Braz J Oral Sci. October/December 2003 - Vol. 2 - Number 7
Genes and tooth development:
reviewing the structure and function of
some key players.
R. M. Scarel-Caminaga 1
S. Pasetto 2
E. Ribeiro da Silva 2
R. C. R. Peres 2
University of the Sacred Heart, Bauru, São
Paulo, Brazil
2
Department of Morphology, Piracicaba
Dental School, State University of
Campinas, Piracicaba, São Paulo, Brazil
1
Abstract
Similar to many other embryonic organs, the mammalian tooth development relies largely on epithelial-mesenchymal interactions. Tooth
development may be divided in multiple stages, where the number,
size and type of teeth are sequentially determined. Teeth are serially
homologous structures, which allow the localization and quantification of the effects of specific gene mutations. Furthermore, it is also
possible to determine the phase of odontogenesis affected by these
conditions. These features make anomalies involving teeth an important system to understand the intricate molecular mechanisms that
regulate developmental process. In this paper we review the structure
and function of some key molecules that participate in tooth development.
Key Words:
tooth morphogenesis, genes, molecular signaling
Received for publication: April 7, 2003
Accepted: September 22, 2003
Correspondence to:
Raquel M. Scarel Caminaga
Universidade do Sagrado Coração – USC
Pró-Reitoria de Pesquisa e Pós-Graduação
Rua Irmã Arminda, 10-50
CEP 17011-160, Bauru – São Paulo – Brazil
e-mail: [email protected]
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Genes and tooth development: reviewing the structure and function of some key players.
Introduction
In the past few years much progress have been made on the
understanding of the mechanisms that control tooth
morphogenesis. Tooth development is controlled by specific
interactions between its epithelial and neural crest-derived
mesenchymal components1-3. The epithelial-mesenchymal
interactions are reciprocal and sequential, and each
component may play important roles in organogenesis,
depending on the organ system and the developmental stage.
Epithelial-mesenchymal interactions govern the development
of all epidermal organs, including teeth, hair follicles and
mammary glands4,5. The initial phases of the development of
these organs is similar, been characterized by the budding of
the lining epithelia towards the underlying mesenchyme. The
development of these organs then diverges to give rise to
specialized organs with vastly different morphologies, cell
types and functions6,7.
The odontogenesis of the mouse is the predominant system
for experimental studies because this animal is suitable for
both genetic and embryological studies. In the mouse
embryo, molar tooth development commences
morphologically at mouse embryonic day 11.5 (E11.5), with a
thickening of the dental epithelium to form the dental lamina
(Fig.1). At E12.5 the dental epithelium invaginates into the
surrounding oral mesenchyme to form a tooth bud. During
the late bud stage, at E13.5, the invaginating epithelium
becomes thick and induces the proliferation and
condensation of the underlying dental mesenchyme8. Cells
at the tip of the molar bud stop dividing, and form the enamel
knot, a morphologically distinct group of cells acting as a
signaling center within the dental ectoderm, which is thought
to control shape. At E14.5 the tooth bud folds at its tip and
forms a cap-resembling structure surrounding the
mesenchymal dental papilla. Continuing growth and folding
in the epithelium determines the shape of the tooth crown
during early bell stage (E16.5). At late bell stage (E18) the
adjacent epithelium and mesenchymal cell layers differentiate
into enamel-secreting ameloblasts and dentin-secreting
odontoblasts, respectively7.
More than 200 genes have been identified in developing
teeth 7,9 . The expression patterns of many genes can be
viewed in a graphical www database (http: //
honeybee.helsinki.fi/toothexp 10 ). Some of those genes
belong to the Hox family, which contains a Homeobox, an
evolutionarily conserved DNA sequence motif found initially
in Drosophila genes 8,11-12. The homeobox encodes the
homeodomain, a DNA-binding motif, and homeotic genes
encode transcription factors13-14. The homeotic genes studied
in mammals are homologues of Drosophila genes. The Hox
clusters are present today from cephalochordates to
mammals, but the orthography of the homeotic genes is little
different among invertebrates, mice and humans; for example,
the msh gene in Drosophila is homologue of mouse Msx1
gene and human MSX1 gene.
