-
Signaling Networks Regulating ToothOrganogenesis and
Regeneration, and theSpecification of Dental Mesenchymal
andEpithelial Cell Lineages
Maria Jussila and Irma Thesleff
Developmental Biology Program Institute of Biotechnology,
Biokeskus 1, P.O. Box 56, University of Helsinki,Helsinki
FIN-00014, Finland
Correspondence: [email protected]
SUMMARY
Teeth develop as ectodermal appendages from epithelial and
mesenchymal tissues. Toothorganogenesis is regulated by an
intricate network of cell–cell signaling during all steps
ofdevelopment. The dental hard tissues, dentin, enamel, and
cementum, are formed by uniquecell types whose differentiation is
intimately linked with morphogenesis. During evolution thecapacity
for tooth replacement has been reduced in mammals, whereas teeth
have acquiredmore complex shapes. Mammalian teeth contain stem
cells but they may not provide a sourcefor bioengineering of human
teeth. Therefore it is likely that nondental cells will have to
bereprogrammed for the purpose of clinical tooth regeneration.
Obviously this will requireunderstanding of the mechanisms of
normal development. The signaling networks mediatingthe
epithelial-mesenchymal interactions during morphogenesis are well
characterized but themolecular signatures of the odontogenic
tissues remain to be uncovered.
Outline
1 Morphogenesis and celldifferentiation during tooth
development
2 Signal networks and signaling centers
3 Regulation of the identity anddifferentiation of
odontogenicmesenchymal and epithelial cell lineages
4 Regulation of tooth replacement, continuousgrowth, and stem
cells in teeth
5 Future challenges: stem cell-basedbioengineering of teeth
6 Concluding remarks
References
Editors: Patrick P.L. Tam, W. James Nelson, and Janet
Rossant
Additional Perspectives on Mammalian Development available at
www.cshperspectives.org
Copyright # 2012 Cold Spring Harbor Laboratory Press; all rights
reserved.
Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425
1
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
Teeth are one of the most diverse organs in vertebrates
bothmorphologically and functionally. Mammalian teeth be-long to
four tooth families: incisors, canine, premolars,and molars, and
they are replaced either once or not atall. Humans have teeth from
all four tooth families and ex-cluding molars, the teeth are
replaced once (Fig. 1A). Thelaboratory mouse (Mus musculus), which
is the most com-mon model animal in tooth development studies, has
amuch derived dentition. It lacks the canine and premolarsand has
only one incisor and three molars separated by atoothless diastema
in each half of the jaw (Fig. 1B). Further-more, the mouse incisors
grow continuously but the teethare not replaced. In contrast,
reptiles, fish, and amphibianscan replace their teeth multiple
times during the life of theanimal. Teeth of these nonmammalian
species are usually
simpler in shape. Thus, during evolution the complexityof tooth
shape has increased, whereas the replacement ca-pacity has been
reduced.
The same conserved signaling pathways that regulatemost aspects
of embryonic development are required fortooth development, and the
core regulatory network seemsto have been in place already when
teeth appeared in evolu-tion (Fraser et al. 2009; Tummers and
Thesleff 2009). It isnoteworthy that teeth develop as epithelial
appendagesand share the same regulatory molecules during the
firststeps of initiation and morphogenesis with other ectoder-mal
organs. However, unlike many other human epithelialappendages,
human teeth have no regenerative capacity.The adult human teeth
contain stem cells that are capableof differentiating to cells
producing the extracellular matrix
Matrix secretion
Alveolar boneDental pulp
Eruption
EnamelDentin
Developing permanent tooth
Morphogenesis
Cap
Enamel knot
Dentalpapilla
Stellatereticulum
Bud
Dentalmesen-chyme
Cell differentiation
Bell
Ameloblasts
Odontoblasts
Secondaryenamel knot
Initiation
Placode
Placode
Mesen-chyme
Epithelium
Mesen-chyme
Dentallamina
EE. J
uuJuur
iri
A B
C
Molars
Premolars
Canine
Incisors
Incisors
Dia
stem
a
Molars
Figure 1. The dental formula of human and mouse, and a schematic
representation of tooth development. The per-manent dentition of
human consists of two incisors, a canine, two premolars, and three
molars in each half of the jaw(A). Mice have one incisor and three
molars separated by a toothless diastema in each half of the jaw
(B). Tooth de-velopment starts from the dental lamina, a thickening
of the epithelium. Individual placodes form within the
dentallamina. The growing epithelium forms a bud and the dental
mesenchyme condenses around the epithelium. Duringmorphogenesis,
the epithelial tissue folds to cap and bell shapes. Primary and
secondary enamel knots in the enamelorgan regulate the growth and
shape of the tooth. During cell differentiation, enamel-secreting
ameloblasts anddentin-secreting odontoblasts mature from the
epithelial and mesenchymal cell compartments. The permanenttooth
develops lingually to the deciduous tooth from an extension of the
dental lamina (C).
M. Jussila and I. Thesleff
2 Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
of tooth-specific mineralized tissues, but so far they havenot
been shown to have morphogenetic potential. The re-placement of
adult teeth in humans by tissue engineeringappears still a distant
goal and it is obvious that moreresearch on stem cell regulation
and the molecular controlof early tooth development is required. In
this article wereview the current knowledge about the mechanisms
in-volved in tooth morphogenesis and replacement, and howthe
epithelial and mesenchymal cell lineages acquire odon-togenic
competence and differentiate into tooth-specificcells depositing
the dental hard tissues, and discuss the fu-ture challenges and
scenarios of tooth bioengineering.
1 MORPHOGENESIS AND CELLDIFFERENTIATION DURING
TOOTHDEVELOPMENT
Teeth are initiated from two tissue components: the
surfaceepithelium and the underlying mesenchyme. The
dentalmesenchyme derives from cranial neural crest cells
thatmigrate into the frontonasal process and first branchialarch.
In mammalsthe epithelium originates from ectoderm,whereas in fish
and some amphibians, pharyngeal teeth de-rive from the
endoderm.
The first sign of tooth development is the formationof the
dental lamina, a horseshoe-shaped epithelial stripealong the
mandible and maxilla (Fig. 3B). The teeth formwithin the dental
lamina, where their development startsfrom placodes, local
thickenings of the epithelium (Figs.1C and 3B,C). Probably all
teeth in one tooth family are in-itiated sequentially from a single
placode. For instance, themouse molars develop successionally,
starting from the firstmolar and followed by initiation of the
second and thirdmolars from a posterior extension of the dental
epithelium.
The individual teeth develop from an epithelial budthat grows
down to the underlying mesenchyme. The neu-ral crest-derived
mesenchyme becomes specified as the den-tal mesenchyme condenses
around the bud and gives rise toall the dental tissues except
enamel. The epithelial bud in-vaginates at its tip and its cervical
loops grow to encompassthe dental papilla mesenchyme, which gives
rise to the den-tal pulp and odontoblasts forming dentin (Fig. 1C).
Themesenchyme surrounding the epithelium and dental pa-pilla
becomes the dental follicle and gives rise to the peri-odontal
tissues and cementoblasts forming the cementum.
During morphogenesis the epithelium acquires cap andbell shapes
and is called enamel organ. It consists of severalcell types: the
inner enamel epithelium facing the dentalpapilla and
differentiating to enamel producing amelo-blasts, the outer enamel
epithelium facing the dental fol-licle, and the stellate reticulum
and stratum intermediumcells in between. The growth and folding of
the inner
enamel epithelium during the bell stage determine thesize and
shape of the tooth crown. The shape becomes fixedwhen the organic
matrices of dentin and enamel mineralizebecause no remodeling of
either dentin or enamel takesplace later.
Root formation is initiated after crown developmentwhen
ameloblast differentiation reaches the future crown-root boundary,
and the cells of the inner enamel epitheliumno longer differentiate
into ameloblasts. Instead they formthe Hertwig’s epithelial root
sheath (HERS) with the outerenamel epithelium. HERS proliferates
and migrates down-ward guiding root formation, and it also induces
the differ-entiation of odontoblasts forming root dentin. HERS has
alimited growth potential, which determines the length ofthe root.
