development of teeth and embryology

1. Development of teeth:
Contents:
 General embryology
 Development of tooth
 Stages of development of tooth
 Cellular components involving
in the tooth development.
 Histophysiology of tooth
formation.

2. GENERAL EMBRYOLOGY
PRENATAL DEVELOPMENT : is divided into 3 successive phases.
PERIOD OF OVUM : It spans the 1st two weeks of fertilization.
This phase mainly involves cellular proliferation and differentiation.
PERIOD OF THE EMBRYO OR MORPHOGENIC PHASE :this phase
Extends from the 14th day to 56th day of intra uterine life and is
characterized mainly by differentiation of the major internal and
external organs.
PERIOD OF FETUS– extends from the
second month till term.
The embryo is now called a fetus.
During this phase only further growth and maturation of the developing
organs occur.

3. CHRONOLOGY OF EVENTS:
2 days after
fertilization
•embryo is in 2 cell stage.
3 days after
fertilization
•embryo is now a morula or in 16 cell stage.
4th day
•blastocyst is formed .
8 th day
•bilaminar germ disc is formed.

4. 14th day
•the prochordal plate and primitive plate is formed
16th day
•formation of intra embryonic mesoderm or trilaminar germ disc
17 day
• notochordal process is seen.
21st day
•neural groove is formed and head fold is seen.
23rd day
•closure of neural tube .

7. MORULA – the ovum after cleavage comes
to have 16 cells.
This 16 cell stage is called as morula .
The outer layer is called as – TROPHOBLAST.
The inner layer is called as – EMBRYOBLAST
BLASTOCYST – fluid from the uterine
cavity seeps into the morula and partially
separates the cells of the inner cell mass
from the trophoblast.
This cyst is called as blastocyst and the
cavity of the cyst as blastocoele.

9. PRIMITIVE STREAK – some of the ectodermal cells
near the tail end of the disc begin to proliferate and form an
elevation that bulges into the amniotic cavity. This elevation is
called the PRIMITIVE STREAK.
.The cells that proliferate in the region of the primitive streak
passes pushing themselves between the ectoderm and
endoderm and form the INTRA EMBRYONIC MESODERM.
The process of formation of primitive streak and intra embryonic
mesoderm is called as GASTRULATION.

10. The embryo is now said to have three germ
layers.
This intra embryonic mesoderm passes
through the disc except in the region of the
prochordal plate .
In the region of the prochordal plate the
ectoderm and endodem remain in contact
and latter form the BUCCOPHARYNGEAL
MEMBRANE

11. FORMATION OF NEURAL TUBE
The ectoderm overlying the notochord
undergoes changes that result in the
formation of the neural tube.
The ectoderm overlying the notochord
becomes thickened to form the neural plate.
The neuralplate becomes depressed along
the midline to form the neural groove.

14. At the same time ,the two edges of the neural
groove come close to each other and eventually
fuse, thus the neural groove becomes the neural
tube .
The neural tube has a enlarged cranial part
and a tubular caudal part.
CRANIAL PART -BRAIN
CAUDAL PART -SPINAL CORD
Since the development of the nervous system is
from the ectoderm , it is called as
NEUROECTODERM

16. The neural crest cells in the head
differentiate to form most of the connective
tissue of the head.
In dental context, the proper migration of
neural crest cells is essential for the proper
development of head and teeth.
Eg: Treacher Collins Syndrome , full
facial development does not occur because
the neural crest cells fail to migrate to the
facial region properly

17. HEAD & TAIL FOLD OF EMBRYO
On the 21st day , the embryo folds in two
planes i.e along the rostrocaudal axis and
the lateral axis.
This head fold is crucial to the formation of
a primitive stomatodeum.
The region of the prochordal plate forms
the buccopharyngeal membrane.

19. MOLECULAR INSIGHTS OF TOOTH
MORPHOGENESIS
WHAT ARE TRANSCRIPTION FACTORS
AND GROWTH FACTORS ?
These are the two principal groups of
molecules that are involved in the
reciprocal exchange of information
between tooth epithelium and
mesenchyme.
TRANSCRIPTION FACTORS : are proteins that
bind to DNA during the start of transcription
of a gene.
They regulate gene expression by facilitating or
inhibiting the enzyme RNA Polymerase .
Mutations involving transcription factors often results in defects of
tooth formation.

