Epithelial mesenchymal interactions (EMIs) are a series of programmed, sequential and reciprocal (complex and multiphase) communications between the epithelium and the mesenchyme with its heterotypic cell population, that result in the differentiation of one or both cell populations. Odontogenesis is the process of tooth development, which involves both ectodermal and mesenchymal components, being the key elements in the development of teeth. In order for the tooth to form, an interactive mechanism between these heterotypic cellular populations is required. For these interactions to occur there should be some or other form of messenger system between epithelium and mesenchyme, further underlining the importance of cell signaling networks and intricacies of physiological growth of an individual. In the process of embryonic development the ectoderm is composed of surface ectoderm, neural crest and neural tube.

1. EPITHELIAL – MESENCHYMAL
INTERACTIONS IN
ODONTOGENESIS
Presented by-
Dr Purva Pihulkar
1

2. Table of contents
1)Introduction
-Epithelial mesenchymal interactions
-Odontogenesis
2)Epithelial mesenchymal interactions in odontogenesis
-Experimental studies
-Branchial arch formation
-Growth factors
-Role of extracellular matrix
-Initiation of tooth
-The route map of development
3) Conclusion
2

3. Introduction
 Epithelial mesenchymal interactions (EMIs)
are a series of programmed, sequential and
reciprocal (complex and multiphase)
communications between the epithelium and
the mesenchyme with its heterotypic cell
population, that result in the differentiation of
one or both cell populations.
3

4.  Odontogenesis is the process of tooth
development, which involves both ectodermal and
mesenchymal components, being the key
elements in the development of teeth.
 In order for the tooth to form, an interactive
mechanism between these heterotypic cellular
populations is required.
4

5.  Epithelial-mesenchymal interactions are the hallmark
of odontogenesis.
 EMI mechanisms can be observed in the dentino-
enamel junction of the tooth.
 The embryonic development of ectoderm derived
appendages undergoes these interactions to give rise
to a large variety of highly specialized organs.
5

6. 6

7.  For these interactions to occur there should be some or other
form of messenger system between epithelium and mesenchyme,
further underlining the importance of cell signaling networks and
intricacies of physiological growth of an individual.
 In the process of embryonic development the ectoderm is
composed of surface ectoderm, neural crest and neural tube.
 This will give rise to the epidermis or skin and the oral epithelium.
7

8. Epithelial mesenchymal
interactions in odontogenesis
8

9. 1)Initiation stage
9
 The localization, identity, shape and size of the teeth are
determined during early stages of tooth development .
 After day 12 of development, first arch epithelium loses
odontogenic potential, which then is assumed by the
ectomesenchyme; thereafter ectomesenchyme can elicit
tooth formation from a variety of epithelia.
 Early signals like Bmp4 (incisor region) and FGF8 (molar
region) from oral ectodermal cells elicit the odontogenic
potentialities of the underlying mesenchyme.

10. 10
 Numerous transcription factors expressed in the
mesenchyme are then activated, including genes coding
for several homeobox divergent transcription factors (e.g.
msx1 [muscle segment homeobox 1], msx2, dlx1 [distal-
less homeobox 1], dlx2.
 Many of these transcription factors, even within the same
family, are coexpressed and participate to functional
redundancy, allowing rescue mechanism in case of
dysfunctions.

11. 11
 Tooth development is arrested at the initiation stage when
msx1 and msx2 or dlx1 and dlx2 are inactivated.
 It indicates that odontogenesis is initiated first by factors
resident in the first arch epithelium influencing
ectomesenchyme but that with time this potential is
transferred to and is assumed by the ectomesenchyme.

12. 2)Placode Formation
12
 One of the key aspects of tooth development is the formation of
ectodermal placodes, thickenings of the epithelium at the
locations of each family of tooth.
 Similar placodes initiate the development of all organs
developing as ectodermal appendages, such as the hair, nails,
salivary, mammary and sebaceous glands.
 Signalling molecules from the four well-known families Tgfβ,
FGFs, hedgehog and Wnts families participate in placode
development.