The disintegration of HERS results in the forma-tion of an
epithelial network called epithelial rests of Malas-sez (ERM) and
this allows the cells of dental follicle to comein contact with
root dentin and their differentiation intocementoblasts depositing
cementum on the root surface.The periodontal ligament that connects
the tooth to thebone is formed by fibroblasts differentiating from
the den-tal follicle cells. In addition, the dental follicle gives
rise toosteoblasts that form the alveolar bone where the fibers
ofthe periodontal ligament are embedded (Nanci 2008).The dental
follicle has an important function later in tootheruption as it
regulates bone remodeling around the tooth(Marks and Cahill
1987).
2 SIGNAL NETWORKS AND SIGNALING CENTERS
All aspects of tooth morphogenesis are regulated
byepithelial-mesenchymal interactions, which are mediatedby the
conserved signaling pathways including Hedgehog(Hh), Wnt,
Fibroblast growth factor (FGF), Transforminggrowth factor b (Tgfb),
Bone morphogenic protein (Bmp),and Ectodysplasin (Eda) (Fig. 2).
Their interactions, targets,and expression patterns have been
elucidated in considerabledetail in teeth
(http://bite-it.helsinki.fi; Bei 2009b; Tum-mers and Thesleff
2009). Epithelial signaling centers playa pivotal role regulating
the different steps of tooth devel-opment. There are three sets of
such centers: the placodes,the primary enamel knots, and the
secondary enamelknots. Their formation is regulated by
epithelial-mesen-chymal interactions and they all express largely
the same ar-ray of multiple growth factors.
All ectodermal organs begin to develop from a placode,and the
molecular mechanisms of tooth placode formationand signaling are
shared to a great extent with placodes ofother organs such as hairs
(Mikkola 2009b). One of theimportant genes regulating placode
formation is the tran-scription factor p63 that is expressed
throughout the sur-face ectoderm. When p63 function is deleted in
mice, the
Signaling Networks Regulating Tooth Development
Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425 3
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
placodes of teeth and other ectodermal appendages do notdevelop,
but the dental lamina forms (Laurikkala et al.2006). Key signaling
pathways including Bmp, Fgf, Notch,and Eda are impaired in the
absence of p63 (Laurikkalaet al. 2006). The importance of the
signaling that takes
place at the placode stage is further highlighted by the
phe-notype of several mouse mutants where tooth developmentstops
before epithelial budding (Bei 2009b).
Ectodysplasin (Eda) is a signal of the tumor necrosisfactor
family and signals through its receptor Edar that islocally
expressed in the placodes of all ectodermal appen-dages as well as
in primary and secondary enamel knots(Mikkola 2009b). Mutations in
the Eda pathway genescause the human syndrome hypohidrotic
ectodermal dys-plasia (HED) manifesting multiple missing teeth as
wellas defects in other ectodermal organs, e.g., sparse hairand
reduced sweating (Mikkola 2009b). Mice lacking func-tional Eda
often lack third molars or incisors and the cusppatterning of
molars is abnormal, indicating a requirementof Eda in the function
of placodes and enamel knots (Pispaet al. 1999). Mice that
overexpress Eda in epithelium (underkeratin14-promotor) develop an
extra tooth in front ofthe molars as well as supernumerary hairs
and mammaryglands (Fig. 5A,B) (Mustonen et al. 2003). The targets
ofEda signaling include molecules from all the other impor-tant
signaling pathways (e.g., Shh, Fgf20, Dkk4, ctgf, Folli-statin)
making Eda a key regulator of ectodermal organdevelopment (Mikkola
2009b).
The primary enamel knot appears in the dental epithe-lium at the
transition from bud to cap stage. In addition todirecting crown
formation, in molars it determines the po-sitions of the secondary
enamel knots which in turn markthe positions of the cusp tips in
the molar crown (Fig. 2B)(Jernvall et al. 2000). Wnts are important
upstream regula-tors of enamel knots as shown by the requirement of
Lef1for Fgf4 expression in the enamel knot (Kratochwil et al.2002)
and the induction of new enamel knots and placodesby forced
activation of Wnt/b-catenin signaling in oralepithelium (Järvinen
et al. 2006; Wang et al. 2009). Morethan a dozen different signal
molecules belonging to allfour conserved signal families are
locally expressed in theprimary and secondary enamel knots. The
enamel knotsinitiate and regulate the folding of the epithelium by
stim-ulating the surrounding epithelium to proliferate throughFgfs
(Fgfs 3, 4, 9, and 20) and remaining nonproliferativethemselves.
They express the cyclin-dependent kinase in-hibitor p21 and lack
Fgf receptors making them insensitiveto the proliferative signals
(Jernvall et al. 1998; Kettunenet al. 1998). The Fgfs also signal
to dental mesenchyme andinduce e.g., Runx2, and Fgf3, which signals
back to epithe-lium illustrating the bidirectional Fgf signaling
betweenepithelium and mesenchyme regulating tooth morpho-genesis
(Klein et al. 2006). Shh from the enamel knotstimulates epithelial
morphogenesis indirectly via the mes-enchyme (Gritli-Linde et al.
2002).
Important aspects of enamel knot signaling are themodulation and
fine-tuning, which affect the patterning
Shh
Fgf9
Fgf3Fgf10
Bmp4Activin
Bmp4
Maintenance of stem cellsand ameloblast production
Enamel knot signals: Fgf3,4,9,20 Shh Wnt3,6,10a,10b Bmp2,4,7
Mesenchymal signals: Fgf3,10 Bmp4 Wnt5a,5b
Cusp patterning and cell differentiation
Renewal
Fgf8 ShhBmp4
Activin FgfsBmp4
Wnts
Pax9, Msx1,2, Runx2, Barx1, Lhx6,7 Dlx1,2,5
Mesenchymal condensationand placode formation
A
B
C
Figure 2. Cross talk between epithelium and mesenchyme
throughthe conserved signaling pathways regulates all aspects of
tooth devel-opment. When tooth development is initiated, signals
from the epi-thelium activate a set of transcription factors in the
mesenchyme,leading to condensation of the mesenchyme and formation
of the ep-ithelial placode (A). The enamel knot is a signaling
center expressingmultiple signaling molecules that induce
reciprocal signals from themesenchyme. Enamel knots determine the
position of the cusps andinitiate differentiation of odontoblasts
(B). Tgfb, Bmp, and Shh sig-naling regulate epithelial-mesenchymal
interactions in the cervicalloop of the mouse incisor. They support
the maintenance and prolif-eration of the stem cells as well as
ameloblast differentiation and en-amel production (C).
M. Jussila and I. Thesleff
4 Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
of the secondary enamel knots and thereby the patternof molar
cusps via lateral inhibition and reaction diffusionmechanisms.
Different shapes of molars can be generatedby mathematical modeling
using parameters of activatingand inhibiting enamel knot signals,
and it has been sug-gested that changes in signaling during
evolution are re-sponsible for the species-specific cusp patterns
(Salazar-Ciudad and Jernvall 2002). This hypothesis has gained
ex-perimental support from phenotypes of transgenic micewhere
signal modulation has resulted in phenotypes resem-bling teeth of
other species. Examples include molarsof K14-Eda resembling
kangaroo teeth, and of Sostdc1knockout (inhibitor of Wnt and Bmp
signaling) resem-bling rhino teeth (Kangas et al. 2004; Kassai et
al. 2005).Furthermore, epithelial deletion of Dicer, which is
requiredfor processing of microRNA (miRNA), results in the
mod-ulation of molar cusp pattern (Michon et al. 2010).
Fine-tuning of the activity of the conserved signalingpathways
controls many other aspects of tooth formationas well. For example,
a supernumerary tooth forms in frontof the first molar in several
mutant mouse lines when sig-naling activity is modulated. These
teeth do not representde novo tooth induction. Instead they form
from activationof the development of a vestigial tooth rudiment
found inwild-type mice in the diastema and represent premolarslost
during the evolution of rodents. Examples are theK14-Eda mouse
(Fig. 5A,B) (Mustonen et al. 2003) andthe Sprouty mutants (Klein et
al. 2006). In the Osr2 knock-out an extra tooth develops lingually
to the first molar(Zhang et al. 2009). This is accompanied by
spreading ofBmp4 expression to the lingual mesenchyme and
resultsprobably from a subsequent broadening of the dental
field(Mikkola 2009a). The relative sizes of the mouse molars
are
influenced by activation and inhibition between succes-sionally
developing teeth (Kavanagh et al. 2007), the sizeand number of
mouse incisors is affected by fine-tuningBmp signaling in the
placodes (Munne et al. 2010), andthe continuous growth and enamel
deposition in incisorscan be modulated by the levels of Fgf,
Activin, and Bmp sig-naling in the epithelial stem cell niche (Fig.
