During early tooth development, multiple signaling molecules are expressed in the dental lamina epithelium and induce the dental mesenchyme. One signal, BMP4, has been shown to induce morphologic changes in dental mesenchyme and mesenchymal gene expression via Msx1, but BMP4 cannot substitute for all the inductive functions of the dental epithelium. To investigate the role of FGFs during early tooth development, we examined the expression of epithelial and mesenchymal Fgfs in wild-type and Msx1 mutant tooth germs and tested the ability of FGFs to induce Fgf3 and Bmp4 expression in wild-type and Msx1 mutant dental mesenchymal explants. Fgf8 expression is preserved in Msx1 mutant epithelium while that of Fgf3 is not detected in Msx1 mutant dental mesenchyme. Moreover, dental epithelium as well as beads soaked in FGF1, FGF2 or FGF8 induce Fgf3 expression in dental mesenchyme in an Msx1-dependent manner. These results indicate that, like BMP4, FGF8 constitutes an epithelial inductive signal capable of inducing the expression of downstream signaling molecules in dental mesenchyme via Msx1. However, the BMP4 and FGF8 signaling pathways are distinct. BMP4 cannot induce Fgf3 nor can FGFs induce Bmp4 expression in dental mesenchyme, even though both signaling molecules can induce Msx1 and Msx1 is necessary for Fgf3 and Bmp4 expression in dental mesenchyme. In addition, we have investigated the effects of FGFs and BMP4 on the distal-less homeobox genes Dlx1 and Dlx2 and we have clarified the relationship between Msx and Dlx gene function in the developing tooth. Dlx1,Dlx2 double mutants exhibit a lamina stage arrest in maxillary molar tooth development (Thomas B. L., Tucker A. S., Qiu M., Ferguson C. A., Hardcastle Z., Rubenstein J. L. R. and Sharpe P. T. (1997) Development 124, 4811–4818). Although the maintenance of molar mesenchymal Dlx2 expression at the bud stage is Msx1-dependent, both the maintenance of Dlx1 expression and the initial activation of mesenchymal Dlx1 and Dlx2 expression during the lamina stage are not. Moreover, in contrast to the tooth bud stage arrest observed in Msx1 mutants, Msx1,Msx2 double mutants exhibit an earlier phenotype closely resembling the lamina stage arrest observed in Dlx1,Dlx2 double mutants. These results are consistent with functional redundancy between Msx1 and Msx2 in dental mesenchyme and support a model whereby Msx and Dlx genes function in parallel within the dental mesenchyme during tooth initiation. Indeed, as predicted by such a model, BMP4 and FGF8, epithelial signals that induce differential Msx1 and Msx2 expression in dental mesenchyme, also differentially induce Dlx1 and Dlx2 expression, and do so in an Msx1-independent manner. These results integrate Dlx1, Dlx2 and Fgf3 and Fgf8 into the odontogenic regulatory hierarchy along with Msx1, Msx2 and Bmp4, and provide a basis for interpreting tooth induction in terms of transcription factors which, individually, are necessary but not sufficient for the expression of downstream signals and therefore must act in specific combinations.

Cam
Y.
,
Neumann
M. R.
,
Oliver
L.
,
Raulais
D.
,
Janet
T.
,
Ruch
J. V.
(
1992
)
Immunolocalization of acidic and basic fibroblast growth factors during mouse odontogenesis.
Int. J. Dev. Biol.
36
,
381
389
. [erratum: (1992). Int. J. Dev. Biol 36, 599f].
Chen
Y.
,
Bei
M.
,
Woo
I.
,
Satokata
I.
,
Maas
R. L.
(
1996
)
Msx-1 Controls Reciprocal Inductive Signalling in Mammalian Tooth Morphogenesis.
Development
122
,
3033
3044
Crossley
P. H.
,
Martinez
S.
,
Martin
G. R.
(
1996
)
Midbrain development induced by FGF8 in the chick embryo.
Nature
380
,
66
68
Dickson
C.
,
Peters
G.