20. GROWTH FACTORS : they are secreted
proteins that are capable of binding to
specific receptors on the cell surface.
Further interaction with both membrane
and cytoplasmic components leads to a
series of complex intracellular events that
results in altered gene expression. These
changes activate cell growth and
differentiation.

22. INITIATION OF TOOTH DEVELOPMENT
How does tooth initiation begin?
Whether the tooth initiation potential lies
in the epithelium or mesenchme ?
Odontogenesis is initiated first by factors
resident in the first arch epithelium.After the
12th day of development , first arch epithelium
loses this odontogenic potential which is then
acquired by the ectomesenchyme.

23. TRANSCRIPTION FACTORS ESSENTIAL FOR TOOTH INITIATION
The earliest mesenchmal markers for tooth
formation are the Lim- homeobox domain
genes – Lhx-6 & Lhx-7 genes.These genes
are expressed in the ectomesenchyme of
the oral half of the first brachial arch as
early as the 9th day.
The expression of FGF-8 establishes the
anteroposterior positioning of the teeth and is also
attributed to be regulating the expression of Lhx-6
& Lhx -7 genes.

24. CONTROL OF TOOTH GERM POSITION
FGF-8 apart from playing a role in the
initiation of tooth also plays a role in
determining the position where the tooth
germs will form.
The PAX-9 gene is one of the earliest
mesenchymal markers defining the
localisation of the tooth germs.

25. TOOTH TYPE DETERMINATION
The determination of specific tooth types at their correct position in
the jaws is referred to as patterning of the dentition .

26. DEVELOPMENT OF TOOTH
The primitive oral cavity or stomadeum is
lined by stratified squamous epithelium
called the ORAL ECTODERM or PRIMITIVE
ORAL EPITHELIUM.
Most of the connective tissue underlying
the oral ectoderm are of neural crest or
ectomesenchymal in origin.
The oral ectoderm contacts the endoderm of
the foregut to form the BUCCOPHARYNGEAL
MEMBRANE
At about the 27th day of gestation , the membrane ruptures and
the primitive oral cavity establishes a connection with the
Foregut.

27. 2 to 3 weeks after the rupture of the
buccopharyngeal membrane when the
embryo is about 6 wks old, certain areas of
the basal cells of the oral ectoderm
proliferate more rapidly than the cells of
the adjacent area which leads to the
formation of the PRIMARY EPITHELIAL
BAND.
At about the 6wk , the primary epithelial band
divides into an INNER LINGUAL PROCESS
called the DENTAL LAMINA & an outer BUCCAL
PROCESS called the VESTIBULAR LAMINA.

29. This dental lamina serves as the
primordium for the ectodemal portion of
the deciduous teeth.
Later during the development of the jaws , the
permanent molars arise directly from a distal
extension of the dental lamina and the
permanent incisors canine and premolars
develop from the lingual extension of the
dental lamina called as the SUCCESSIONAL
LAMINA.

30. Structures involved in tooth
formation:
• Dental papilla
• Dental follicle/sac
• Enamel organ

31. VESTIBULAR LAMINA
• Labial and buccal to the dental lamina in each arch another
epithelial thickening develops independently to form the
VESTIBULAR LAMINA OR LIP FURROW BAND
• The cells of the vestibular lamina enlarge rapidly and then
degenerate to form a cleft that becomes the vestibule between the
teeth bearing area and the cheek.

32. TOOTH DEVELOPMENT
At certain points along the dental lamina
each representing the location of the future
deciduous teeth , the ectodermal cells
multiply rapidly and form little knobs that
grow into the underlying mesenchyme.
Each of these little outgrowths represent
the enamel organ of the tooth bud of a
deciduous teeth.
All these enamel organs do not form at the same
time and the first to appear are those of the
mandibular anterior region.

33. A tooth germ consists of an :
ECTODERMAL COMPONENT
ECTOMESENCHYMAL COMPONENT
DEVELOPMENTAL STAGES:
The developmental history of a tooth is
divided into several morphological stages
for descriptive purposes.
They are : 1)BUD STAGE
2)CAP STAGE
3)BELL STAGE

34. BUD STAGE
It is represented by the first epithelial
incursion into the ectomesenchyme
of the jaw.
The epithelial cells show little change in
shape and function.
In the bud stage ,the enamel organ consists
of peripherally located low columnar cells
and centrally located polygonal cells.