2C) (Wang et al.2007).
3 REGULATION OF THE IDENTITY ANDDIFFERENTIATION OF
ODONTOGENICMESENCHYMAL AND EPITHELIAL CELLLINEAGES
Classical recombination experiments have shown that
theodontogenic potential shifts from the epithelium to mes-enchyme
in mouse teeth between embryonic days 11 and12, i.e., around the
time of placode formation (Fig. 3).When epithelium of the first
branchial arch from an E9-11 mouse embryo was recombined with
second arch mes-enchyme, a tooth formed (Fig. 3A) (Mina and Kollar
1987).Similarly, first arch epithelium from an E9-10 embryo
in-duced tooth formation when recombined with cranial neu-ral crest
cells that normally form the dental mesenchymeand, interestingly,
also when combined with premigratorytrunk neural crest cells
(Lumsden 1988). At E12 the epithe-lium no longer has inductive
potential and now the firstarch mesenchyme can induce tooth
formationwhen recom-bined with second arch epithelium (Fig. 3A).
The mesen-chyme from E13 or older tooth germs has the informationon
the tooth identity as the shape of the tooth in the recip-rocal
recombinations between incisor and molar epithe-lium and mesenchyme
will form according to origin of
B
C
Pitx2
Pitx2
E11
E12.5
* *
AgeE11 E13E12E10
odon
toge
nic
pote
ntia
l (%
)
100
50
0
A
Epithelium Mesenchyme t
t
Figure 3. Shift of the odontogenic potential from epithelium to
mesenchyme between dental lamina and placodestages as shown by
reciprocal tissue recombinations (Mina and Kollar 1987). Epithelium
is capable of inducing toothdevelopment when recombined with
nondental mesenchyme until E11 stage of mouse development. At E12
theodontogenic potential has shifted to mesenchyme, and it can
induce tooth development when recombined with non-dental epithelium
(A). Pitx2 is expressed in the dental lamina of the mouse lower jaw
at E11 (t ¼ tongue) (B). At E12.5the Pitx2 expression is restricted
to the placode epithelium of the incisors (arrows) and molars
(asterisks) (C).
Signaling Networks Regulating Tooth Development
Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425 5
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
the mesenchyme (Kollar and Baird 1969). In addition, thedental
papilla can induce tooth formation when recom-bined with limb
epithelium (Kollar and Baird 1970). Ithas been proposed, based on
in vitro experiments, thatthe incisor versus molar identity of
teeth is determined bythe level of Bmp signaling (Tucker et al.
1998). However,this conclusion was challenged recently by the
observationthat inhibition of Bmp signaling caused partial
splittingof the incisor placode resulting in the formation of
twofused incisors rather than incisor to molar transformation(Munne
et al. 2010).
The molecular basis of odontogenic competence inearly jaw
epithelium and later in the condensed dental mes-enchyme remains
elusive. As all the genes that are knownto regulate tooth
development are also expressed in otherdeveloping organs, it seems
that there is no single tooth-specific gene that defines the
odontogenic tissues. Cur-rently only few genes such as Sonic
hedgehog (Shh) andthe transcription factor Pitx2 are known to be
restrictedto the dental lamina (Fig. 3B,C) (Keränen et al. 1999).
Itis not known how the dental lamina becomes established,and to
date there is no mouse mutant reported where thedental lamina would
be missing. All teeth develop withinthe dental lamina, even in
micewhere extra teeth are induced.Activation of Wnt signaling in
the epithelium induces super-numerary placodes throughout the
surface epithelium andthey give rise to various epithelial
appendages. Yet extra teethform only in the region of dental arches
and mostly in con-nection with other teeth (Järvinen et al. 2006;
Wang et al.2009). These observations indicate that the
odontogeniccompetence is present only in the oral region.
It is likely that spatiotemporal patterns of the
epithelialsignals are involved in the shift of competence to
mesen-chyme (Fig. 2A). In addition to Shh, which is restrictedto
the dental lamina, many Wnt ligands are expressed inthe oral
epithelium (Sarkar and Sharpe 1999). Wnt/b-cat-enin signaling
regulates Fgf8 expression in the early epithe-lium (Wang et al.
2009), and placodes do not form in miceoverexpressing the Wnt
inhibitor Dkk1 (Andl et al. 2002).Bmp4 and Fgf8 are expressed in
the jaw epithelium in over-lapping patterns before any
morphological sign of toothdevelopment. Bmp4 is expressed more
distally at the sitewhere molars will develop and Fgf8 more
proximally inthe incisor region (Neubüser et al. 1997). Epithelial
Bmpsinduce the expression of Bmp4 in the mesenchyme beforebud stage
correlating with the shift in the odontogenic po-tential (Vainio et
al. 1993). Also, Wnt/b-catenin signalingin the incisor mesenchyme
stimulates the expression ofBmp4 that in turn regulates Shh in the
epithelium (Fujimoriet al. 2010). Furthermore, Wnt signaling was
shown to berequired in the molar mesenchyme for the bud to cap
stagetransition and primary enamel knot formation (Chen
et al. 2009). The signals induced in the mesenchyme byepithelial
Fgfs and acting reciprocally to the epithelium in-clude Activin,
Fgf3, and Fgf10 (Ferguson et al. 1998; Ket-tunen et al. 2000).
These signals regulate the subsequentepithelial morphogenesis and
the enamel knot formation(Fig. 2A).
The shift of the odontogenic competence from epithe-lium to
mesenchyme is accompanied by the induction ofimportant
transcription factors in the dental mesenchyme(Fig. 2A). The
deletion of the function of several of theseeither alone or
together with another transcription factorin the same family
results in tooth arrest at placode orbud stage. All four signal
pathways have been shown tobe involved in the regulation of these
transcription factors(Bei 2009b). For example, Bmp4 induces the
expression ofMsx1 and Fgf8 induces the expression of Pax9 (Vainio
et al.1993, Neubüser et al. 1997). Other targets of Bmp and
Fgfsignaling in mesenchyme at this stage include Lhx6,7,Barx1,
Dlx1,2, and Runx2 (Bei 2009b, Tummers and Thes-leff 2009). The Shh
mediators Gli2 and Gli3 are expressed inthe mesenchyme and are
required for tooth formation(Hardcastle et al. 1998). In addition,
the expression ofLef1, a Wnt effector, shifts from the epithelium
to the mes-enchyme together with the shift in the odontogenic
poten-tial and is regulated by Bmp4 in mesenchyme (Kratochwilet al.
1996). Perhaps a combination of these transcriptionfactors
constitutes the code for the odontogenic identityof the
mesenchyme.
The differentiation of the tooth-specific cell types is
in-timately linked with epithelial morphogenesis. Odonto-blasts and
cementoblasts differentiate from the lineage ofdental mesenchyme,
the odontoblasts from the dental pa-pilla, and cementoblasts from
the dental follicle, whereasameloblasts differentiate from the
epithelial lineage. Theyare responsible for the formation and the
deposition ofthe extracellular matrices of the tooth-specific
mineralizedtissues, dentin, cementum, and enamel, respectively. It
isnot known exactly at which stage of tooth formation thecells
become committed, but the final steps of odontoblastand ameloblast
differentiation have been analyzed in detailduring the bell stage
of tooth formation.