In the tooth development, besides transcription factors, there
are other important molecules that play central roles in the
process, like growth factors and extracellular matrix (ECM)15.
These groups of molecules have combined expression in
dynamic patterns between epithelial and mesenchymal
tissues as a “molecular cascade”, which promotes the
development of the tooth. Some principal representatives of
each group of molecules are described below.
Fig. 1 – Schematic representation of the odontogenesis of molar
mouse tooth, and the expression of molecules in the epithelial and
mesenchymal dental tissues (based on ZHAO et al.28).
1. Transcription Factors
Transcription factors are molecules that interact with DNA
modulating the expression of a gene. Inactivation in genes
that encode transcription factors can modify the phenotype,
especially if they are expressed in the early stages of
embryogenesis16. In case of Homeobox genes, which present
domains conserved filogenetically, a mutation can attenuate
the interaction of this molecule with target DNA17.
1.1 MSX (Muscle Segment Box )
The Msx gene family consists of three physically unlinked
members in the mammalian genome. The Msx3 is the most
primitive gene in mice because its expression (only in the
dorsal neural tube) resembles the expression pattern of msh
in Drosophila. The Msx1 and Msx2 genes have arisen by
two successive gene duplication events acquiring their
expression properties. During mid gestation, Msx1 and Msx2
expressions occur at almost all sites of epithelialmesenchymal tissue interactions 8. At E11.5, Msx1 is coexpressed with Msx2 in the dental mesenchyme (Fig. 1).
However, the Msx1 is expressed quite broadly (in high levels)
in the mesenchyme and the Msx2 expression is restricted to
the mesenchyme around the tooth-forming regions. Msx1 is
also strongly expressed in the developing molar and incisor
tooth germs in a distal-to-proximal gradient in mesenchyme
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Genes and tooth development: reviewing the structure and function of some key players.
of the mandibular and maxillary processes. The expression
of Msx1 in the dental mesenchymal tissue remains high
throughout the cap stage, being down regulated at the early
bell stage (E16.5). The Msx2 gene is expressed at early cap
stage in the enamel knot, in the internal enamel epithelium
and the dental papilla mesenchyme. With odontoblast
differentiation, Msx2 becomes strongly expressed in the
odontoblastic cells8. Mice deficient for Msx1 exhibit an arrest
in molar tooth development at E13.5 bud stage, while mice
deficient for Msx2 exhibit defects in cuspal morphogenesis,
root formation and enamel organ differentiation8,16.
In humans, the role of MSX genes in craniofacial development
has been highlighted by the identification of MSX1 and MSX2
mutations, which are associated with alteration of the
normality. The G®C transversion at the homeobox region of
the MSX1 gene results in a substitution of an Arginine to
Proline in the conserved domain of the protein. It is the cause
of the specific patterns of tooth agenesis 17, however, the
mutation in the MSX1 gene cannot explain all types of tooth
agenesis18. A mutation in the homeodomain of the MSX2
gene was associated with dominant craniosynostosis19.
1.2 PAX (Paired Box )
The mammalian Pax gene family is formed by nine members
that present a conserved DNA-binding motif, named paired
domain20. Pax proteins have been implicated as regulators of
organogenesis and as key factors in maintaining pluripotency
of stem cell populations during development. Mutations of
the Pax genes cause profound developmental defects in
organisms as diverse as flies, mice and humans21, 22.
The genes of the Pax family can be grouped into paralogous
relationships: Pax1 and 9, Pax2, 5 and 8, Pax3 and 7, Pax4
and 6, are most closely related both in sequence and in
expression pattern8. The Pax1 and Pax9 are expressed in
developing tooth. The strong expression of the Pax9 found
in the mesenchyme from E12.5-E14.5, appears to be an early
signal of tooth development23 (Fig.1). Pax9 knockout mice
exhibit an arrest in molar tooth development at the bud stage,
and it is phenotypically similar to that observed in Msx1
mutants 24.
The human PAX9 gene is located in chromosome 14q12-q13.