(
1987
)
Potential oncogene product related to growth factors.
Nature
326
,
833
–.
Finch
P. W.
,
Cunha
G. R.
,
Rubin
J. S.
,
Wong
J.
,
Ron
D.
(
1995
)
Pattern of keratinocyte growth factor and keratinocyte growth factor receptor expression during mouse fetal development suggest a role in mediating morphogenetic mesenchymal-epithelial interactions.
Dev. Dyn
203
,
223
240
Grigoriou
M.
,
Tucker
A.
,
Sharpe
P. T.
,
Pachnis
V.
(
1998
)
Expression and regulation of Lhx6 and Lhx7, a novel subfamily of LIM homeodomain encoding genes, suggests a role in mammalian head development.
Development
125
,
2063
2074
Heikinheimo
M.
,
Lawshe
A.
,
Shackleford
G. M.
,
Wilson
D. B.
,
MacArthur
C. A.
(
1994
)
Fgf-8 expression in the post-gastrulation mouse suggests roles in the development of the face, limbs and central nervous system.
Mech. Dev
48
,
129
138
Igarashi
M.
,
Finch
P. W.
,
Aaronson
S. A.
(
1998
)
Characterization of recombinant human fibroblast growth factor (FGF)-10 reveals functional similarities with keratinocyte growth factor (FGF-7).
J. Biol. Chem
273
,
13230
13235
Jernvall
J.
,
Kettunen
P.
,
Karavanova
I.
,
Martin
L. B.
,
Thesleff
I.
(
1994
)
Evidence for the role of the enamel knot as a control center in mammalian tooth cusp formation: non-dividing cells express growth stimulating Fgf-4 gene.
Int. J. Dev. Biol
38
,
463
469
Jernvall
J.
,
Aberg
T.
,
Kettunen
S.
,
Keranen
S.
,
Thesleff
I.
(
1998
)
The life history of an embryonic signalling center: BMP-4 induces p21 and is associated with apoptosis in the mouse tooth enamel knot.
Development
125
,
161
169
Jessell
T. M.
,
Goodman
C. S.
(
1996
)
Development: neural development: are there any surprises left?.
Curr. Opin. Neurobiol
6
,
1
2
Johnson
R. L.
,
Tabin
C. J.
(
1997
)
Molecular models for vertebrate limb development.
Cell
90
,
979
990
Kettunen
P.
,
Thesleff
I.
(
1998
)
Expression and function of FGFs-4,-8 and-9 suggest redundancy and repetitive use as epithelial signals during tooth morphogenesis.
Dev. Dyn
211
,
256
268
Kratochwil
K.
,
Dull
M.
,
Farinas
I.
,
Galceran
J.
,
Grosschedl
R.
(
1996
)
Lef1 expression is activated by BMP-4 and regulates inductive tissue interactions in tooth and hair development.
Genes Dev
10
,
1382
1394
Lee
S. M. K.
,
Danielian
P. S.
,
Fritzsch
B.
,
McMahon
A. P.
(
1997
)
Evidence that FGF8 signalling from the midbrain-hindbrain junction regulates growth and polarity in the developing midbrain.
Development
124
,
959
969
Lumsden
A. G. S.
(
1988
)
Spatial organization of the epithelium and the role of neural crest cells in the initiation of the mammalian tooth germ.
Development
103
,
155
169
Maas
R. L.
,
Chen
Y.
,
Bei
M.
,
Woo
I.
,
Satokata
I.
(
1996
)
The role of Msx genes in mammalian development.
Ann. NY Acad. Sci
785
,
171
181
Maas
R. L.
,
Bei
M.
(
1997
)
The genetic control of early tooth development.
Crit. Rev. Oral. Biol
8
,
4
31
MacKenzie
A.
,
Leeming
G. L.
,
Jowett
A. K.
,
Ferguson
M. W. J.
,
Sharpe
P. T.
(
1991
)
The homeobox gene Hox 7. 1 has specific regional and temporal expression patterns during early murine craniofacial embryogenesis, especially tooth development in vivo and in vitro.