The mesenchyme is first induced to differentiate
intoodontoblasts by the inner enamel epithelium. The
differen-tiation starts from the cusp tips and proceeds downward
tocervical and intercuspal directions. Signals in Tgfb/Bmpfamilies
have been implicated in odontoblast induction(Ruch et al. 1995),
and it was shown recently that the condi-tional loss of Smad4, a
mediator of Tgfb/Bmp signaling,from the dental papilla prevents the
terminal differen-tiation of odontoblasts and dentin deposition (Li
et al.2011a). As the formation of enamel knots is
temporallyassociated with the initiation of odontoblast
differentiation
M. Jussila and I. Thesleff
6 Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
at the cusp tips, the enamel knot signals have been sug-gested
to play a role (Fig. 2B) (Thesleff et al. 2001). Oneof these
signals, Wnt10b, was suggested to regulate the ex-pression of
dentin sialophosphoprotein (Dspp) and odonto-blast differentiation
(Yamashiro et al. 2007). The localizationof Wnt reporter activity
in odontoblasts is also in line withthe role of Wnts in the process
(Suomalainen and Thesleff2010). In addition, the basement membrane
is importantfor the polarization and differentiation of the
odontoblastsand serves presumably as a reservoir of signal
molecules(Thesleff and Hurmerinta 1981; Ruch et al. 1995). Dentinis
composed mainly of type I collagen, dentin phosphopro-tein, and
Dspp, and mutations in these genes cause dentino-genesis imperfecta
in humans (Shields et al. 1973).
After the odontoblasts have been induced to differenti-ate, they
signal back to the epithelium. The signals fromthe mesenchyme
involved in the ameloblast inductioninclude Bmp2, Bmp4, and Tgfb1
(Fig. 4) (Coin et al.1999; Wang et al. 2004). In addition, Shh from
the epithe-lial stratum intermedium cells is required to support
ame-loblast differentiation and maturation (Dassule et al.
2000;Gritli-Linde et al. 2002). Other epithelial growth
factorsregulating ameloblasts are TFGb1, Wnt3, Eda, and
Follista-tin (Bei 2009a). Ameloblasts express transcription
factorssuch as Sp6 and Msx2 that have been shown to play a
role in amelogenesis in mice (Bei 2009a). Mutations
inameloblast-specific genes including ameloblastin, ameloge-nin,
enamelin, and Mmp20 cause human amelogenesisimperfecta (Bei 2009a).
Very little is known about the mo-lecular regulation of
cementoblast development. Bmp sig-naling was reported to induce
cementoblast differentiationfrom dental follicle cells, whereas Wnt
signaling promotestheir proliferation (Zhao et al. 2002; Nemoto et
al. 2009).
4 REGULATION OF TOOTH REPLACEMENT,CONTINUOUS GROWTH, AND STEM
CELLSIN TEETH
As the mouse teeth are not replaced, relatively little isknown
about the mechanisms of tooth replacement in +mammals. Histological
observations in nonmodel animalsindicate that replacement teeth
develop from the dentallamina associated with their predecessors
(Luckett 1993;Järvinen et al. 2009). The ferret (Mustela putorius
furo) re-places its incisors, canines, and premolars, and it was
shownthat the deciduous teeth are connected to each other by
acontinuous dental lamina, and the permanent teeth startto grow
from the lingual side of each deciduous tooth asan extension of the
dental lamina (Fig. 5C–E) (Järvinenet al. 2009). Similarly, in the
reptiles the replacement tooth
Pulp
Dentin
Enamel
Lingual - enamel free
Labial - enamel
D
B
C
A
Bmp4
E16
*
Amelogenin
NB
Ameloblastin
Bmp4 bead
Cervicalloop a
o
Figure 4. Bmp4 is one of the signals regulating ameloblast
induction. A schematic view of the postnatal mouse in-cisor shows
the asymmetrical deposition of enamel only on the labial side of
the tooth and the cervical loop stem cellniche (A). Amelogenin
protein is present in the ameloblasts (a) and in the first enamel
matrix on the labial side ofnewborn (NB) incisor (arrow) but not on
the lingual side [asterisk; o, odontoblasts]) (B). Bmp4 is
expressed in themesenchyme and is intense in the odontoblasts
(arrows) of the developing incisor at E16. The white line
surroundsthe epithelium (C). A bead soaked in Bmp4 protein induces
ameloblastin expression in E16 incisors (D). (B and Dreprinted,
with permission, from Wang et al. 2004.)
Signaling Networks Regulating Tooth Development
Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425 7
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
arises from an outgrowth of the dental lamina each time
theprevious tooth has grown to a certain size (Richman andHandrigan
2011). In contrast, in the fish species studied,there seems to be
no successional dental lamina, and thenew teeth are initiated
directly from the epithelium of the pre-vious tooth or from the
oral epithelium (Smith et al. 2009).
Wnt signaling has been associated with tooth replace-ment both
in mammals and reptiles and may be a key factorregulating tooth
renewal across vertebrates (Järvinen et al.2009; Richman and
Handrigan 2011). In the ferret, the ex-pression of Sostdc1, an
inhibitor of Wnt and Bmp signaling,marks the border between the
deciduous tooth and thedental lamina that gives rise to the
permanent tooth (Fig.5E) (Järvinen et al. 2009). The expression of
Axin2, a feed-back inhibitor of Wnt signaling, was also detected in
themesenchyme between the tooth and the growing dentallamina
(Järvinen et al. 2009). During snake tooth replace-ment, there is
Wnt activity in the tip of the dental lamina
and it is promoted by Shh and Bmp signaling from the mes-enchyme
(Richman and Handrigan 2011).
The phenotypes of some human syndromes and theirmouse models
support the role of Wnt signaling in toothreplacement. Mutations in
the human AXIN2 gene causeoligodontia, which specifically affects
permanent teeth(Lammi et al. 2004). On the other hand,
supernumeraryteeth are common in familial adenomatous
polyposis(FAP), which is caused by mutations in APC, an
inhibitorycomponent of the Wnt pathway, and the patients also
de-velop odontomas, benign tumors composed of numeroussmall teeth
(Wang and Fan 2011). A similar phenotype isseen in mice when Wnt
signaling is activated in the epithe-lium either by deletion of APC
or stabilization of b-catenin(Fig. 5F–H) (Järvinen et al. 2006;
Liu et al. 2008; Wanget al. 2009). Sp62/2 (Epiprofin) mutants have
a similarphenotype but this gene has not been associated with
hu-man conditions (Nakamura et al. 2008; Wang and Fan
m1 m3m2
A
F
C
B
G
D
H
E
Sostdc1
Fgf20
Pitx2
Fgf20
Shh E14.5
E14.5
E33
C
dC dC
C
P3 dP3
wt
E14.5
-cat Δx3K14/+ -cat Δex3K14/+
K14-Eda K14-Eda
E33 E35
dl dl dl
Figure 5. Supernumerary teeth, tooth replacement, and continuous
tooth renewal. Overexpression of ectodysplasinin the surface
epithelium results in development of a supernumerary tooth (arrow)
in front of the molars (m1-3) inthe K14-Eda mice (A). The rudiment
of the supernumerary tooth (blue arrow) in front of the first molar
(red arrow)can be visualized by Shh expression in E14.5 lower jaw
of K14-Eda embryo (B). The permanent canine (C) of theferret
develops as an extension of the dental lamina (dl) on the lingual
side of the deciduous canine (dC) at E33 (C).Both deciduous and
permanent canine express Pitx2 in the epithelium (D). Sostdc1 is
expressed in the intersectionbetween the dental lamina and the
deciduous third premolar (dP3) at the time when permanent P3 is
initiated in theE35 ferret embryo (arrow) (E). Stimulation of Wnt
signaling by stabilized b-catenin in mouse oral epithelium leadsto
the development of multiple small teeth from a single E14 mutant
tooth germ cultured under the kidney capsule(F). Fgf20 is expressed
in the enamel knots of upper and lower molars of E14.5 wild-type
mouse embryos (G). Multi-ple enamel knots expressing Fgf20 have
been induced in the dental epithelium of b-catDex3K14/+ embryos (H
).
M. Jussila and I. Thesleff
8 Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
2011). The teeth in the Wnt gain-of-function mouse modelswere
shown to develop successionally from previous teeth re-sembling the
continuous generation of the simple-shapedreplacement teeth in fish
and reptiles. This led to a sugges-tion, that Wnt signaling may
have been involved in the re-duction in the replacement capacity
and in the gain intooth complexity during evolution (Järvinen et
al. 2006).
Interestingly, the phenotypes of two syndromes suggestthat the
capacity for continued tooth replacement can beunlocked in humans.
