doi: 10.1242/10.1242/dev.00334
FGF signaling through FGFR1 is required for olfactory bulb morphogenesis
Jean M. Hébert1,*,
Mary Lin1,
Juha Partanen2,
Janet Rossant3 and
Susan K. McConnell1,
1 Department of Biological Sciences, Stanford University, Stanford, CA 94305,
USA
2 Institute of Biotechnology, University of Helsinki, FIN-00014, Finland
3 S. Lunenfeld Research Institute, University of Toronto, Toronto M5G 1X5, ON,
Canada
* Present address: Department of Neuroscience, Albert Einstein College of
Medicine, Bronx, NY 10461, USA
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Fig. 7. Expression of FGF receptor mRNAs in E12.5 wild-type and
Fgfr1-deficient mice. Sagittal sections showing in situ hybridization
in control (A,B) and mutant (A',B') brains for Fgfr2,
(A,A') and Fgfr3 (B,B'). (C) FGF2 induces Fos
expression in control cells from the dorsolateral telencephalon. FGF2-induced
Fos expression is reduced but not eliminated in
Fgfr1-deficient cells. Gapdh is used as a loading control
(G3PDH in figure). Scale bar: 0.25mm.
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Fig. 2. Telencephalons that lack Fgfr1 do not develop normal olfactory
bulbs. Ventral views of E13.5 control (A) and mutant (A') brains showing
a lack of OB development in the mutant (arrow; anterior is upwards). Scale
bar: 0.75 mm. Dorsal views of E16.5 control (B) and mutant (B') brains;
OBs are absent from the mutant (arrow). At P0, control brains (C) have well
developed OBs, whereas mutants (C') have a small bulb-like protrusion
(arrow). Scale bar for B-C': 1 mm. Cresyl Violet stained sagittal
sections through the brains of control (D) and mutant (D') mice at E16.5
showing a lack of cellular organization in the anterior mutant telencephalon
(arrowhead); the anterior telencephalon of mutants is no longer arranged in
layers as it is in control embryos. Cresyl Violet stained sections through
control (E) and mutant (E') brains at P0 showing a small bulb-like
protrusion (arrowhead). cp, cortical plate; OB, olfactory bulb; vz,
ventricular zone. Scale bar for D-E': 0.75 mm.
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Fig. 3. Patterns of gene expression that distinguish anterior and posterior regions
of the telencephalon are for the most part maintained the in
Fgfr1-deficient mice. RNA in situ hybridization on sagittal sections
of E12.5 telencephalons using Emx2 (A,A') and Pax6
(B,B') probes. In control embryos (A,B), Emx2 and Pax6
form opposing gradients in the cerebral cortex. Emx2 and
Pax6 gradients are also established in mutants (A',B').
Scale bar: 0.4 mm. (C,C') Cdh8 expression at E16.5; the
anterior domain of Cdh8 expression is reduced in mutants (C')
compared with control (C) (arrowheads). (D,D') Ephrin A5 expression at
P0; in control animals (D), ephrin A5 is expressed most strongly in the
presumptive sensorimotor cortex
(Mackarehtschian et al., 1999 )
and this location of strongest expression is unchanged in
Foxg1-Cre;Fgfr1flox/flox mutants (D').
Scale bar for C-D': 0.6 mm.
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Fig. 4. The specification of anterior cell types appears normal in the
Fgfr1-deficient telencephalon. RNA in situ hybridization of sagittal
sections of control (A,B) and mutant (A',B') brains with a probe
against ephrin A5; anterior expression is maintained in mutants (arrow) at
E12.5 (A,A') and E16.5 (B,B'; note that expression at this age is
no longer organized in layers). Sections of E12.5 control (C) and mutant
(C') brains probed for Pou3f1 shows expression in both control
and mutant anterior telencephalon (arrow). (D) Gad67 marks
interneurons migrating anteriorly into the olfactory bulb of P0 control (D)
and mutant (D') brains. Sections of control (E,F) and mutant
(E',F') brains at E12.5 (E,E') and E16.5 (F,F')
hybridized to a Tbr1 probe; the accumulation of
Tbr1+ neurons at the anterior tip of the brain occurs
normally in mutants, although the cells are displaced ventrally (arrowheads).
Scale bar in E', 0.4 mm; in F', 0.6 mm. (G,G') Nuclei of
cells in the olfactory bulb region of E13.5 embryos stained with Syto11.
(H,H') The same sections stained with antibodies against RC-2 to
visualize radial glial cells. Scale bar: 0.05 mm.
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Fig. 5. Neurons at the anterior end of the Fgfr1-deficient telencephalon
make connections characteristic of mitral cells. DiI labeling of sensory axons
from the olfactory epithelium (oe) at E12.5 shows that axons have reached the
anterior telencephalon (tel) in both control (A) and Fgfr1-deficient
(A') mice. Scale bar: 0.1 mm. (B,B') DiI labeling of the olfactory
epithelium (oe) at E18.5 reveals that sensory axons have penetrated the
anterior telencephalon in both control (B) and mutant (B') mice;
ventricular and basal borders of the telencephalic neuroepithelium are
indicated (broken lines). Scale bar: 0.15 mm. (C) Retrograde labeling of
mitral cells in the olfactory bulb of control mice following DiI injection
into lateral (piriform) cortex (in which DiI penetrated the tract). Similar
cells are labeled in Fgfr1 mutants (C'). Scale bar: 0.1 mm.
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Fig. 6. The failure of olfactory bulb development in Foxg1-Cre/+;
Fgfr1fx/fx mice involves a failure of cells in the anterior telencephalon
to show decreased levels of proliferation. BrdU incorporation in E12.5 control
(A,B) and mutant (A',B') telencephalons at the site of normal
olfactory bulb development (A,A') and in the cerebral cortex
(B,B'). Levels of BrdU incorporation are reduced at the anterior tip of
the telencephalon in control whereas in the mutant levels remain high
(arrowheads). Scale bar: 0.08 mm. (C) Quantitation of BrdU incorporation.
Unlike cells in the anterior telencephalon of controls, cells in the anterior
telencephalon of mutants continue to proliferate at rates comparable with
those in neocortex (asterisk, P<0.0001).
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Fig. 8. Model of the steps involved in early OB morphogenesis. In the first step,
axons from the olfactory epithelium (OE) project to and make contact with the
anterior telencephalon (tel.). In a second step that is dependent on FGF
signaling, a reduction in the number of proliferating cells (dots) at the
anterior end of the telencephalon occurs (around the axons from the OE),
concurrent with an increase in the number of neurons (not shown). In the third
step, the decrease in cell proliferation causes initial bulb evagination
(purple).
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© The Company of Biologists Ltd 2003
500 bp
EcoRI-BamHI genomic DNA fragment containing exon 15. (B) RNA
in situ hybridization analysis for Fgfr1 expression in a sagittal
section through the telencephalon of an E12.5 control embryo. Fgfr1
is expressed throughout the neuroepithelium lining the telencephalic ventricle
(tel) as well as in the facial mesenchyme (mes). Anterior is leftwards, dorsal
is upwards. (B') Fgfr1 expression is lost from the
telencephalon in mutant embryos. Scale bar: 0.25 mm. (C) Sagittal section
through the nose of an E12.5 control embryo. Fgfr1 expression is
widespread in the mesenchyme (mes) and, compared with the mesenchyme, is at a
lower level in the olfactory epithelium (oe). (C') In the E12.5 mutant
embryo, Fgfr1 expression is also lost from the olfactory epithelium,
but not the surrounding mesenchyme, as expected given the recombination
pattern observed previously with Foxg1-Cre mice
(Hébert and McConnell,
2000