345A series of loss-of-function experiments identified Fgf4 and Fgf8 as endogenous
mesoderm inducing signals in frogs and fish. Morpholino knockdown experiments
in the frog showed that fgf4 and fgf8b, an alternatively spiced isoform of Fgf8, are
both required for to specify mesodermal cell fates (Fisher et al. 2002 ; Fletcher et al.
2006 ). At high doses, the fgf8b morpholino phenocopies embryos expressing the
dominant negative Fgf receptor (Amaya et al. 1991 ). Interestingly, the Fgf8a iso-
form differs from Fgf8b by only 11 amino acids is necessary and sufficient to induce
neural fates, but lacks the ability to induce mesoderm (Fletcher et al. 2006 ). In
zebrafish, fgf8 single mutants have defects in the somites associated with later roles
in mesoderm differentiation (Reifers et al. 1998 ). When fgf24 function is reduced in
fgf8 mutants, the embryos lack all posterior trunk mesoderm (Draper et al. 2003 ).
This indicates that Fgf8 and Fgf24 function redundantly to specify posterior/ventral
mesoderm in zebrafish.
Convergence-extension movements are blocked in frogs and fish when Fgf sig-
naling is inhibited (Amaya et al. 1991 ; Griffin et al. 1995 ; Nutt et al. 2001 ).
Expression of the Fgf antagonist, Sprouty2 inhibits the movements of gastrulation
without affecting the specification of mesoderm and endoderm (Nutt et al. 2001 ).
This suggests that the role of Fgf signaling in controlling cell movements is inde-
pendent of its role inducing mesoderm.
7.6.5 Fibroblast Growth Factor in Amniotes
In chicken, fgf2, -3, -4, -8, -12, -13, and -18 are all expressed in the primitive streak
with slightly different spatial and temporal dynamics, while fgf8 is initially expressed
broadly in the primitive streak before being excluded from the node and anterior
streak at later stages (Shamim and Mason 1999 ; Lawson et al. 2001 ). fgf8 is also
expressed in Koller’s sickle before the primitive streak is apparent (Lawson et al.
2001 ). fgf4 and fgf8 label distinct subpopulations of cells within the streak (Karabagli
et al. 2002 ). Cells close to the midline of the streak express fgf4, while cells express
fgf8 as they leave the streak and migrate laterally. Misexpression of Fgf8b or Fgf4
induces a secondary primitive streak that contains both mesoderm and a node
(Bertocchini et al. 2004 ). Injection of Fgf4 into the subgerminal space induces all
marginal cells to adopt mesodermal fates (Alev et al. 2013 ). In addition, mesoderm
does not form when Fgf signaling is blocked, and Fgf antagonists prevent the ability
of cVg1 to induce an ectopic body axis (Mitrani et al. 1990a; Bertocchini et al.
2004 ). This indicates that Fgf is necessary and sufficient to induce mesoderm in the
chicken. Fgf4 can also induce mesoderm when overexpressed in turtle embryos,
suggesting that this function is conserved in reptiles (Alev et al. 2013 ).
Fgf signaling also controls the polonaise movements during gastrulation by che-
motaxis (Yang et al. 2002 ; Hardy et al. 2011 ). A fluorescent assay to detect cell
migration revealed that cells within the epiblast were attracted towards a bead
soaked with Fgf4 protein (Yang et al. 2002 ). Cells were not attracted to beads soaked
with Fgf2 and were repelled by beads soaked with Fgf8. This suggests that Fgf4 acts
as a chemoattractant drawing cells in the epiblast toward the primitive streak where
7 Establishment of the Vertebrate Germ Layers