13. 13

14. 3)Experimental studies
 The specific features of the EMIs are explained by the
studies on recombinant DNA, which focus on the
combination of two or more sources of tissues/DNA.
 These studies often involve the combination of DNA
from different organisms and depend on the ability to
cut and recombine DNA molecules at certain points
identified by specific sequences of nucleotide bases.
14

15.  In 2010, Del Moral et al. maintained that the roles
of the epithelium and the mesenchyme in these
signaling process can be determined using studies
on recombinant tissues.
 The proliferative response of epithelial cells via the
keratinocyte growth factor was investigated by
studies on recombinant tissues,which provided
some evidence that was used to study the EMIs.
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16. 16

17. 17

18.  The experimental procedures in these studies are based on a
culture of the bud stage of the tooth development process to
evaluate the role of EMI molecules.
 In 2003, Garant reported that the results from recombinant studies
revealed that the tooth continues to develop in the following two
combinations:
i) dental epithelium with dental mesenchyme and
ii) dental epithelium and skin mesenchyme.
18

19. 19

20.  On the contrary, no tooth development occurred in cultures
obtained with the dental epithelium only,
i) the dental mesenchyme,
ii) the skin epithelium with dental mesenchyme.
 For the EMIs to occur, some form of the messenger system is
also required between the epithelium and the mesenchyme.
 The messenger system could be anyone of the following seven:
20

21. Factors involved in growth and
development
1. Direct cell-cell communication
2. Matrix vesicles between two cell populations.
3. Ions like K
+
and Ca
2+
.
4. Extracellular matrix component like collagen IV, I, III, fibronectin, tenascin, E-
cadherin, laminin.
5. Molecular diffusion and transfer of information by substances like bone
morphogenetic protein (BMP-2, 4, 6, 7) FGF, EGF, TGF.
6. Autocrine and paracrine regulators.
7. Messenger RNA.
21

22.  In 2003, Irma Thesleff reported the genes
involved in signaling tooth development:pitx2,
p21, Msx2, Lef1, Edar, Lhx6, Lhx7, Dix1, Dix2,
Pax9, Gli1, Gli2,Gli3, Barx1 and Runx2.
 These genes have a role in the differentiation
and the mineralization during the tooth
development process.
22

23.  Therefore EMIs in odontogenesis are considered to be a
programmed series of events, which are controlled by
various genes, growth factors and extracellular
molecules.
 Hierarchical and sequential functional interactions
between the epithelium and the mesenchyme are
considered to be a hallmark of embryonic development.
23

24. 4)GROWTH FACTORS
 These were primarily important for signaling of
various developmental steps.
 The well regarded signals belong to the
families of Fibroblast Growth Factor (FGF),
Epidermal Growth Factor (EGF) and
Transforming Growth Factor (TGF)
24

25. Fibroblast Growth Factor (FGF)
25
 FGF belongs to a large family of heparin binding proteins that
were known to mediate the growth and differentiation of cells
from a wide variety of developmental origins.
 At the time of odontogenic initiation this expression becomes
restricted to the area of the presumptive dental epithelium and
persists until the beginning of bud stage.
 An important role by this factor was found in the differentiation
of ameloblasts as FGF4 and FGF9 were revealed in the inner
enamel epithelium.

26. Epidermal Growth Factor (EGF)
26
 EGF is very important for signaling and synergistic
effect of different factors.
 It has got a vivid role in development of an embryo
and highly essential for interaction between key
players of development.
 EGF signaling would be responsible for activation
of major tissues like submandibular glands.

27. Transforming Growth factor
(TGF)
27
 TGF and its receptor Edar were involved in multiple signaling systems
during developmental regulative mechanisms .
 Another idea was that TGF-β and Shh signaling from Hertwig's epithelial
root sheath induces the differentiation of root progenitor cells and thereby
has a direct control in modulating and transforming the formation of
odontoblasts.
 It was also established that during tooth morphogenesis, TGF-β
signaling controls odontoblast maturation and dentin formation

28. 5)Role of Extracellular Molecules
(Ecm) and Matrix
28
 ECM was present in interactions in the morphogenesis and
differentiation of developing tooth including budding of oral
epithelium and condensation of neural crest cells around the
bud.
 The interactions mediated by the basement membrane were
regulated by the differentiation of mesenchymal cells into
odontoblasts and these molecules were elaborated at that
time.
 The structural components of ECM and components affect
cellular structure.

29. 29
 Also, they were involved in regulation of interactions. The first
extra cellular matrix molecule to appear during embryonic
development is basement membrane.
 Their function includes mediation of signals for sustained and
proper development.
 By binding to specific matrix receptors on the cell surface, the
extracellular matrix molecules exert their effects on the cells
and structural components, thereby completing the circle of
organized events that finalizes the nature in which these
events turn out .