36. CAP STAGE
At this stage, the tooth forming potential is
transferred from the epithelium to the
dental papilla.
As the tooth bud continues to grow, it takes
on the shape of a cap or it resembles a cap
sitting on a ball of condensed
mesenchyme.
Formation of enamel knots are seen in this stage. Secondary enamel
knots are seen only in the molars which has multiple cusps.

37. 1)DENTAL FOLLICLE
2)ENAMEL ORGAN
3)STELLATE RETICULUM
4)DENTAL PAPILLA
5)ENAMEL KNOTS

38. A vertical extension of the enamel knot
occurs which is called as the ENAMEL
CORD.
Where the enamel cord extends to meet
the outer enamel epithelium, it is called as
ENAMEL SEPTUM, for it would divide the
stellate reticulum into two parts.
The outer enamel epithelium at the point of
meeting shows a small depression called ENAMEL
NAVEL.
Recent studies show that the enamel knot
is an organizational centre which
orchestrates cuspal morphogenesis.

39. BELL STAGE
As the invagination of the epithelium
deepens and its margins continue to grow,
the enamel organ assumes a bell shape.
The bell stage is characterised by few
important events :
1)Morphodifferentiation
2)Histodifferentiation
3)Dental lamina breaks up
4)Formation of Stratum Intermedium
5)Formation of cervical loop

42. During the bell stage, crown shape is
determined.
The folding of the enamel organ to form
different crown shapes is due to
differential rates of mitosis and differences
in cell differentiation time.
The point at which the inner enamel epithelium
differentiates first represents the site of future cusp
development or growth centre.
The pressure exerted by the continuous
cell division upon these differentiating
cells cause these cells to be pushed out
into the enamel organ in the form of a cusp
tip.

43. Cells seen in the bell stage are :
1)INNER ENAMEL EPITHELIM
2)STRATUM INTERMEDIUM
3)STELLATE RETICULAM
4)OUTER ENAMEL EPITHELIUM
INNER ENAMEL EPITHELIUM: The cells of the inner
enamel epithelium just prior to amelogenesis
differentiate into tall columnar cells, the
AMELOBLASTS.
The cells of the inner enamel epithelium
exert an organizing influence on the
underlying mesenchymal cells in the
dental papilla which later differentiate into
odontoblast

44. STRATUM INTERMEDIUM
A few epithelial cells between the inner
enamel epithelium and the stellate
reticulum form the STRATUM INTERMEDIUM.
These cells show high activity of the
enzyme Alkaline Phosphotase.
This layer is essential for enamel
formation.
It is absent in the part of the tooth germ that forms
the root portions of the tooth and hence enamel is
not found in the root.

45. STELLATE RETICULUM
The stellate reticulum expands further
mainly by a further increase in the amount
of intercellular fluid.
Just before enamel formation begins, the
stellate reticulum collapses reducing the
distance between the centrally situated
ameloblast and the nutrient capillaries
near the outer enamel epithelium.

46. OUTER ENAMEL EPITHELIUM
Just preparatory to and during the
formation of enamel, the smooth surface of
the outer enamel is laid in folds.
Between the folds the mesenchyme of the
dental sac forms papilla, that has capillary
loops which provide nutritional supply for
the avascular enamel organ.
This compensates for the loss of the nutritional
supply from the dental papilla owing to the
formation of mineralized dentin.

47. DENTAL PAPILLA
Before the IEE begins to produce enamel
the peripheral cells of the mesenchymal
dental papilla differentiate into
odontoblast and acquire the specific
potential to produce dentin.
Lamina fibroreticularis
The basement membrane that separates the enamel organ
and dental papilla just prior to dentin formation is called as
MEMBRANA PREFORMATIVA.

48. DENTAL SAC
The dental sac shows a circular
arrangement of its fibres and resembles a
capsular structure.
With the development of root, the fibres of
the dental sac differentiate into the
periodontal fibres that become embedded
into the developing alveolar bone.

49. ADVANCED BELL STAGE
This stage is characterized by the
commencement of mineralization and root
formation.
During the advanced bell stage, the
boundary between IEE and odontoblast
outline the future DENTINOENAMEL
JUNCTION.
Dentin is deposited first at the future cusp tip
and then proceeds pulpally and apically.
After the first layer of dentin is formed ,the ameloblast, which
is already formed from the IEE lays down enamel over the
dentin in the future incisal and cuspal areas.