Mutations in this gene were implicated in tooth agenesis. A
family with autosomal dominant oligodontia, affecting
premolars and molars, presented a frameshift mutation in the
paired domain of the PAX925. In another case, two members
of a small family showed absence of all molars and premolars
both in maxilla and mandible, as well as some incisors. That
was caused by a deletion encompassing the PAX9 locus in
one chromosome, while the other was normal. These findings
suggest that PAX9 is a dosage-sensitive gene in humans,
with haploinsufficiency causing hypodontia26.
1.3 DLX (Distal-Less Box )
341
In mammals the Dlx gene family consists of six members which
are arranged in three closely linked pairs: Dlx1 and 2, Dlx7
and 3, Dlx6 and 527. The expression patterns of Dlx genes in
developing mouse molar tooth are summarized in Figure 1.
Dlx1 and Dlx2 genes are expressed in both maxillar and
mandibular processes. The expression domain of Dlx1 in
mandibular arch is more distally restricted than those of Dlx2,
Dlx5 and Dlx6. The expression of Dlx3 and Dlx7 in the distallabial region of mandibular arch is located clearly away from
the tooth-forming area28. In the mouse E12-14, Dlx1 and Dlx2
genes are expressed in dental mesenchyme and epithelium,
respectively, coinciding with mesenchymal Msx1 and
epithelial Msx2 expression8. At the bud stage Dlx2 and Dlx3
were expressed in the labial dental epithelium, Dlx2 is also
expressed in the mesenchyme. At the cap and early bell
stages, Dlx1 and Dlx6 were expressed in the dental follicle.
Dlx2, Dlx3, Dlx5 and Dlx7 were detected in the dental papilla.
During cap and bell stages all six Dlx genes exhibit complex
expression patterns in the developing teeth (Fig.1). Dlx5 was
expressed in the cervical loop. The expression of Dlx2 and
Dlx3 suggests a role for these genes in ameloblast
differentiation 28. The role of Dlx3 in amelogenesis was
supported by mutations of human DLX3 gene that are
associated with enamel hypoplasia29. Inactivation of Dlx5
affects maturation of the dental enamel30. In E10.5 murine
embryos, none of these Dlx genes were expressed in the
lingual side of the medial nasal process, the region from which
upper incisors develop. This situation was changed at E16E17 where all six Dlx members were expressed in the lower
incisal region28.
1.4 LEF (Lymphoid Enhancer Factor)
Genes belonging to the Lef family have the capacity to induce
a sharp bend in the DNA helix and produce proteins
characterized as High Mobility Group (HMG)31-32. Common
properties of HMG domain proteins include interaction with
the minor groove of the DNA helix, binding to irregular DNA
structures, and the capacity to modulate DNA structure by
bending 32-33.
Studies suggest an essential role for Lef1 in the formation of
several organs and structures that require inductive tissue
interactions34. LEF1 is a cell type-specific transcription factor
expressed in lymphocytes of the adult and during
embryogenesis. It is expressed in neural crest,
mesencephalon, tooth germs, whisker follicles and other
sites35-38. This gene was mapped in human chromosome 4
(q23-q25) and in mouse chromosome 3 near Egf (Epidermal
Growth Factor) gene. The human LEF1 is a 54-KDa nuclear
protein that binds to a functionally important site in the Tcell receptor alpha and contributes to its maximal enhancer
activity39.
The Lef1 (-/-) mutant mice lack teeth, mammary glands,
whiskers and hair, but show no obvious defects in lymphoid
Braz J Oral Sci. 2(7):339-347
Genes and tooth development: reviewing the structure and function of some key players.
cell populations at birth. Tooth development is initiated in
Lef1 (-/-) embryos, however it is arrested before the formation
of a mesenchymal dental papilla. Likewise, development of
body hair follicles and mammary glands are incomplete or
abrogated before morphogenesis. All organs that are affected
by the mutation in the Lef1 gene share a requirement for
tissue interactions between ectoderm derived epithelium and
mesenchyme40.
The Lef1 gene expression in E10-E16 mouse embryos was
assessed by in situ hybridization and immunohistochemistry.
