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they ingress, while Fgf8 acts as a chemorepellant, driving cells away after they exit
the streak. Cell movements are blocked when the Fgf pathway is pharmacologically
inhibited (Hardy et al. 2011 ). Since Fgf4 can induce mesoderm even in the absence
of polonaise movements, these results indicate that Fgf signals have two indepen-
dent roles during gastrulation. They direct the movements of cells into and out of the
streak and they specify cells to adopt mesodermal fates.
Similar results were obtained in the mouse. Fgf2 can induce cultured ectodermal
cells to become mesoderm (Burdsal et al. 1998 ). Paraxial mesoderm fails to form in
fgfr-1−/− mutants, and cells accumulate in the primitive streak due to defects in
EMT, resulting in an expansion of axial mesoderm (Deng et al. 1994 ; Yamaguchi
et al. 1994 ; Ciruna and Rossant 2001 ). The failure of EMT is caused by persistent
E-cadherin expression in fgfr-1−/− mutants. In chimeras, fgfr-1−/− mutant cells dis-
play defective movements within a wild type primitive streak and an inability to
delaminate from the epiblast (Ciruna et al. 1997 ). This indicates that Fgf signaling
is required for the specification of mesodermal fates and for the migration of pre-
sumptive mesoderm and endodermal cells into the primitive streak in the mouse, as
it is in the chicken. These roles are also conserved in teleosts, since medaka hdf
mutants, which lack both the maternal and zygotic function of fgfr1, display defects
in the migration of axial mesoderm during gastrulation in addition to the lack of
trunk and tail mesoderm (Shimada et al. 2008 ).
Both fgf4 and fgf8 are expressed in the primitive streak (Niswander and Martin
1992 ; Crossley and Martin 1995 ). fgf4 null mutant embryos had severe proliferation
defects and failed to develop post-implantation, whereas fgf8 mutants die after gas-
trulation, failing to form mesoderm (Feldman et al. 1995 ; Meyers et al. 1998 ). The
early phase of fgf4 expression is expressed normally in fgf8−/− mutants, but it is lost
in the primitive streak during gastrulation (Sun et al. 1999b). In these embryos, cells
move into the primitive streak and ingress, but fail to exit the streak or become
mesoderm. This indicates that Fgf8 and/or Fgf4 are required both for mesoderm
formation and to direct the cell movements of gastrulation into and out of the primi-
tive streak. A role for Fgf4 in mesoderm formation and epiblast migration has not
been firmly established in the mouse, however, because its early requirement in the
blastocyst complicates analysis of its later functions.
7.6.6 Conclusion
These studies showed that the molecular mechanism of germ layer formation is
highly conserved in vertebrates despite the dramatically differently morphologies of
the blastula and gastrula stage embryos. Experiments in frogs, fish, chicken, and
mice all converged to identify activation of the Activin/TGF-β and RTK pathways
as important steps in vertebrate germ layer formation. In all vertebrates, members of
the Nodal-related gene family are expressed in the presumptive mesoderm and
endoderm. These factors can induce mesoderm and endoderm in a dosage depen-
dent manner when overexpressed, and are required in all vertebrates for formation
W. Tseng et al.