The supernumerary teeth were sug-gested to represent a third
dentition in cleidocranial dys-plasia (CCD) and in a novel
craniosynostosis syndrome,caused by mutations in the transcription
factor RUNX2and interleukin receptor IL11RA, respectively (Jensen
andKreiborg 1990; Nieminen et al. 2011). Unfortunately, themouse
models of these syndromes do not show supernum-erary teeth, likely
because the mouse teeth are not normallyreplaced, and they are
therefore not suitable for studies ontooth replacement (D’Souza et
al. 1999; Nieminen et al.2011). Runx2 has been associated with Fgf
as well as Wntsignaling in tooth development as the induction of
theWnt inhibitor Dkk1 in dental mesenchyme by epithelialFgf4
requires Runx2 (James et al. 2006).
Putative epithelial stem cells have been identified duringtooth
replacement in gecko, a reptile (Handrigan et al.2010). These cells
reside in the lingual side of dental laminaand express some known
stem cell marker genes. It is possi-ble that the reduction of tooth
replacement capacity inmammals to maximally one replacement has
involved deple-tion of such stem cells and that they are maintained
in thecleidocranial dysplasia and craniosynostosis syndromes.
Although the human teeth do not regenerate and theirdevelopment
is completed already during adolescence, thereare stem cells in the
adult teeth. Human dental mesenchymalstem cells were first isolated
from the dental pulp and whentransplanted they formed dentin
(Gronthos et al. 2000).Stem cells in periodontal ligament were
shown to producecementum and periodontal ligament-like structures
(Seoet al. 2004). Similar stem cells were also characterized in
ex-foliated deciduous teeth and third molars (Rodriguez-Loza-no et
al. 2011). Epithelial stem cells may reside within theepithelial
cell rests of Malassez as these cells can be inducedto
ameloblastlike cells (Shinmura et al. 2008).
Some mammals have teeth that grow continuouslythroughout life
and thus have stem cells. The most studiedsuch tooth is the mouse
incisor, which harbors epithelialstem cells in a niche situated in
the cervical loop at its prox-imal end (Fig. 2C) (Harada et al.
1999). The mouse incisorhas an asymmetric structure with enamel
deposited only onthe labial side, whereas the lingual side is
covered by dentin(Fig. 4A). The asymmetry arises from differences
betweenthe lingual and labial cervical loops as only the larger
labial
cervical loop contains label-retaining stem cells and transi-ent
amplifying (TA) cells (Harada et al. 1999; Seidel et al.2010). The
stem cells reside within the stellate reticulumcells in the core of
cervical loop, which is surroundedby dental mesenchyme. The progeny
of stem cells invadesthe basal epithelium and proliferates as TA
cells before dif-ferentiating into ameloblasts (Figs. 2C and
4A,B).
Mesenchymal signals play key roles in the regulation ofthe
epithelial stem cells and their progeny (Fig. 2C). Fgf10is the key
mesenchymal signal required for epithelial stemcell maintenance and
proliferation, and Fgf3 has a partlyredundant function as
stimulator of TA cell proliferation(Wang et al. 2007). Mesenchymal
Fgfs act in a regulatoryloop with epithelial Fgfs, notably Fgf9,
and stimulation ofFgf signaling by deleting the function of Sprouty
genes resultsin extensive growth of the incisors and ectopic
deposition ofenamel on the lingual surface (Klein et al. 2008).
Lingualenamel also forms in Follistatin knockout mice,
whereasenhanced Follistatin expression results in complete
absenceof enamel from labial side as well as in growth
inhibition(Wang et al. 2004). Follistatin is expressed in the
lingual epi-thelium and it antagonizes Activin function in the
cervicalloop epithelium while, interestingly, inhibiting Bmp4
func-tion in the zone of differentiation, which prevents
enamelformation. It was shown that Bmp signaling is required
forameloblast differentiation (Fig. 4) (Wang et al. 2004).
Ac-cordingly, when the Bmp inhibitor Noggin is overexpressedin
epithelium, the mouse incisors grow extensively and lackenamel
(Plikus et al. 2005). Bmps and Activin function in aregulatory
network with Fgf3, which is inhibited by Bmp4,which is in turn
repressed by Activin that is strongly ex-pressed in the labial
dental mesenchyme but not on the lin-gual side. This contributes to
the asymmetric production ofTA cells only in the labial cervical
loop (Wang et al. 2007).
Fgf and Shh signaling play a role in the postnatal ho-meostasis
of the TA cell production in the mouse incisorsbut not in stem cell
survival (Parsa et al. 2010; Seidelet al. 2010). On the other hand,
Wnt signaling activity isnot detected in the stem cells in the
cervical loops (Suoma-lainen and Thesleff 2010). Some stem cell
marker genes,such as Lgr5, Bmi1, Oct3/4, and Yap, have been
localizedin the incisor stem cells (Suomalainen and Thesleff
2010;Li et al. 2011b). Despite the intense investigations on
thesignal pathways regulating the incisor stem cell niche,
thecharacterization of these stem cells is still on its way.
5 FUTURE CHALLENGES: STEM CELL-BASEDBIOENGINEERING OF TEETH
Different scenarios have been proposed for bioengineeringof
human teeth. One possibility could be the direct induc-tion of
tooth development in the jaws with activators such
Signaling Networks Regulating Tooth Development
Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425 9
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
as Wnt and Eda. However, although supernumerary teethare induced
in mouse models and in human syndromesby modulation of signal
pathways, it is not likely that thisapproach would function in the
adult jaws. The main rea-son is that the supernumerary teeth in
mice—as well as inthe rare human syndromes—form from the tissue
associ-ated with developing teeth and such teeth would not
bepresent in adult jaws anymore. The stem cells discoveredin adult
teeth described above, including the mesenchymalstem cells in
dental papilla, pulp, and periodontal ligamenthave the capacity to
generate cells forming dentin, cementum,and periodontal ligament,
but it is unlikely that they havemorphogenetic potential, and even
less likely that the cellscould be targeted in vivo to undergo
tooth morphogenesis.
A more realistic approach would be to engineer a toothin vitro
and implant it to patient’s mouth. It has been pro-posed that such
teeth could be generated by growing cells intooth-shaped scaffolds.
However, taking into account thecomplex structure and organization
of the hard and softtissues of teeth and the fact that tooth size
and shapeemerge during the multistep process of morphogenesis
to-gether with the periodontal tissue attaching the tooth to
thebone, it is difficult to imagine how a functional tooth couldbe
developed within a scaffold. Therefore, the preferableway would be
to trigger the initiation of tooth developmentprogram in progenitor
cells and let the tooth develop itself.
The classical tooth bud transplantation and tissue
re-combination experiments have shown that the programfor tooth
morphogenesis is present very early in the jawsand that a tooth bud
can form a complete tooth whentransplanted to various ectopic
sites. The proof of principleexperiments in mouse have already
shown that even disso-ciated embryonic dental epithelial and
mesenchymal cellscan regenerate a tooth germ in vitro and that this
forms afunctional tooth when implanted to the jaw of an adultmouse
(Nakao et al. 2007; Oshima et al. 2011).
To use such an approach in human therapy, one wouldneed to
replace the embryonic dental cells with adult cellspreferably with
tooth forming capacity. Obviously, bothepithelial and mesenchymal
cell lineages are needed, butbased on the classical recombination
experiments onlyone cell type needs to have the odontogenic
potential.Although there is some evidence that adult mouse
bonemarrow stem cells can form a tooth together with embry-onic
branchial arch epithelium (Ohazama et al. 2004), itis probable that
in the recombinations the nonodontogenicmesenchyme should have
properties of cranial neural crestand the epithelium should be
ectodermal. The inductivepotential of the current human dental stem
cell lines hasnot been explored. However, although dental stem
cellsfrom adult teeth could perhaps be used for tooth
bioengin-eering because they are likely to share characteristics
with
embryonic dental tissue, collecting dental stem cells fromadults
is challenging as it would imply sacrificing a toothfrom the
patient needing a new tooth.
Odontogenic cells might be produced from adult so-matic cells by
iPS cell technology or direct reprogramming(Hanna et al. 2010).
Thus they could be first reprogram-med to embryonic stem cells by
iPS technology and pro-grammed further to dental epithelial or
mesenchymal cellfates. Oral mucosal epithelium and stromal cells
could befeasible sources for reprogramming because their
develop-mental history is likely more similar to dental tissues.
Alter-natively, the oral mucosal cells or other adult somatic
cellscould perhaps be directly converted to tooth epitheliumand
mesenchyme as recently shown in other tissues (Zhouet al. 2008;
Hanna et al. 2010).