At E12, Lef1 is expressed in the condensing mesenchyme
around the invaginated epithelial tooth bud. At E13, Lef1 is
expressed in the mesenchyme condensed and in the
immediately adjacent basal cells of the epithelium. During
the subsequent cap and bell stages of tooth development
(E14-E16) Lef1 transcripts are continuously detected in both
tissues, including the mesenchymal papilla and
preodontoblasts, and in the epithelium-derived
preameloblasts (Fig.1)37.
The expression of Lef1 in the epithelium seems to be critical
for the induction of the mesenchyme between E13 and E14
to form dental papilla, but is dispensable for both the initiation
of tooth development and the epithelial and mesenchymal
cytodifferentiation. Although Lef1 is expressed at the earliest
stages of tooth development, Lef1 (-/-) mouse embryos
initiate the formation of tooth germs34. The first visible defect
in tooth development of Lef1 deficient embryos can be
detected at the late bud stage around E13 when the dental
papilla fails to form. In particular, the mutant dental epithelium
does not form the enamel knot and fails to adopt the cap
shape at later stages34. Epithelial Lef1 expression seems to
be necessary for the induction of the mesenchyme,
presumably for the formation of the dental papilla, but is
dispensable for further cytodifferentiation40.
1.5 Other important Transcription Factors
The Sonic hedgehog (Shh) gene is a vertebrate homologue
of the gene hedgehog (hh), which controls the Drosophila
segment polarity and organogenesis41. The Shh gene exerts
its short- and long-range effects by activating downstream
gene expression 42 . During tooth development, Shh is
expressed in dental epithelium from E11.5 throughout bud
stage and in the enamel knot at cap stage43. The Patched
(Ptc) gene, originally identified as a Drosophila segment
polarity gene44, encodes a transmembrane protein, which
serves as Shh receptor. Interestingly, ectopic Shh expression
leads to ectopic Ptc expression in several vertebrate
developing organs 43 . In developing tooth, Shh may be
involved in this process by activating the expression of its
receptor Ptc and the transcription factor Gli1. Therefore, the
Gli1 is a component of the Shh signaling pathway. The Gli1
gene resides downstream of Shh but upstream to Ptc. In
addition, Gli1 expression was detected in both dental
epithelium and dental mesenchyme from E11.5 to E14.5,
coinciding with Ptc expression pattern43.
2. Growth Factors
Growth factors constitute an important class of signaling
molecules. A growth factor produced by one cell may either
affect the behavior of another cell in its vicinity (paracrine),
or it may have an autocrine effect. The effects of growth
factors are always mediated through binding to specific cell
surface receptors45. Signaling interactions, which determine
the location, identity, size, and shape of teeth, take place
during early stages of tooth development. The most studied
signals belong to the families of FGF (Fibroblast Growth
Factor), EGF (Epidermal Growth Factor) and TGF
(Transforming Growth Factor). Each family consists of several
signals encoded by different genes46.
2.1 FGF (Fibroblast Growth Factor)
Fibroblast growth factors (FGFs) are a large family of heparin
proteins, which have potent morphogenetic effects in several
organs and are potent stimulators of cell proliferation. FGFs
induce cell division both in dental mesenchyme and
epithelium at several stages of tooth morphogenesis47-48. FGFs
also prevent apoptosis in dental mesenchyme, mimicking
the effect of epithelium49. Ten members of FGF family were
already identified. These molecules have very similar
biological effects. There is a possible functional redundancy
between the co-expressed FGFs50-51. FGFs 2, 4, 8 and 9 genes
have similar effects on dental mesenchyme in vitro. They all
stimulate cell proliferation as well as expression of Msx1 in
E11-E15 dental mesenchyme48.
Fgf8 is expressed in the ectoderm covering the tooth-forming
region at the initiation stage. At E10.5, expression of Fgf8 in
the mandibular epithelium is responsible for inducing the
expression of Pax9 in the mesenchyme at the prospective
sites of odontogenesis52-53. Fgf9 is co-expressed with Fgf8
in the oral ectoderm at initiation stage. It is likely that Fgf8
and Fgf9 act together to control the expression of FGF
responsive genes, including Pax9, in early branchial arch
mesenchyme 54. It was suggested that FGF signaling was
required for development of both molar and incisor teeth
and that the specification of tooth mesenchyme involves at
least two FGF-dependent steps.