30. 7)THE ROUTE MAP OF DEVELOPMENT
 Growth does not occur in one cycle.
 Multiple channels of cell network co-exist to promote
signal propagation between cells, suggesting a type
of cell to exhibit its functional capacities.
 Teeth develop as ectodermal appendages in
vertebrate embryos and their early development
resembles morphologically as well as molecularly.
30

31.  Interactions between the ectoderm and underlying
mesenchyme constitute a central mechanism
regulating the morphogenesis of all these organs.
 During tooth initiation the ectoderm thickens and
forms a placode that buds to the underlying
neural-crest derived mesenchyme.
31

32.  Paracrine signal molecules of several conserved families
mediate cell communication during tooth development.
 Most of them belong to the transforming growth factor β (TGFβ),
fibroblast growth factor (FGF), Hedgehog and Wnt families.
 Although the signals mostly regulate interactions between the
ectoderm and mesenchyme, they also mediate communication
within one tissue layer.
32

33.  Ectodysplasin, a recently identified signal molecule
in the tumor necrosis factor (TNF) family, and its
receptor Edar mediate signalling between
ectodermal compartments in tooth germs.
 A characteristic feature of tooth development is the
reiterated appearance of transient signalling centers
in the epithelium during key morphogenetic steps.
33

34. 34

35.  These signalling centers express more than ten different
signal molecules including SHH (sonic hedgehog) and
several BMPs (bone morphogenetic proteins, belonging
to the TGFβ superfamily), FGFs and Wnts.
 The first signalling centers appear in the dental placodes
when epithelial budding begins.
 Subsequently, at the bud-to-cap transition, the enamel
knot signalling centers appear.
35

36.  These regulate the advancing morphogenesis of
the tooth crown and control the initiation of the
secondary enamel knots at the sites of epithelial
foldings that mark cusp formation.
 An early signalling event in tooth development is
the induction of the odontogenic mesenchyme by
BMPs and FGFs from the epithelium.
36

37.  Tissue recombination studies have shown that epithelial
signals induce in the mesenchyme the competence to
instruct subsequent tooth morphogenesis.
 BMPs and FGFs induce the expression of several
mesenchymal transcription factors, many of which are
necessary for the continuation of tooth development.
37

38.  The first epithelial signals induce in the mesenchyme
the expression of reciprocal signal molecules,
including activin, FGF and BMP4 which act back on
the epithelium and regulate the formation of the dental
placode.
 In addition, Wnts and the TNF signal ectodysplasin,
secreted by ectodermal cells, regulate placode
development.
38

39. 39

40.  The placodal signals then regulate budding of the
epithelium and condensation of the mesenchymal
cells.
 They maintain the expression of earlier induced
transcription factors in the mesenchyme and induce
the expression of new genes such as the transcription
factor Runx2 and the signal Fgf3, which regulate
epithelial morphogenesis from bud to cap stage.
40

41. 41
 Runx2-deficient mice show arrested tooth development at
the bud stage.
 The Wnt signalling pathway is involved in the tooth
replacement cycle with the development of the primary and
then permanent dentition.
 This signalling pathway also leads to the formation of
supernumerary teeth through the multiplication of signalling
centres via budding from the dental epithelium.

42.  At this time mesenchymal BMP4 is required for
the formation of the enamel knot at the tip of the
bud.
 It induces the expression of p21, which is
associated with the exit of the knot cells from the
cell cycle.
42

43.  The Edar receptor is also induced in the enamel
knot, making the cells responsive to the TNF
signal ectodysplasin, which is expressed in the
flanking epithelium of the tooth bud.
 Ectodysplasin-edar signalling regulates the
formation and perhaps the signalling activity of the
enamel knot.
43

44. 44
 The underlying condensing mesenchyme controls the growth
and folding of the epithelium.
 Mesenchymal signals induce, within the enamel organ, the
formation of signalling centres called enamel knots –
transitory structures which produce numerous signalling
molecules at the cap stage.
 The primary enamel knot is indispensable to crown
development. These signals pass towards the mesenchyme
and within the epithelium regulate tooth shape.

45. 45
 Shh is one signal essential for epithelial
proliferation, but its direct action seems to
target the mesenchyme, where it induces a
feedback loop signal towards the epithelium.
 Wnt and Bmp signalling regulate the formation
of enamel knots.

