the
enamel organ.
The increasing fluid increases the volume of
the extracellular compartment of the
enamel organ, so that the cells of the organ
are forced apart.
Since the cells retain connections with each
other through desmosomal attachment,
they become star shaped.
21.
22. .
Cells at center of enamel
organ secret GAGs
GAGs are hydrophilic so attract fluids
into extracellular space between cells
Cells are pushed further apart but are
still firmly attached by desmosomes
Cells become star shaped and
so called stellate reticulum
23. 2. Dental papilla
The dental papilla is the condensation of
ectomesenchymal cells; it is partly
enclosed by the concave inner dental
epithelium and is separated from it by a
delicate basement membrane.
The cells of dental papilla show division
and become condensed with active
budding of capillaries &mitotic figures.
The peripheral part of the dental papilla
appears free from cells but contains their
cytoplasmic processes and fine
agryrophilic fibers. This layer appears
narrow light and called cell free (acellular)
zone.
24.
25. 3. Dental Sac:
The dental sac is the condensation of the ectomesenchymal cells
surrounding the enamel organ and dental papilla.
In this zone, gradually a denser and more fibrous layer develops.
26. 4. Transitory structures:
• During cap stage, some transient
structures are present.
• Are not necessarily present in every
tooth germ.
• Or present at the same time.
27. The enamel knot:
localized cellular condensation in the
region of the inner enamel epithelium,
at the center of the tooth germ in
sections of molar cap stage tooth germ
This knot projects in part toward the
underlying dental papilla showing
knob like enlargement, which
disappears in the bell stage by
apoptosis.
28.
29. The enamel cord (enamel septum):
A stand of cells running from the enamel
knot to the outer enamel epithelium.
It meets O.E.E at a point of a small
depression termed enamel navel as it
resembles the umbilicus that seems to
divide the dental organ into two halves.
30. • The function of these two structures is not known, but it may be
1. Determining the initial position of the first cusp of the tooth during
the crown pattern information.
2. It may give another type of the cells. Both structures disappear
before enamel formation begins.
3. May act as reservoir of dividing cells for the growing dental organ.
4. Enamel cord may involve in the process by which the cap stage is
transformed into the bell stage (act as a mechanical tie).
31. Enamel niche
Created by the plane of section…
Cutting through a curved lateral
lamina so that the mesenchyme
appear to be surrounded by
dental epithelium
33. This stage show great changes in:
Dental lamina.
Every member of enamel organ.
Dental papilla.
Dental sac.
34. 1.Dental lamina:
• Mesenchyme invades main dental lamina and
divides it into:
1. Lateral dental lamina that carry the enamel
organ of the deciduous tooth. It is present
labially to the dental lamina proper.
2. The dental lamina proper (Successional)
carrying the tooth germ of the permanent
successors.
1
2
36. 2.Enamel organ:
Outer dental epithelium:
The cells of outer enamel epithelium decrease
in height and become low cuboidal.
This to facilitates entrance of nutrients to
enamel organ
37. Inner dental epithelium
These cells show elongation reaching 40 microns in length.
Most of this elongation is towards the dental papilla resulting in the
disappearance of the cell free zone.
As the inner dental epithelial cells get in contact with the peripheral
connective tissue cells of the dental papilla.
They exert an organizing influence on the undifferentiated mesenchymal
cells to differentiate into odontoblasts; this process is known as induction.
Also the inner dental epithelium shows alteration in their functional polarity
by migration of the nucleus and mitochondria to the proximal end of the cell,
while the Golgi bodies and centrioles migrate to the distal end(reversed
polarity).
At this stage the inner dental epithelium begins to be arranged on the
basement membrane of future amelodentinal junction (ADJ).
39. Stellate reticulum
At this stage the stellate cells expand
further apart mainly by the increase
of the intercellular fluid.
40. Stratum intermedium:
It is composed of 2 – 3 layers of squamous epithelial cells, and is
differentiated in between the inner dental epithelium and stellate
reticulum.
This layer is derived from the cells inside the enamel organ perhaps
from the inner dental epithelium or from the enamel knot, which
insinuated themselves in this place.
The stratum intermedium may show mitotic division and attached to
the inner dental epithelium and stellate reticulum by desmosomes.
This layer seems to be essential for enamel formation as they contain
the enzyme alkaline phosphatase which is essential for mineralization of
enamel and also the well developed cytoplasmic organelles, acid
mucopolysacharides and glycogen deposits indicate a high degree of
metabolic activity.
41.
42.
43. Cervical loop
(zone of reflection):
The inner enamel epithelium meets
the outer enamel epithelium at a
stable rim and this functional zone
is known as cervical loop.
At this region the inner enamel
epithelium continues to cover the
outer surface of the enamel organ
to a certain distance.
44. 3. Dental papilla:
At the end of the early bell stage the differentiated odontoblasts at the
periphery of the dental papilla assume a cuboidal form then become
columnar in shape and acquire the specific potential to produce dentine.
The basement membrane that separates the enamel organ and dental
papilla just prior to dentin formation is called membrana preformativa.
4. Dental sac:
Close to the end of the early bell stage the dental sac shows a circular
arrangement of its fibers resembling a capsular structure.
Its inner layer is more vascular and shows a plexus of nerve fibers and lies
close to the outer dental epithelium and dental papilla.
The outer layer of the dental sac lies against the wall of the bony crypt.
The fibers of this capsule may merge with fibers of the lamina propria of
the gingiva.