Development
111
,
269
285
MacKenzie
A.
,
Ferguson
M. W. J.
,
Sharpe
P. T.
(
1991
)
Hox-7 expression during murine craniofacial development.
Development
113
,
601
611
MacKenzie
A.
,
Ferguson
M. W. J.
,
Sharpe
P. T.
(
1992
)
Expression patterns of the homeobox gene, Hox-8, in the mouse embryo suggest a role in specifying tooth initiation and shape.
Development
115
,
403
420
Mansour
S. L.
,
Goddard
J. M.
,
Capecchi
M. R.
(
1993
)
Mice homozygous for a targeted disruption of the proto-oncogene int-2 have developmental defects in tail and inner ear.
Development
117
,
13
29
Mina
M.
,
Kollar
E. J.
(
1987
)
The induction of odontogenesis in non-dental mesenchyme combined with early murine mandibular arch epithelium.
Arch. Oral Biol
32
,
123
127
Neubuser
A.
,
Peters
H.
,
Balling
R.
,
Martin
G. R.
(
1997
)
Antagonistic interactions between FGF and BMP signalling pathways: a mechanism for positioning the sites of tooth formation.
Cell
90
,
247
255
Nieminen
A.
,
Pirnen
S.
,
Thesleff
I.
,
Peltonen
L.
(
1996
)
Gene defect in hypodontia: exclusion of EGF, EGFR, and FGF-3 as candidate genes.
J. Dent. Res
75
,
1346
1352
Ornitz
D. M.
,
Xu
J.
,
Colvin
J. S.
,
McEwen
D. G.
,
MacArthour
C. A.
,
Coulier
F.
,
Gao
G.
,
Goldfarb
M.
(
1996
)
Receptor specificity of the fibroblast growth factor family.
J. Biol. Chem
25
,
15292
15297
Orr-Urtreger
A.
,
Yayon
A.
,
Givol
D.
,
Yarden
Y.
,
Lonai
P.
(
1991
)
Developmental expression of two murine fibroblast growth factor receptors, flg and bek.
Development
113
,
1419
1434
Orr-Urtreger
A.
,
Bedford
M. T.
,
Burakova
T.
,
Arman
E.
,
Zimmer
Y.
,
Yayon
A.
,
Givol
D.
,
Lonai
P.
(
1993
)
Developmental localization of the splicing alternatives of fibroblast growth factor receptor-2 (FGFR2).
Dev. Biol
158
,
475
486
Peters
K. G.
,
Werner
S.
,
Chen
G.
,
Williams
L. T.
(
1992
)
Two FGF receptor genes are differentially expressed in epithelial and mesenchymal tissues during limb formation and organogenesis in the mouse.
Development
114
,
233
243
Qiu
M.
,
Bulfone
A.
,
Martinez
S.
,
Meneses
S.
,
Shimamura
K.
,
Pedersen
R. A.
,
Rubenstein
J. L. R.
(
1995
)
Null mutation of Dlx-2 results in abnormal morphogenesis of proximal first and second branch arch derivatives and abnormal differentiation in the forebrain.
Genes Dev
9
,
2523
2538
Qiu
M.
,
Bulfone
A.
,
Ghattas
I.
,
Menses
J. J.
,
Sharpe
P. T.
,
Presley
R.
,
Pedersen
R. A.
,
Rubenstein
J. L. R.
(
1997
)
Role of the Dlx homeobox genes in proximodistal patterning of the branchial arches: mutations of Dlx-1, Dlx-2 and Dlx-1 and-2 alter morphogenesis of proximal skeletal and soft tissue structures derived from the first and second arches.
Dev. Biol
185
,
165
184
Richman
J. M.
,
Herbert
M.
,
Matovinovic
E.
,
Walin
J.
(
1997
)
Effect of fibroblast growth factors on outgrowth of facial mesenchyme.
Dev. Biol
189
,
135
147
Sassoon
D.
,
Rosenthal
N.