The knowledge lacking at the moment is the molecularsignatures
of the epithelial and mesenchymal lineages thatcould be used in
reprogramming. The key genes likelyinclude transcription factors
expressed by the early embry-onic tissues such as Pitx2 in the
branchial arch epitheliumand Msx1,2, Dlx1,2,5, Runx2, Pax9,
Lhx6,7,8, and Prx1,2in the odontogenic mesenchyme (Fig. 2A)
(Thesleff andTummers 2008). So far these are onlyeducated guesses
basedon expression patterns and mutant phenotypes
(http://bite-it.helsinki.fi; Bei 2009b).
Finally, it should be noted that it is unrealistic to aim
atgenerating a perfect tooth crown, because it is rather ob-vious
that the right shape and size of the crown as well asthe color and
proper structure of enamel cannot be gener-ated by bioengineering.
Therefore, the crown needs to becompleted prosthetically. The most
important aspect ofthe bioengineered tooth would be a functional
root pro-viding physiological anchorage of the tooth to jaw
bone.However, initiation of root formation, without a
precedingcrown appears impossible, at least by mimicking
normaldevelopmental mechanisms. The physiological anchorageis
lacking in the titanium implants that otherwise are suc-cessfully
used for tooth replacement. To this end interestingexperiments have
been performed in minipigs, where stemcells from the root apical
papilla and periodontal ligamentstem cells were used. When the
cells were seeded on a root-shaped cylindrical
hydroxyapatite/tricalcium phosphatescaffold, and implanted in the
jaw, dentin, cementum, andperiodontal ligament were generated and a
structure resem-bling root developed (Sonoyama et al. 2006). It is
not yetknown whether the bioengineered root has adequate
phys-iological properties to be used in clinical tooth
replacement.
6 CONCLUDING REMARKS
Studies on the laboratory mouse have given a great deal
ofknowledge on the molecular regulation of tooth initiation,
M. Jussila and I. Thesleff
10 Advanced Online Article. Cite this article as Cold Spring
Harb Perspect Biol doi: 10.1101/cshperspect.a008425
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
morphogenesis, and stem cell maintenance. However, be-fore the
building of teeth by tissue engineering becomes areality, more
detailed understanding of the process of toothdevelopment and
regeneration is required. In particular,the gene regulatory
networks during cell lineage specifi-cation in dental epithelium
and mesenchyme need to beunderstood more thoroughly and the origins
of the twotypes of progenitor cells to be used for tooth
bioengineer-ing should be determined.
ACKNOWLEDGMENTS
We thank Emma Juuri, Otso Häärä, Elina Järvinen, andAapo
Kangas for providing illustrations.
REFERENCES
Andl T, Reddy ST, Gaddapara T, Millar SE. 2002. WNT signals are
re-quired for the initiation of hair follicle development. Dev Cell
2:643–653.
Bei M. 2009a. Molecular genetics of ameloblast cell lineage. J
Exp Zool BMol Dev Evol 312B: 437–444.
Bei M. 2009b. Molecular genetics of tooth development. Curr Opin
GenetDev 19: 504–510.
Chen J, Lan Y, Baek JA, Gao Y, Jiang R. 2009. Wnt/b-catenin
signalingplays an essential role in activation of odontogenic
mesenchyme dur-ing early tooth development. Dev Biol 334:
174–185.
Coin R, Haikel Y, Ruch JV. 1999. Effects of apatite,
transforming growthfactor b-1, bone morphogenetic protein-2 and
interleukin-7 on ame-loblast differentiation in vitro. Eur J Oral
Sci 107: 487–495.
Dassule HR, Lewis P, Bei M, Maas R, McMahon AP. 2000. Sonic
hedge-hog regulates growth and morphogenesis of the tooth.
Development127: 4775–4785.
D’Souza RN, Åberg T, Gaikwad J, Cavender A, Owen M, KarsentyG,
Thesleff I. 1999. Cbfa1 is required for epithelial-mesenchymal
inter-actions regulating tooth development in mice. Development
126:2911–2920.
Ferguson CA, Tucker AS, Christensen L, Lau AL, Matzuk MM, Sharpe
PT.1998. Activin is an essential early mesenchymal signal in tooth
devel-opment that is required for patterning of the murine
dentition. GenesDev 12: 2636–2649.
Fraser GJ, Hulsey CD, Bloomquist RF, Uyesugi K, Manley NR,
StreelmanJT. 2009. An ancient gene network is co-opted for teeth on
old and newjaws. PLoS Biol 7: e31.
Fujimori S, Novak H, Weissenbock M, Jussila M, Goncalves A,
Zeller R,Galloway J, Thesleff I, Hartmann C. 2010. Wnt/b-catenin
signaling inthe dental mesenchyme regulates incisor development by
regulatingBmp4. Dev Biol 348: 97–106.
Gritli-Linde A, Bei M, Maas R, Zhang XM, Linde A, McMahon AP.
2002.Shh signaling within the dental epithelium is necessary for
cell prolif-eration, growth and polarization. Development 129:
5323–5337.
Gronthos S, Mankani M, Brahim J, Robey PG, Shi S. 2000.
Postnatal hu-man dental pulp stem cells (DPSCs) in vitro and in
vivo. Proc NatlAcad Sci 97: 13625–13630.
Handrigan GR, Leung KJ, Richman JM. 2010. Identification of
putativedental epithelial stem cells in a lizard with life-long
tooth replacement.Development 137: 3545–3549.
Hanna JH, Saha K, Jaenisch R. 2010. Pluripotency and cellular
repro-gramming: Facts, hypotheses, unresolved issues. Cell 143:
508–525.
Harada H, Kettunen P, Jung HS, Mustonen T, Wang YA, Thesleff I.
1999.Localization of putative stem cells in dental epithelium and
their asso-ciation with Notch and FGF signaling. J Cell Biol 147:
105–120.
Hardcastle Z, Mo R, Hui C-C, Sharpe PT. 1998. The Shh signalling
path-way in tooth development: Defects in Gli2 and Gli3 mutants.
Develop-ment 125: 2803–2811.
James MJ, Järvinen E, Wang XP, Thesleff I. 2006. Different
roles of Runx2during early neural crest-derived bone and tooth
development. J BoneMiner Res 21: 1034–1044.
Järvinen E, Salazar-Ciudad I, Birchmeier W, Taketo MM, Jernvall
J, The-sleff I. 2006. Continuous tooth generation in mouse is
induced by ac-tivated epithelial Wnt/b-catenin signaling. Proc Natl
Acad Sci 103:18627–18632.
Järvinen E, Tummers M, Thesleff I. 2009. The role of the dental
laminain mammalian tooth replacement. J Exp Zool B Mol Dev Evol
312B:281–291.
Jensen BL, Kreiborg S. 1990. Development of the dentition in
cleidocra-nial dysplasia. J Oral Pathol Med 19: 89–93.
Jernvall J, Åberg T, Kettunen P, Keränen S, Thesleff I. 1998.
The life historyof an embryonic signaling center: BMP-4 induces P21
and is associ-ated with apoptosis in the mouse tooth enamel knot.
Development125: 161–169.
Jernvall J, Keränen SV, Thesleff I. 2000. Evolutionary
modification of de-velopment in mammalian teeth: Quantifying gene
expression patternsand topography. Proc Natl Acad Sci 97:
14444–14448.
Kangas AT, Evans AR, Thesleff I, Jernvall J. 2004.
Nonindependence ofmammalian dental characters. Nature 432:
211–214.
Kassai Y, Munne P, Hotta Y, Penttilä E, Kavanagh K, Ohbayashi
N, TakadaS, Thesleff I, Jernvall J, Itoh N. 2005. Regulation of
mammalian toothcusp patterning by ectodin. Science 309:
2067–2070.
Kavanagh KD, Evans AR, Jernvall J. 2007. Predicting evolutionary
pat-terns of mammalian teeth from development. Nature 449:
427–432.
Keränen SV, Kettunen P, Åberg T, Thesleff I, Jernvall J. 1999.
Gene expres-sion patterns associated with suppression of
odontogenesis in mouseand vole diastema regions. Dev Genes Evol
209: 495–506.
Kettunen P, Karavanova I, Thesleff I. 1998. Responsiveness of
developingdental tissues to fibroblast growth factors: Expression
of splicing alter-natives of FGFR1, -2, -3, and of FGFR4; and
stimulation of cell prolif-eration by FGF-2, -4, -8, and -9. Dev
Genet 22: 374–385.