46.  Signals from the enamel knot affect both epithelial
and mesenchymal cells and subsequent reciprocal
interactions between the mesenchyme and
epithelium are responsible for the maintenance of
the enamel knot as well as for the subsequent
morphogenesis of the epithelium.
46

47.  An SHH signal from the enamel knot is needed for the
growth of the epithelial cervical loops flanking the enamel
knots.
 The enamel knot signals also regulate the patterning of the
tooth crown by influencing the initiation of the secondary
enamel knots that express most of the same signal
molecules as the primary enamel knots.
47

48. 48

49.  They form in an exact sequence and
determine the sites where the epithelial sheet
folds and cusp development starts.
 Their development is regulated by signals from
earlier formed primary and secondary enamel
knots together with mesenchymal signals.
49

50. 50

51.  Conceivably this involves mechanisms of lateral
inhibition and activators and inhibitors.
 Recently a gene network model was presented that
can reproduce both the reiteration of the epithelial
signalling centers and their gene expression patterns
as well as the resulting tooth morphologies of
different mammalian species (Salazar-Ciudad and
Jernvall, 2002)
51

52.  It is obvious that the signalling networks regulating
tooth morphogenesis are much more complex.
 For example, there are numerous specific inhibitors of
signals that have central roles in modulating locally the
signalling activities.
 Also, the different signalling pathways are integrated at
various levels and have synergistic as well as
counteractive effects.
52

53.  Nevertheless, the model illustrates the general principle
of the development of multicellular organisms, namely
that the cells and tissues communicate via conserved
signal molecules which are used reiteratively during
advancing morphogenesis.
 The variation in cellular responses to the same signals
in different tissues and at different times is caused by
the different histories of the cells determining their
competence to receive and respond to the signals.
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54. 54

55. Dental Anomalies
55
 Acquired dental anomalies are relatively common and
provide real markers of gene–environment relationships,
pointing to pre- and post-natal pathological developmental
pathology, easily accessible and readable via fixed teeth
morphology and appearance throughout mineralization.
 Missing teeth (anomalies of tooth number) concern tooth
agenesis, hypodontia, oligodontia and anodontia and are
associated with mechanisms regulating tooth patterning as
well as the transition from the bud to the cap stage.

56. 56
 In the mouse, inactivation of different genes leads to the arrest of tooth
development.
 Genes shown to be involved in arrest of tooth development in mice include
dlx1, dlx2, lef1, pax9, pitx2, runx2 and the activin-coding gene .
 In humans, mutations in these same genes, MSX1, PAX9 and PITX2, also
result in missing teeth according to specific patterns.

57. Conclusion
 The completion of epithelial mesenchymal interactions and
transitions brings about varied changes in the developing
embryo and tooth development, thereby increasing the
potential of individual cell to grow and substantiate the
necessary development required for tooth formation.
 The complex mechanism involved are to be studied
exhaustively in order to overcome the difficulties arising due
to the improper knowledge of pathogenesis of a specific
odontogenic lesion thus jeopardizing treatment options.
57

58.  The expansive field of dentistry requires genetic research and
molecular level of contemplation to highlight the significance of
biological processes, thus reflecting on the overall management
protocols.
 A paradigm shift is needed in this regard to appropriately digress
the meshwork of intricacies associated with any developmental
scenarios.
 The “end of the tunnel” should surely be reached once sufficient
knowledge and expertise is gained in further increasing our
knowledge on several new pathways and factors attached to
developmental scenarios.
58

59. References
59
 Epithelial-mesenchymal signalling regulating tooth
morphogenesis Irma Thesleff(2003) Journal of Cell Science
116, 1647-1648
 Reiterative signaling and patterning in mammalian tooth
morphogenesis. Jernvall, J. and Thesleff, I. (2000). Mech.
Dev. 92, 19-29
 The role of growth factors in tooth development. Thesleff, I.
and Mikkola, M. (2002). Int. Rev. Cytol. 217, 93-135.
 Odontogenesis, Anomalies and Genetics. Bloch-Zupan, A.,
Sedano, H. O., & Scully, C. (2012). Dento/Oro/Craniofacial
Anomalies and Genetics, 1–8.

60. 60
 Epithelial – Mesenchymal Interactions in Tooth Development
and the Significant Role of Growth Factors and Genes with
Emphasis on Mesenchyme – A Review.Puthiyaveetil J S V,
Kota K,chakkarayan R,chakkarayan J, thodiyil A K P. J Clin
and Diag Res. 2016 Sep, Vol-10(9): ZE05-ZE09
 The epithelial-mesenchymal interactions: insights into
physiological and pathological aspects of oral tissues.A.B.
Rajendra Santosh and T.J. Jones. Oncology Reviews 2014;
8:239
 Tencate’s oral histology 9th edition

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