Other members of FGF family, Fgf3, Fgf4, and Fgf10, are also
expressed at different stages of odontogenesis48,55. Fgf4 is
only expressed in the enamel knot cells while Fgf3 is expressed
in the mesenchyme at the late bud stage (Fig.1). During cap
and bell stages, both Fgf3 and Fgf10 are intensely expressed
in dental papilla55.
FGFs function as mitogens in cultured dental tissues. The
abundance of FGF receptor expression (FGFR1c, 1b and 2b
isoforms) in the cervical loops and in dental papilla
mesenchyme (FGFR1c) suggests that these are target tissues,
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Genes and tooth development: reviewing the structure and function of some key players.
and this is also in line with the distribution of dividing cells
in tooth germs56-57.
2.2 EGF (Epidermal Growth Factor)
EGF has been found in embryonic tissues and its expression
is related to epithelial proliferation58-59. In fact, EGF is a mitogen
for ectodermal, mesodermal and endodermal cells stimulating
proliferation of embryonic cells during morphogenesis in
vivo and in vitro 60-61. EGF interacts specifically with its
receptor. EGF-R is a transmembrane glycoprotein, which is
present in many cell types derived from all three germ
layers60,62. EGF and EGF-R appear to have been well conserved
both structurally and functionally throughout evolution63.
Egf is expressed at E-9 and E-10 in mouse mandible
immediately before the appearance of dental lamina, revealing
that Egf expression is necessary for the formation of dental
lamina and subsequent development of teeth 64-65. The
initiation of odontogenesis is totally inhibited by blocking
the production of Egf in situ66. Partanem, Thesleff67 studied
the expression pattern of Egf at various stages of tooth
development using I-labeled Egf. It was shown that Egf binds
to mouse embryonic molar epithelium and mesenchyme from
gestational days 13-17 and that the distribution of binding
sites changes dramatically (Fig.1). Binding to epithelium
occurs during bud stage, but the condensing mesenchymal
cells around the epithelial bud do not bind Egf. At the cap
stage the dental mesenchyme cells bind Egf. This result
indicates that Egf stimulates or maintains proliferation of
undifferentiated cells during embryonic development. The
detection of Egf mRNA in both epithelium and mesenchyme
suggests that the action of Egf in epithelial proliferation could
be a result of either autocrine or paracrine mechanisms.
2.3 TGF-b (Transforming Growth Factor-b)
Members of the Transforming Growth Factor-beta (TGF-b)
superfamily regulate cell proliferation, differentiation, and
apoptosis, controlling the development and maintenance of
most tissues68. TGF-b is a growth factor that takes place in
the cascade of signaling events during early tooth
development15,69.
During bud stage of molar mouse development, Tgf-b is
expressed in dental epithelium and mesenchyme (Fig.1). The
dental mesenchyme is rapidly proliferating at bud and cap
stages while dental epithelium intensely expresses Tgf-b1
mRNA15,70. The mesenchyme itself also expresses Tgf-b1
although at lower levels. Thus, local expression of Tgf-b1 in
dental epithelium may regulate cell proliferation in the
underlying dental mesenchyme contributing to the
determination of tooth morphology 15,71. Tgf-b1 acts as a
paracrine and autoinducing factor and participates in the
epithelial-mesenchymal interactions. In the cap stage Tgf-b
is expressed by cells from the inner enamel epithelium, enamel
knot, stellate reticulum and dental papilla. During early bell
343
stage Tgf-b is expressed by the inner enamel epithelium cells
but at late bell stage the expression is restricted to
preodontoblasts 15,71. At E19 Tgf-b1 mRNA was detected
transiently in stratum intermedium cells before the
differentiation of ameloblasts, and by secretory
odontoblasts. Tgf-b1 synthesized by stratum intermedium
may regulate the initiation of ameloblast differentiation. The
expression of Tgf-b1 in ameloblasts is restricted to a short
period, only the ameloblasts at the tips of the cusps that
remain non-secretory continue to express Tgf-b115,72.
Besides the function of the TGF-b in regulation of cell
proliferation in the underlying dental mesenchyme, another
function of TGF-b during tooth morphogenesis is the
regulation of matrix deposition, which is explained below.