Kettunen P, Laurikkala J, Itäranta P, Vainio S, Itoh N,
Thesleff I. 2000. As-sociations of FGF-3 and FGF-10 with signaling
networks regulatingtooth morphogenesis. Dev Dyn 219: 322–332.
Klein OD, Minowada G, Peterkova R, Kangas A, Yu BD, Lesot H,
PeterkaM, Jernvall J, Martin GR. 2006. Sprouty genes control
diastema toothdevelopment via bidirectional antagonism of
epithelial-mesenchymalFGF signaling. Dev Cell 11: 181–190.
Klein OD, Lyons DB, Balooch G, Marshall GW, Basson MA, Peterka
M,Boran T, Peterkova R, Martin GR. 2008. An FGF signaling loop
sus-tains the generation of differentiated progeny from stem cells
in mouseincisors. Development 135: 377–385.
Kollar EJ, Baird GR. 1969. The influence of the dental papilla
on the de-velopment of tooth shape in embryonic mouse tooth germs.
J EmbryolExp Morph 21: 131–148.
Kollar EJ, Baird GR. 1970. Tissue interactions in embryonic
mouse toothgerms. II. The inductive role of the dental papilla. J
Embryol Exp Morph24: 173–186.
Kratochwil K, Dull M, Farinas I, Galceran J, Grosschedl R. 1996.
Lef1 ex-pression is activated by BMP-4 and regulates inductive
tissue interac-tions in tooth and hair development. Genes Dev 10:
1382–1394.
Kratochwil K, Galceran J, Tontsch S, Roth W, Grosschedl R. 2002.
FGF4, adirect target of LEF1 and Wnt signaling, can rescue the
arrest of toothorganogenesis in Lef1(2/2) mice. Genes Dev 16:
3173–3185.
Lammi L, Arte S, Somer M, Järvinen H, Lahermo P, Thesleff I,
Pirinen S,Nieminen P. 2004. Mutations in AXIN2 cause familial tooth
agenesisand predispose to colorectal cancer. Am J Hum Genet 74:
1043–1050.
Laurikkala J, Mikkola ML, James M, Tummers M, Mills AA, Thesleff
I.2006. P63 regulates multiple signalling pathways required for
ectoder-mal organogenesis and differentiation. Development 133:
1553–1563.
Signaling Networks Regulating Tooth Development
Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425 11
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
Li J, Huang X, Xu X, Mayo J, Bringas P Jr, Jiang R, Wang S, Chai
Y. 2011a.SMAD4-mediated WNT signaling controls the fate of cranial
neuralcrest cells during tooth morphogenesis. Development 138:
1977–1989.
Li L, Kwon HJ, Harada H, Ohshima H, Cho SW, Jung HS. 2011b.
Expres-sion patterns of ABCG2, Bmi-1, Oct-3/4, and Yap in the
developingmouse incisor. Gene Expr Patterns 11: 163–170.
Liu F, Chu EY, Watt B, Zhang Y, Gallant NM, Andl T, Yang SH, Lu
MM,Piccolo S, Schmidt-Ullrich R, et al. 2008. Wnt/b-catenin
signaling di-rects multiple stages of tooth morphogenesis. Dev Biol
313: 210–224.
Luckett WP. 1993. An ontogenetic assessment of dental homologies
intherian mammals. In Mammal phylogeny: Mesozoic
differentiation,multituberculates, monotremes, early therians, and
marsupials (ed. Sza-lay FS, Novacek MJ, McKenna MC), p. 183.
Springer, New York.
Lumsden AG. 1988. Spatial organization of the epithelium and the
role ofneural crest cells in the initiation of the mammalian tooth
germ. De-velopment 103: 155–169.
Marks SC Jr, Cahill DR. 1987. Regional control by the dental
follicle of al-terations in alveolar bone metabolism during tooth
eruption. J OralPathol 16: 164–169.
Michon F, Tummers M, Kyyrönen M, Frilander MJ, Thesleff I.
2010.Tooth morphogenesis and ameloblast differentiation are
regulatedby micro-RNAs. Dev Biol 340: 355–368.
Mikkola ML. 2009a. Controlling the number of tooth rows. Sci
Signal2: e53.
Mikkola ML. 2009b. Molecular aspects of hypohidrotic ectodermal
dys-plasia. Am J Med Genet A 149A: 2031–2036.
Mina M, Kollar EJ. 1987. The induction of odontogenesis in
non-dentalmesenchyme combined with early murine mandibular arch
epithe-lium. Arch Oral Biol 32: 123–127.
Munne PM, Felszeghy S, Jussila M, Suomalainen M, Thesleff I,
Jernvall J.2010. Splitting placodes: Effects of bone morphogenetic
protein andActivin on the patterning and identity of mouse
incisors. Evol Dev12: 383–392.
Mustonen T, Pispa J, Mikkola ML, Pummila M, Kangas AT,
Pakkasjärvi L,Jaatinen R, Thesleff I. 2003. Stimulation of
ectodermal organ develop-ment by Ectodysplasin-A1. Dev Biol 259:
123–136.
Nakamura T, de Vega S, Fukumoto S, Jimenez L, Unda F, Yamada Y.
2008.Transcription factor epiprofin is essential for tooth
morphogenesisby regulating epithelial cell fate and tooth number. J
Biol Chem 283:4825–4833.
Nakao K, Morita R, Saji Y, Ishida K, Tomita Y, Ogawa M, Saitoh
M, To-mooka Y, Tsuji T. 2007. The development of a bioengineered
organgerm method. Nat Methods 4: 227–230.
Nanci A. 2008. Ten Cate’s Oral Histology: Development, Structure
andFunction. Mosby Elsevier, St Louis.
Nemoto E, Koshikawa Y, Kanaya S, Tsuchiya M, Tamura M,
SomermanMJ, Shimauchi H. 2009. Wnt signaling inhibits cementoblast
differen-tiation and promotes proliferation. Bone 44: 805–812.
Neubüser A, Peters H, Balling R, Martin GR. 1997. Antagonistic
interac-tions between FGF and BMP signalling pathways: A mechanism
forpositioning the sites of tooth formation. Cell 90: 247–255.
Nieminen P, Morgan NV, Fenwick AL, Parmanen S, Veistinen V,
MikkolaML, Giraud A, Judd L, Arte S, Brueton LA, et al. 2011.
Inactivation ofIL11 signaling causes craniosynostosis, delayed
tooth eruption andsupernumerary teeth. Am J Hum Genet 89:
67–81.
Ohazama A, Modino SA, Miletich I, Sharpe PT. 2004.
Stem-cell-basedtissue engineering of murine teeth. J Dent Res 83:
518–522.
Oshima M, Mizuno M, Imamura A, Ogawa M, Nakao K, Yamazaki
H,Morita R, Ikeda E, Takano-Yamamoto T, Kasugai S, et al. 2011.
Func-tional tooth regeneration using a bioengineered tooth unit as
a matureorgan replacement therapy. PLoS One 6: e21531.
Parsa S, Kuremoto K, Seidel K, Tabatabai R, Mackenzie B, Yamaza
T,Akiyama K, Branch J, Koh CJ, Al Alam D, et al. 2010. Signaling
byFGFR2b controls the regenerative capacity of adult mouse
incisors.Development 137: 3743–3752.
Pispa J, Jung H, Jernvall J, Kettunen P, Mustonen T, Tabata MJ,
Kere J,Thesleff I. 1999. Cusp patterning defect in Tabby mouse
teeth andits partial rescue by FGF. Dev Biol 216: 521–534.
Plikus MV, Zeichner-David M, Mayer JA, Reyna J, Bringas P,
ThewissenJG, Snead ML, Chai Y, Chuong CM. 2005. Morphoregulation of
teeth:Modulating the number, size, shape and differentiation by
tuningBmp activity. Evol Dev 7: 440–457.
Richman JM, Handrigan GR. 2011. Reptilian tooth development.
Genesis49: 247–260.
Rodriguez-Lozano FJ, Bueno C, Insausti CL, Meseguer L, Ramirez
MC,Blanquer M, Marin N, Martinez S, Moraleda JM. 2011.
