Tooth development

1. Tooth development II
Dr. Samah khaled

2. 5. Late bell stage:
1. Dental lamina:
The breakpoint that makes enamel organ
transfers from early to late bell is
Deposition of the first layer of dentine
Mesenchyme of dental sac invades the lateral
dental lamina leading to its break down.
Remnants of the dental lamina may persist in
the gingival and the jaw and they are called
epithelial rests of Serres (Serres' pearls).
These remnants may form:
 Eruption cyst: small cyst over the developing
tooth that may delay eruption.
 May give rise to odontoma.
 May be activated to form supernumerary
teeth.

3. (Serres' pearls)

4. Eruption cyst Odontoma Supernumerary tooth

5. 2. Enamel organ:
It increases in size.
Its cervical portion gives
rise to epithelial root sheath
of Hertwig (HERS).

6. A. Outer dental epithelium:
Nutrition to enamel organ enter via 2 routs:
Dental papilla
Dental sac through O.E.E
When the first layer of dentin is laid down the
nutrition of the dental organ via the dental papilla
will stop.
1. O.E.E become flattened. They become low
cuboidal with high nuclear/cytoplasmic ratio (little
cytoplasm).
2. Folding of the smooth surface of outer enamel
epithelium to increase its surface area.
3. At the region of these folds the dental sac sends
many capillary loops, to provide a rich nutritional
supply.
4. O.E.E develop microvilli, cytoplasmic, vesicles and
increased number of mitochondria at the end
facing the capillary loops for active transport of
materials.

7. B. Inner dental epithelium:
 Under the influence of the first formed dentin layer the
inner enamel epithelium will be stimulated to be
differentiated into tall columnar cells that will produce
enamel matrix: called ameloblasts.
 Ameloblasts 4 – 5 microns in diameter
40 microns in length
in cross section they are
hexagonal.
 They are attached by junctional complexes laterally and
by desmosomes to the stratum intermedium.
 The boundary between inner dental epithelium and
odontoblasts outlines the future ADJ.

8. Induction
Inner dental epithelium
Proximal end
Distal end
Odontoblasts
8
Cell free
zone
Cell rich
zone
CentrioleNucleus Golgi apparatus Mitochondria

9. Reciprocal Induction
Odontoblast
Dentin matrix
Ameloblast
Enamel matrix
Dentin

10. STRATUM
INTER.
STELLATE
RETICULUM
INNER
DENT.
EPITH.
PREDENTIN
ODONTOBLASTS
LATE BELL STAGE
( Appostion stage)

12. Stellate reticulum:
The space needed for the developing enamel will be gained
from:
 The shrinkage of the stellate reticulum by the loss of the
intercellular fluid.
Becomes hardly distinguished from the stratum intermedium.
The shrinkage begins at the height of the cusp or the incisal
edge and progresses cervically.
Stratum intermedium:
 These layers show strong reaction for alkaline phosphatase
enzyme, which is needed for the mineralization of the enamel.
 Also, the presence of well developed cytoplasmic organelles,
acid mucopolysacharides and glycogen deposits indicate a high
degree of metabolic activity.

13. Root formation
Single rooted Multirooted

14. 1. The single rooted tooth:
When the enamel and dentine formation have
reached the future amelocemental junction.
The cervical loop (the inner & outer enamel
epithelium begin to grow deeper into the
surrounding ectomesenchyme of the dental sac.
 It elongates and moves away from the newly
completed crown area to enclose more of the dental
papilla tissue forming Hertwig's epithelial root
sheath (HERS).

15.  At the first the epithelial root sheath bends at
the future cemento enamel junction into
horizontal plane known as epithelial
diaphragm.
 This diaphragm produces narrowing of the wide
cervical opening of the tooth germ.
 The plane of the diaphragm remains relatively
fixed during the root development and growth.
 The proliferation of the cells of the epithelial
diaphragm is accompanied by proliferation of
the cells of connective tissue of the pulp, which
occurs in the area adjacent to the diaphragm.

16. The epithelial sheath of Hertwig proliferates coronally
to the diaphragm in a vertical direction.
The cells of the inner enamel epithelium forming the
sheath of Hertwig remain short and induce the
undifferentiated mesenchymal cells of the dental
papilla to differentiate into odontoblasts.
So the function of the root sheath is to mold the
shape of the root and initiate dentine formation.
After dentine deposition the connective tissue of the
dental sac proliferates and invades the Hertwig
epithelial root sheath dividing it into a network of
epithelial strands.
These strands are moved away from the surface of
dentine so that the connective tissue cells of dental
sac in contact with the outer surface of dentine are
differentiated into cementoblasts which deposit
cementum on the dentine surface.