2.4 BMPs (Bone Morphogenetic Proteins)
Bone Morphogenetic Proteins belong to the Transforming
Growth Factor-beta (TGF-b) superfamily. These are
homodimeric proteins originally defined by their ability to
induce bone formation in vitro. Presently, the mammalian
BMP family is formed by eight members, which may be
grouped into three subclasses based upon amino acid
similarity. The different BMP genes are important for
determination of bone shape and organ morphogenesis. The
subgroup consisting of the vertebrate Bmp2 and Bmp4 genes
is most closely related to the prototypical decapentaplegic
(dpp) gene in Drosophila, with @ 75% amino acid similarity.
These genes, have a 95% identity, and are key factors for
initiation and morphogenesis of teeth8,45. Bmp2 and Bmp4
are genes which play key roles in morphogenesis73-74. The
expression of Bmp4 was detected very early during mouse
tooth development in the thickened presumptive dental
epithelium. It shifted to the condensed mesenchyme during
bud stage. Bmp2 is also expressed in early dental epithelium,
where it stays until late cap stage, when expression shifts
dental papilla. Bmp2 is expressed in the central mesenchymal
cells of dental papilla during early bell stage. Bmp2 and Bmp4
are expressed by differentiated odontoblasts and they are,
therefore, potential inducers of ameloblast differentiation
(Fig.1)45. BMPs act as early signals, which stimulate other
“master genes”, and their involvement in a cascade of
reciprocal signaling events has been cleared up. Some
aspects of those signaling events in tooth development are
focused forward in this review.
3. Extracellular Matrix Molecules
Extracellular matrix (ECM) is involved in the epithelialmesenchymal interactions in the morphogenesis and
differentiation of developing tooth. Morphogenesis and cell
differentiation can be disturbed by mutations in collagen
and proteoglycan genes8. Functional studies in vitro have
shown that the integrity of the basement membrane is a
prerequisite for tooth epithelial morphogenesis 75. In the
Braz J Oral Sci. 2(7):339-347
Genes and tooth development: reviewing the structure and function of some key players.
developing tooth, the epithelial basement membrane contains
collagen types I, III, and IV, and also laminin, fibronectin and
various proteoglycans76. These molecules are expressed at
the same time when interactions mediated by the basement
membrane regulate the differentiation of mesenchymal cells
into odontoblasts 77 . Below, Tenascin and Syndecan are
considered in more detail, since their expressions have been
extensively analysed in tooth development, and their
correlations with other molecules such as Msx1 and Bmp4
are investigated.
3.1 Syndecan
Syndecan is a cell-surface proteoglycan identified in
mammary epithelial cells. This molecule contains both
heparan and chondroitin sulfate moieties and encodes an
extracellular, a trans-membrane and a cytoplasmic domain78.
The extracellular domain of mouse syndecan1 binds to
components of the ECM, including collagen types I, III, and
V, tenascin and fibronectin79. Syndecan binds tenascin, which
could mediate the interactions between cells and ECM and
contribute to the aggregation of the mesenchymal cells80.
In the E11 mouse molar tooth, syndecan is expressed in the
dental epithelium and some expressions are detected in the
underlying mesenchyme. Syndecan expression in dental
mesenchyme increases significantly from the bud to cap stage
and decreases rapidly during bell stage, then disappears
completely (Fig.1)81.
3.2 Tenascin
Tenascin is a large extracellular matrix glycoprotein and is a
receptor for syndecan. Although tenascin-C is widely
expressed in embryonic tissues, Saga et al.82 reported that
mutant mice lacking tenascin-C develop normally into fertile
adults. A possible explanation is the presence of four other
glycoproteins structurally related to tenascin. Therefore,
gene redundancy may also explain the lack of phenotype in
the tenascin-C mutants. Dental epithelium has been shown
to induce tenascin-C in early dental mesenchyme, around
the epithelial invaginating tooth bud at E12. Tenascin-C
expression temporarily disappears during the cap stage, but
reappears during bell stage (E16) in dental papilla
mesenchyme and persists until the time of odontoblast
differentiation (E18) (Fig.1). It is also expressed in the pulp
of erupted teeth83.