50. Enamel formation then proceeds
coronally and cervically from the DEJ to
the surface.
In addition, the cervical portion of the
enamel organ gives rise to the HERTWIGS
EPITHELIAL ROOT SHEATH.
The HERS outlines the future root and its
responsible for the shape, length, size and
number of roots.

51. DEVELOPMENT OF ROOT.
The development of the root begins after
enamel and dentin has reached the future
Cemento-enamel junction.
The cervical loop of the enamel organ
forms the HERTWIGS EPITHELIAL ROOT
SHEATH , which molds the shape of the
roots and initiates radicular dentin
formation.
HERS consists of only the outer and inner enamel
epithelium. The cells of the inner enamel epithelium
remain short and do not produce enamel.
Prior to the beginning of root formation, the root sheath forms
the epithelial diaphragm .

52. The free end of the diaphragm does not grow into the connective tissue,
but the epithelium proliferates coronal to the epithelial diaphragm.
The differentiation of the odontoblasts and the
formation of dentin follow the lengthening
of the root sheath.
At the same time, the connective tissue of the
dental sac surrounding the root sheath proliferates
and invades the epithelium dividing it into a
network of epithelial strands.
The epithelium is then moved away the surface of the dentin , so that
the connective tissue cells come into contact with the outer surface of
the dentin and differentiate into odontoblasts that deposit a layer of
cementum on the surface of dentin.

54. FORMATION OF A MULTIROOTED
TEETH
During the growth of the enamel organ ,
the expansion of the cervical opening
occurs in such a way that long tongue like
extensions of the horizontal diaphragm
develop.
Before division of the root trunk occurs ,
the free ends of the horizontal epithelial
flaps grow towards each other and fuse.
Thus the single cervical opening of the coronal
enamel organ is divided into 2 or 3 openings.

55. If the continuity of the HERS is broken
before the dentin formation , a defect in
the dentinal wall of the pulp ensues.
These defects are seen on the pulpal floor
corresponding to the furcation areas and this
accounts for the development of the
accessory canals on the periodontal surface
of the root.

57. HISTOPHYSIOLOGY OF TOOTH
FORMATION
INITIATION : this stage is first noticed in a
6wk old fetus.
Initiation refers to the ability of the dental
lamina to respond to certain factors that
induce tooth development.
PROLIFERATION : is only a further
multiplication of the cells which were
initiated in the initiation stage and results
succesively in the bud, cap and bell stages
of the enamel organ.

58. HISTODIFFERENTIATION : in this phase ,
the cells in the proliferative stage undergo
definitive morphologic and functional
changes and acquire their functional
assignment.
The cells differentiate and give up their
capacity to multiply as they assume their
new function.
This phase reaches its highest development in the bell
stage of the enamel organ , just preceding the
beginning of formation and apposition of enamel and
Dentin.

59. MORPHODIFFERENTIATION : the
morphologic pattern or basic form and
relative size of the future tooth is
established by morphodifferentiation.
The advanced bell stage marks the
important stage of morphodifferentiation
of the crown outlining the future
Dentino-enamel junction.
In conformity with this pattern , the
ameloblasts , odontoblasts and
cementoblasts deposit enamel , dentin and
cementum respectively and thus give
completed tooth its characteric form and
size.

60. APPOSITION : is the deposition of the
organic matrix of the hard dental
structures .
It is characterized by regular and rhythmic
deposition of the extracellular matrix,
which accounts for the layered appearance
of enamel and dentin.

61. CALCIFICATION : occurs with an influx of
mineral salts within the previously
developed tissue matrix.
Calcification begins with the precipitation
of enamel on the cusp tips and incisal
edges and progresses cervically.
Therefore the older or more mature enamel is
found at the cusp tips or incisal edges and the
newly formed enamel at the cervical region.

62. Fate of the different structural layers of tooth germ:
HERS cell rests of mallasez
Periapical cyst
Residual cyst
Enamel pearls
Outer enamel and
inner enamel
epithelium
Reduced enamel
epithelium
Junctional epithelium
and gingival col,
Dentigerous cyst,
Eruption cyst
Dental lamina
OKC,gingival cysts,
Lateral periodontal
cyst,
Glandular
odontogenic cysts.

63. Initiation Oligodontia, Supernumerary
teeth
Histo differentiation Amelo genesis
Morpho
differentiation
Dens invaginatus, Dens
evaginatus, Taurodontism
Apposition Enamel hypoplasia

65. Next seminar:
 Enamel:
Composition, function and
structure.