17. The rapid epithelial sheath proliferation, dentine formation and epithelial sheath
destruction explain the fact that the HERS cannot be seen as a continuous layer in
the developing root.
The wide apical foreman is reduced first to the width of the diaphragmatic
opening and later is further narrowed by apposition of dentine and cementum at
the apex of the root.

18.  The epithelial strands may undergo degeneration; remnants may persist in the
periodontal ligament in the form of network or isolated islands known as
epithelial rests of Malassez.
 Epithelial rests of Malassez are source of epithelial lining of dental cyst that
develops in reaction to inflammation of periodontal ligament.

19. 2. The multirooted teeth:
The deciduous and permanent molars and some premolars have
more than one root.
Their roots are formed like the single rooted tooth till it reaches
the level of bi-or trifurcation where the epithelial diaphragm
proliferates horizontally producing tongue like extensions (2
tongue extensions in case of 2 rooted tooth and 3 in the three
rooted tooth).
 At the region of future bifurcation of the roots the free ends of
these horizontal epithelial extensions grow towards each other
and fuse dividing the wide opening into 2 or 3 partitions.
 The odontoblasts differentiate along the diaphragm, and the
pulpal surface of the extended epithelial bridges form dentin, and
on the periphery of each opening, root development follows in
the same way as described for single rooted teeth.

21. Clinical considerations
Tooth formation is dependent on both
oral epithelial and adjacent
mesenchymal cells for development.
Factors such as:
1. X-rays.
2. Nutritional deficiencies.
3. Drugs.
Change the ability of these cells to
function, thus affecting tooth
development.

22. 1. Enamel pearls
 If the cells of the epithelial root sheath of
Hertwig remain adherent to the dentin
surface, they may differentiate into
ameloblasts and produce enamel, called
enamel pearls.
 They appear as small, spherical enamel
projections especially at the cemento-enamel
junction (CEJ) or in the furcation area in
molars.
 They usually cause periodontal problems.
 Enamel pearl in deciduous teeth may cause
delayed exfoliation of primary teeth because
of slower process of enamel resorption. This
may lead to deviation of erupting permanent
molars.

23. 2. Bare dentin
If the epithelium root sheath of
Hertwig is delayed in its
separation from the dentin, a
zone of the root is devoid of
cementum.
In about 10% of teeth, the
cemento-enamel junction
consists only of a layer of dentin
without enamel and cementum.
Dentin is sensitive when
exposed to oral environment
patient feel pain with different
foods and drinks.
Enamel
Cementum
Dentin

24. 3. Intermediate cementum:
 If the continuity of the Hertwig's root
sheath is broken after odontoblastic
differentiation and before dentine
formation, intermediate cementum is
developed.
 It occurs at apical 2/3 of premolars
and molars roots and rare in incisors
and deciduous teeth.

25. 4. Accessory root canal
1. If the epithelial root sheath of Hertwig is broken
before odontoblastic differentiation and dentin
formation a defect in the dentinal wall of the pulp
chamber or root canal will result.
2. Disturbance in the fusion of the tongue like
extension of the diaphragm causes defects in the
pulpal floor.
3. Large blood vessel may disturb the course of the
root sheath lead to accessory root canal.
 These defects account for the development of
accessory root canals opening on the periodontal
surface of the root.

26. The stages of tooth development

27. 1. Initiation
The dental lamina and tooth buds represent
that part of the oral epithelium that has powers
for tooth formation.
Histophysiological stages of tooth development
Absence of either single tooth or
multiple teeth (partial anodontia)
most frequently, upper 2, upper
and lower 8,due to lack of
initiation
Complete lack of teeth
(anodontia) as in Ectodermal
Dysplasia syndrome.
Abnormal initiation may result
in development of single or
multiple supernumerary teeth.

28. 2. Proliferation
Proliferative activity is found in dental lamina, bud, cap,
early bell stage and late bell stage where the dental
matrices are not yet deposited. The unequal growth by
mitotic division causes regular changes in size and
proportions of the growing tooth germ.
Any disturbance in this stage causes dental problems
ranging from; absence of teeth, disturbed tooth
formation, or giving supernumerary teeth

29. 3. Histodifferentiation.
This phase reaches its highest development in the bell
stage just before the beginning of matrix formation.
So the differentiation of ameloblasts and stratum
intermedium is essential for enamel formation, also
the differentiation of odontoblasts is important for
the producing of dentine.
In case of vitamin A deficiency, ameloblasts fail to
differentiate properly so their organizing influence is
disturbed and atypical dentin called osteodentin is
formed.

30. 4. Morphodifferentiation
The morphologic pattern and relative size of the future tooth
is started at the early bell stage when the inner enamel
epithelium arranges them on the basement membrane. At
late bell stage the morphodifferentiation is established by
outlining the future amelodentinal junction and
amelocemental junction. The morphology of the tooth is
performed at this stage by differential growth of the
formative cells just prior to matrix deposition.