(
1993
)
Detection of messenger RNA by in situ hybridization.
Meth. Enzymol
225
,
384
404
Satokata
I.
,
Maas
R.
(
1994
)
Msx-1 deficient mice exhibit cleft palate and abnormalities of craniofacial and tooth development.
Nature Genetics
6
,
348
356
Suzuki
A.
,
Ueno
N.
,
Hemmati-Brivanlou
A.
(
1997
)
Xenopus msx1 mediates epidermal induction and neural inhibition by BMP4.
Development
124
,
3037
3044
Tajbakhsh
S.
,
Sporle
R.
(
1998
)
Somite development: constructing the vertebrate body.
Cell
92
,
9
16
Thesleff
I.
,
Sharpe
P.
(
1997
)
Signalling networks regulating dental development.
Mech. Dev
67
,
111
123
Thesleff
I.
,
Vaahtokari
A.
(
1992
)
The role of growth factors in determination and differentiation of the odontoblastic cell lineage.
Proc. Finn. Dent. Soc
88
,
357
368
Thesleff
I.
,
Vaahtokari
A.
,
Partanen
A.-M.
(
1995
)
Regulation of organogenesis. Common molecular mechanisms regulating the development of teeth and other organs.
Int. J. Dev. Biol
39
,
35
50
Thomas
B. L.
,
Tucker
A. S.
,
Qiu
M.
,
Ferguson
C. A.
,
Hardcastle
Z.
,
Rubenstein
J. L. R.
,
Sharpe
P. T.
(
1997
)
Role of Dlx1 and Dlx2 genes in patterning of murine dentition.
Development
124
,
4811
4818
Turecková
J.
,
Sahlberg
C.
,
Aberg
T.
,
Ruch
J. V.
,
Thesleff
I.
,
Peterkova
R.
(
1995
)
Comparison of expression of the Msx1, Msx-2, BMP-2 and BMP-4 genes in the mouse upper diastemal and molar primordia.
Int. J. Dev. Biol
39
,
459
468
Vaahtokari
A.
,
Aberg
T.
,
Thesleff
I.
(
1996
)
Apoptosis in the developing tooth: association with an embryonic signalling center and suppression by EGF and FGF-4.
Development
122
,
121
129
Vainio
S.
,
Karavanova
I.
,
Jowett
A.
,
Thesleff
I.
(
1993
)
Identification of BMP-4 as a signal mediating secondary induction between epithelial and mesenchymal tissues during early tooth development.
Cell
75
,
45
58
Vogel
A.
,
Roberts-Clarke
D.
,
Niswander
L.
(
1995
)
Effect of FGF on gene expression in chick limb bud cells in vivo and in vitro.
Dev. Biol
171
,
507
520
Xu
P.-X.
,
Woo
I.
,
Her
H.
,
Beier
D. R.
,
Maas
R. L.
(
1997
)
Mouse Eya homologues of the Drosophila eyes absent gene require Pax-6 for expression in lens and nasal placode.
Development
124
,
219
231
Xu
X.
,
Weinstein
M.
,
Li
C.
,
Naski
M.
,
Cohen
R. I.
,
Ornitz
D. M.
,
Leder
P.
,
Deng
C.
(
1998
)
Fibroblast growth factor receptor 2 (FGFR2)-mediated reciprocal regulation loop between FGF8 and FGF10 is essential for limb induction.
Development
125
,
753
765
Zhang
H.
,
Hu
G.
,
Wang
H.
,
Sciavolino
P.
,
Iler
N.
,
Shen
M. M.
,
Abate-Shen
C.
(
1997
)
Heterodimerization of Msx and Dlx homeoproteins results in functional antagonism.
Mol. Cell. Biol
17
,
2920
2932
Zhou
P.
,
Byrne
C.
,
Jacobs
J.
,
Fuchs
E.
(
1995
)
Lymphoid enhancer factor 1 directs hair follicle patterning and epithelial fate.
Genes Dev
9
,
570
583
This content is only available via PDF.