Mesenchymalstem cells derived from dental tissues. Int Endod J 44:
800–806.
Ruch JV, Lesot H, Begue-Kirn C. 1995. Odontoblast
differentiation. Int JDev Biol 39: 51–68.
Salazar-Ciudad I, Jernvall J. 2002. A gene network model
accounting fordevelopment and evolution of mammalian teeth. Proc
Natl Acad Sci99: 8116–8120.
Sarkar L, Sharpe PT. 1999. Expression of wnt signalling pathway
genesduring tooth development. Mech Dev 85: 197–200.
Seidel K, Ahn CP, Lyons D, Nee A, Ting K, Brownell I, Cao T,
Carano RA,Curran T, Schober M, et al. 2010. Hedgehog signaling
regulates thegeneration of ameloblast progenitors in the
continuously growingmouse incisor. Development 137: 3753–3761.
Seo BM, Miura M, Gronthos S, Bartold PM, Batouli S, Brahim J,
YoungM, Robey PG, Wang CY, Shi S. 2004. Investigation of
multipotentpostnatal stem cells from human periodontal ligament.
Lancet 364:149–155.
Shields ED, Bixler D, el-Kafrawy AM. 1973. A proposed
classification forheritable human dentine defects with a
description of a new entity.Arch Oral Biol 18: 543–553.
Shinmura Y, Tsuchiya S, Hata K, Honda MJ. 2008. Quiescent
epithelialcell rests of Malassez can differentiate into
ameloblast-like cells. J CellPhysiol 217: 728–738.
Smith MM, Fraser GJ, Mitsiadis TA. 2009. Dental lamina as source
ofodontogenic stem cells: Evolutionary origins and developmental
con-trol of tooth generation in gnathostomes. J Exp Zool B Mol Dev
Evol312B: 260–280.
Sonoyama W, Liu Y, Fang D, Yamaza T, Seo BM, Zhang C, Liu H,
Gron-thos S, Wang CY, Wang S, et al. 2006. Mesenchymal stem
cell-mediatedfunctional tooth regeneration in swine. PLoS One 1:
e79.
Suomalainen M, Thesleff I. 2010. Patterns of Wnt pathway
activity in themouse incisor indicate absence of Wnt/b-catenin
signaling in the epi-thelial stem cells. Dev Dyn 239: 364–372.
Thesleff I, Hurmerinta K. 1981. Tissue interactions in tooth
develop-ment. Differentiation 18: 75–88.
Thesleff I, Tummers M. 2008. Tooth organogenesis and
regeneration. InStemBook (ed. The Stem Book Research Community).
doi/10.3824/stembook.1.37.1.
Thesleff I, Keränen S, Jernvall J. 2001. Enamel knots as
signaling centerslinking tooth morphogenesis and odontoblast
differentiation. AdvDent Res 15: 14–18.
Tucker AS, Matthews KL, Sharpe PT. 1998. Transformation of tooth
typeinduced by inhibition of BMP signalling. Science 282:
1136–1138.
Tummers M, Thesleff I. 2009. The importance of signal pathway
modu-lation in all aspects of tooth development. J Exp Zool B Mol
Dev Evol312B: 309–319.
Vainio S, Karavanova I, Jowett A, Thesleff I. 1993.
Identification ofBMP-4 as a signal mediating secondary induction
between epithelialand mesenchymal tissues during early tooth
development. Cell 75:45–58.
Wang XP, Fan J. 2011. Molecular genetics of supernumerary tooth
forma-tion. Genesis 49: 261–277.
Wang XP, Suomalainen M, Jorgez CJ, Matzuk MM, Werner S, Thesleff
I.2004. Follistatin regulates enamel patterning in mouse incisors
byasymmetrically inhibiting BMP signaling and ameloblast
differentia-tion. Dev Cell 7: 719–730.
M. Jussila and I. Thesleff
12 Advanced Online Article. Cite this article as Cold Spring
Harb Perspect Biol doi: 10.1101/cshperspect.a008425
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
Wang XP, Suomalainen M, Felszeghy S, Zelarayan LC, Alonso MT,
PlikusMV, Maas RL, Chuong CM, Schimmang T, Thesleff I. 2007. An
inte-grated gene regulatory network controls stem cell
proliferation inteeth. PLoS Biol 5: e159.
Wang XP, O’Connell DJ, Lund JJ, Saadi I, Kuraguchi M, Turbe-Doan
A,Cavallesco R, Kim H, Park PJ, Harada H, et al. 2009. Apc
inhibition ofWnt signaling regulates supernumerary tooth formation
during em-bryogenesis and throughout adulthood. Development 136:
1939–1949.
Yamashiro T, Zheng L, Shitaku Y, Saito M, Tsubakimoto T,
TakadaK, Takano-Yamamoto T, Thesleff I. 2007. Wnt10a regulates
dentin sia-lophosphoprotein mRNA expression and possibly links
odontoblast
differentiation and tooth morphogenesis. Differentiation 75:
452–462.
Zhang Z, Lan Y, Chai Y, Jiang R. 2009. Antagonistic actions of
Msx1 andOsr2 pattern mammalian teeth into a single row. Science
323:1232–1234.
Zhao M, Xiao G, Berry JE, Franceschi RT, Reddi A, Somerman MJ.
2002.Bone morphogenetic protein 2 induces dental follicle cells to
differen-tiate toward a cementoblast/osteoblast phenotype. J Bone
Miner Res17: 1441–1451.
Zhou Q, Brown J, Kanarek A, Rajagopal J, Melton DA. 2008. In
vivo re-programming of adult pancreatic exocrine cells to b-cells.
Nature 455:627–632.
Signaling Networks Regulating Tooth Development
Advanced Online Article. Cite this article as Cold Spring Harb
Perspect Biol doi: 10.1101/cshperspect.a008425 13
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/
-
published online March 13, 2012Cold Spring Harb Perspect Biol
Maria Jussila and Irma Thesleff Epithelial Cell
LineagesRegeneration, and the Specification of Dental Mesenchymal
and Signaling Networks Regulating Tooth Organogenesis and
Subject Collection Mammalian Development
Mouse EmbryoThe Dynamics of Morphogenesis in the Early
HadjantonakisJaime A. Rivera-Pérez and Anna-Katerina
Development
Neural Progenitors during Mammalian Cortical Cell Division Modes
and Cleavage Planes of
Fumio Matsuzaki and Atsunori ShitamukaimicroRNAs as
Developmental Regulators
Kathryn N. Ivey and Deepak SrivastavaBlood and Lymphatic Vessel
Formation
Victoria L. Bautch and Kathleen M. CaronDevelopment of the
Endochondral Skeleton
Fanxin Long and David M. Ornitz DevelopmentTranscriptional
Networks in Liver and Intestinal
Karyn L. Sheaffer and Klaus H. KaestnerAdipogenesis
Kelesha Sarjeant and Jacqueline M. StephensPluripotency in the
Embryo and in Culture
Jennifer Nichols and Austin SmithMolecular Mechanisms of Inner
Ear Development
Doris K. Wu and Matthew W. Kelley Development and
RegenerationSignaling and Transcriptional Networks in Heart
Benoit G. BruneauPolarity in Mammalian Epithelial
Morphogenesis
Julie Roignot, Xiao Peng and Keith Mostov Cell
DifferentiationSignals and Switches in Mammalian Neural Crest
Shachi Bhatt, Raul Diaz and Paul A. TrainorEye Development and
Retinogenesis
Whitney Heavner and Larysa PevnyHematopoiesis
Michael A. Rieger and Timm SchroederPrimordial Germ Cells in
Mice
Mitinori Saitou and Masashi YamajiEmbryonic AxesEstablishing
Blastocyst Cell Lineages and Intercellular Interactions, Position,
and Polarity in
P.L. TamRobert O. Stephenson, Janet Rossant and Patrick
http://cshperspectives.cshlp.org/cgi/collection/ For additional
articles in this collection, see
Copyright © 2012 Cold Spring Harbor Laboratory Press; all rights
reserved
on April 5, 2021 - Published by Cold Spring Harbor Laboratory
Press http://cshperspectives.cshlp.org/Downloaded from
http://cshperspectives.cshlp.org/cgi/collection/http://cshperspectives.cshlp.org/cgi/collection/http://cshperspectives.cshlp.org/