31. In Morphodifferentiation stage, endocrine disturbances affect size or form
of teeth without affecting ameloblasts and odontoblasts functions
resulted in underdeveloped or extra structures in the tooth. Also delayed
eruption in hypopituitarism and hypothyroidism causes small clinical
crown mistaken as small anatomical crown.
Twining
Extra roots or cusps Peg or mal formed tooth

33. 5. Apposition
The apposition is the deposition of the matrix of the hard
structures. It is confined to the late bell stage. The matrix is
deposited by the cells along the site outlined the formative
cells at the end of morphodifferentiation. The dentine and
enamel matrices are deposited in a rhythmic manner.
Enamel hypocacification Enamel hypoplasia

34. Function of the enamel organ
Outer enamel epithelium:
1. It limits the boundary of the enamel organ.
2. Active transport of materials specially, after hard dental
tissue formation as it becomes folded to facilitate the
passage of nutrient material to the enamel organ.
3. It picks up the calcium salts from the dental sac to either
the stellate reticulum or stratum intermedium.
4. It forms with the inner dental epithelium the epithelial
root sheath of Hertwig which is responsible for root
formation.

35. Stellate reticulum:
1. Acts as a buffer against physical forces that may distort the
configuration of the developing amelodentinal junction
giving rise to gross morphologic changes, so maintains
tooth shape
2. It seems to permit only a limited flow of nutritional
elements from the outlying blood vessels to the formative
cells, so it acts as a store house for the nutritive materials.
3. It keeps room for the developing enamel and supports its
production as they shrink and lose their intercellular fluid
to bring the ameloblasts close to the blood vessels
situated outside the outer enamel epithelium.

36. Stratum intermedium:
1. The function of this layer is not understood.
It is believed to control fluid diffusion into and
out of the ameloblasts i.e. transfer Ca from
blood vessels to ameloblasts.
2. It provides the enamel organ with proteins
(alkaline phosphatase, Ca-Mg ATPase) needed
for mineralization.
3. Those cells are histologically and histochemically
distinct from the cells with inner enamel
epithelium, but both layers considered as a
single functional unit responsible for supporting
the production and mineralization of enamel.

37. Inner enamel epithelium:
1. Morphodifferentiation function as it determines the form
and size of the crown and root portions of the tooth
2. Organizing function as it exerts an organizing influence
induction on the undifferentiated cells of the dental papilla
to differentiate into odontoblasts.
3. Formative functions as it differentiated into ameloblasts (by
reciprocal induction) that lays down enamel matrix and
helps in its mineralization.
4. This layer is arranged in a pattern to determine the future
morphology of the amelodentinal junction (A.D.J) and the
crown.
5. It forms with the outer dental epithelium the epithelial root
sheath of Hertwig which is responsible for root formation.
6. Nutritive function as it shares in the transport of the
nutritive materials from the dental papilla to the enamel
organ before dental hard tissue formation.

38. 7. Protective function as after the full
enamel thickness is deposited it
secretes an organic layer called
primary enamel cuticle.
A protective covering to enamel of
unerupted tooth against resorption
and preventing precipitation of
cementum.
7. Protective function also as it forms
with the other layers of the dental
organ the reduced enamel
epithelium which protects the
enamel surface until the tooth
erupts.
8. Contribute to the formation of the
dentogingival junction of erupted
teeth.

40. Function of dental papilla and dental sac
The dental papilla gives rise to:
1. Dentine
2. Dental pulp
The dental sac gives rise to:
1. Cementum
2. Periodontal ligament
3. Alveolar bone proper

41. Function and fate of dental lamina:
1. The maxillary and mandibular dental laminae give rise to a total 52 tooth buds or tooth
germ, 20 for the primary teeth and 32 for the permanent teeth in 3 phases.
2. Initiation of the entire deciduous dentition that occurs during the 2nd month (I.U.L), at
first the enamel organ of the deciduous teeth maintains a broad connection to the
dental lamina in the cap stage.
3. in the early bell stage it begins to breakdown by mesenchymal invasion, which first
penetrates its central portion and divides it into the dental lamina proper and lateral
dental lamina.
4. At the late bell stage complete disintegration of the dental lamina occurs.

42. 5. The dental lamina proper proliferates at its deeper margin and gives rise to
successional lamina which forms the primordium of the permanent successors
lingual to the enamel organs of the deciduous teeth, and occurs about
5(M.I.U.L) for the permanent central incisors and 10 months of age for the
second premolars.
6. The permanent molars which have no deciduous predecessors arise directly
from the distal extension of the dental lamina distal to the primary second
molars. The time of initiation of the first permanent molar is about 4(M.I.U.L),
for the second molar it is one year and the third molar four years.
7. It is thus evident that the activity of the dental lamina extends over a period of
about five years and disintegrates completely or remains as epithelial rests of
Serres.

43. Thank you