Regulation of ECM by signaling molecules
Several growth factors induce syndecan and tenascin in cell
cultures, such as TGF-b1 and FGFs84-87. TGF-b1 can promote
the synthesis of ECM proteins. It can also modify cell surface
matrix receptors and prevent degradation of ECM88-89. It has
been suggested that TGF-b1 induces the expression of type
1 collagen gene90. BMP2 and BMP4 mimic some of the effects
of dental epithelium on mesenchyme, including stimulation
of expression of the transcription factors as Msx1 and Msx2.
However, BMP2 and BMP4 do not stimulate tenascin and
syndecan expression in the early dental mesenchyme91.
“Molecular Cascade” of Reciprocal Signaling Events in
Tooth Development
Some aspects of the molecular signaling that regulate tooth
development are compiled in the Figure 1. It is possible to
observe that the molecular signals are expressed in different
stages of odontogenesis.
Regarding initiation stage, a genetic model explaining the
regulation of the expressed genes has been proposed by
Bei, Maas92 and by Zhang et al.43 (Fig.2). The expression of
Msx1 in the dental mesenchyme is initially induced by
epithelially derived Bmps and Fgfs. Interestingly, Bmp4 cannot
induce Fgfs, neither the contrary, suggesting that Bmp4 and
Fgf8 act by independent pathways in inducing dental
mesenchyme. While both Fgf8 and Bmp4 can induce Msx1
in dental mesenchyme, only Bmp4 can induce Msx2
expression. Bmp4 and Fgf8 induce Dlx2 expression in dental
mesenchyme while only Fgf8 can induce Dlx1 expression.
Fgf8 stimulates Dlx1, Dlx2 and Pax9 expression in dental
mesenchyme. The arrest of tooth development in Msx1
mutant mice was associated with a down-regulation of Bmp4,
Fgf3, Lef1, Ptc, Dlx2 and syndecan1 in the molar
mesenchyme. This suggests that Msx1 is placed upstream
of those genes. In addition, epithelial Shh induces Gli1
expression in the mesenchyme. Gli1 may activates Ptc
expression by interacting with the product of Msx1, which is
induced in the mesenchyme by epithelially derived Bmp484,93.
Mesenchymal Bmp4, which induces Lef1, requires Msx1
products to induce Ptc expression and provides a positive
feedback signal for maintenance of Msx1 expression94.
Recent studies have clarified some aspects of the molecular
signaling that occur during the bud stage of odontogenesis.
The down-regulation of Bmp4 in molar mesenchyme, causes
down-regulation of Lef1 and Dlx2 in the epithelial bud. This
was deduced from the observations that addition of
exogenous BMP4 could partly rescue the tooth phenotype
and induce Lef1 and Dlx2 expression in the Msx1 mutant
molar tooth germ92. Interestingly, the ectopic expression of
BMP4 in the dental mesenchyme of Msx1 mutants cannot
rescue the expression of Fgf3 and syndecan-1 genes 28 .
Moreover, mesenchymal BMP4 is also required for the
maintenance of Shh and Bmp2 expression in dental
epithelium 95 and may be responsible for inducing the
formation of enamel knot in tooth epithelium56. On the other
hand, overexpression of BMP4 in the wild type molar
mesenchyme represses Shh and Bmp2 expression in the
enamel knot, suggesting that Shh and Bmp2 may not be
critical signals in regulating the formation of tooth cusps28.
Similar to many other embryonic organs, the mammalian tooth
development relies largely on epithelial-mesenchymal
344
Braz J Oral Sci. 2(7):339-347
Genes and tooth development: reviewing the structure and function of some key players.
interactions. Tooth development may be divided in multiple
stages, where the number, size and type of teeth are
sequentially determined. Teeth are serially homologous
structures, which allow the localization and quantification
of the effects of specific gene mutations. Furthermore, it is
also possible to determine the phase of odontogenesis
affected by these conditions. These features make tooth
development an important system to understand the intricate
molecular mechanisms that regulate development provide a
link between development and evolutionary genetics26, 96.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Fig. 2 – Schematic representation of the molecular signaling in
odontogenesis of molar mouse tooth.
21.
22.
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