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Xenopus laevis embryogenesis is controlled by the inducing activities of Spemann's organizer. These inducing activities are separated into two distinct suborganizers: a trunk organizer and a head organizer. The trunk organizer induces the formation of posterior structures by emitting signals and directing morphogenesis. Here, we report that the fibroblast growth factor receptor (FGFR) signaling pathway, also known to regulate posterior development, performs critical functions within the cells of Spemann's organizer. Specifically, the FGFR pathway was required in the organizer cells in order for those cells to induce the formation of somitic muscle and the pronephros. Since the organizer influences the differentiation of these tissues by emitting signals that pattern the mesodermal germ layer, our data indicate that the FGFR regulates the production of these signals. In addition, the FGFR pathway was required for the expression of chordin, an organizer-specific protein required for the trunk-inducing activities of Spemann's organizer. Significantly, the FGFR pathway had a minimal effect on the function of the head organizer. We propose that the FGFR pathway is a defining molecular component that distinguishes the trunk organizer from the head organizer by controlling the expression of organizer-specific genes required to induce the formation of posterior structures and somitic muscle in neighboring cells. The implications of our findings for the evolutionarily conserved role of the FGFR pathway in the functions of Spemann's organizer and other vertebrate-signaling centers are discussed.
FIG. 1. Inhibiting the FGFR pathway in the organizer cells
blocked trunk/tail development and somitic muscle formation. (A)
Blastomere map of a 16-cell-stage Xenopus embryo. AB1 blastomeres
will give rise to A1 and B1 daughter cells and CD1
blastomeres give rise to the C1 and D1 daughter cells of the
32-cell-stage embryo. The A1, B1, C1, and D1 cells all contribute to
the organizer as well as giving rise to the anterior structures of the
embryo. The anterior (A) and posterior (P), formerly dorsal and
ventral, origins of embryonic structures are indicated (Lane and
Sheets, 2000; Lane and Smith, 1999). (B–D) Both CD1 blastomeres
of 16-cell-stage Xenopus embryos were injected with the mRNAs
indicated. Embryos were scored at the early tadpole stage for
defects in trunk and tail development. The morphology of uninjected
(B), HAVØ-FGFR-expressing (C), and DN-FGFR-expressing
(D) embryos at stage 31–35. Embryos expressing the DN-FGFR
exhibit severe posterior reductions and develop with an open
blastopore. Head development in these embryos was relatively
normal, the cement gland is marked with an asterisk (*). (E–G) One
AB1 and one CD1 blastomere of 16-cell-stage embryos were injected
with the mRNAs indicated. Muscle formation in uninjected
(E), HAVØ-FGFR-expressing (F), and DN-FGFR-expressing (G) embryos
at stage 31–35. All embryos were analyzed for somitic muscle
using wholemount immunocytochemistry and the 12/101 antibody,
shown in blue. Embryos were coinjected with the mRNA
encoding b-galactosidase; b-galactosidase activity is stained red.
The embryos expressing the DN-FGFR in the organizer cells
exhibited significant defects in muscle formation, ranging from no
muscle detected (the embryo on the left) to a .50% reduction in
muscle (embryo on the right).
FIG. 2. Trunk and muscle induction by the organizer required the FGFR pathway. Organizer tissues from embryos expressing
b-galactosidase and either the DN-FGFR or the HAVØ-FGFR were transplanted to the presumptive posterior region (ventral) of
unmanipulated host embryos. (A, B) Embryos receiving transplants were examined at the early tadpole stage for the presence of secondary
axes containing both an ectopic head and trunk. Ectopic heads were judged by the presence of at least one eye (red arrows) and a cement
gland (indicated by the asterisk). Ectopic trunk formation was assessed morphologically by the presence of a dorsal fin and significant tissue
separating the ectopic head from the trunk and head of the 1° axis. (A) Organizer grafts expressing HAVØ-FGFR induced complete secondary
axes containing trunks and heads with eyes (red arrows) and cement glands (asterisk). (B) DN-FGFR-expressing organizer grafts only induced
ectopic heads with cement glands and eyes; these tissues did not induce ectopic trunks. (C, D) Embryos receiving organizer grafts were
analyzed for somitic muscle induced by the grafted tissue using either immunocytochemistry (shown in blue) or by in situ hybridization
with a muscle actin probe (data not shown). Embryos were stained for b-galactosidase activity (shown in red) to detect the transplanted
organizer cells. (C) Organizer grafts expressing HAVØ-FGFR induced ectopic trunks containing muscle. Note the absence of
b-galactosidase-expressing cells in the ectopic muscle induced by the HAVØ-FGFR-expressing organizers (magnified image in Fig. 2C). (D)
DN-FGFR-expressing organizer grafts did not induce ectopic trunks containing muscle. There was no muscle surrounding the
b-galactosidase and DN-FGFR-expressing cells (magnified image in Fig. 2D). The results shown are representative of four separate
experiments.
FIG. 3. Inhibiting the FGFR pathway in the organizer cells
blocked formation of the pronephros. One AB1 and one CD1
blastomere of 16-cell-stage Xenopus embryos were injected with
the mRNAs indicated. All embryos were analyzed at stage 38â40
for the presence of pronephric ducts using whole-mount immunocytochemistry
and the 4A6 antibody, shown in blue (red arrow).
Pronephric duct formation in uninjected (A) and HAVÃ-FGFRexpressing
stage 38â40 embryos (B). Pronephric duct formation
was inhibited on the injected side of embryos expressing DN-FGFR
(C) but was normal on the contralateral side of the same embryo
(D). The cement gland is marked with an asterisk (*). The results
shown are representative of two separate experiments.
FIG. 4. The FGFR pathway regulated expression of the chordin
gene. Total RNA was isolated from stage-10.25 embryos injected
with mRNA encoding the DN-FGFR or the HAVÃ-FGFR. The
RNA was fractionated by denaturing agarose gel electrophoresis
and analyzed by blot hybridization using probes to mRNAs expressed
in the organizer cells. The results shown are representative
of two separate experiments. Both results for the analysis of noggin
expression are shown. The arrowhead marks the position of the
noggin mRNA. The asterisk (*) indicates the position of the
injected DN-FGFR or the HAVÃ-FGFR mRNAs that hybridize to
the noggin probe.
