271mediated by Dact1 (Dishevelled-binding antagonist of beta-catenin 1; Suriben et al.
2009 ). Thus, whereas mice may exhibit a reduced role for cell movement- driven medio-
lateral intercalation during primitive streak formation (see above), a role for Wnt/PCP
components may have been retained at the level of EMT in the forming primitive streak.
Wnt/PCP signaling exerts its effects on intercalating cells by coordinately regu-
lating bipolar cell shape and polarization of actin dynamics to generate tensile
forces along the cell’s mediolateral axis. The mechanisms that orient this bilateral
protrusive activity perpendicular to the elongating anteroposterior axis are not well
understood. Experiments using dissociated Xenopus animal cap cells treated with
graded Activin doses or the recombination of anterior versus posterior notochord
explants showed that apposition of tissue from different axial levels was necessary
for convergent extension along a perpendicular axis, whereas recombination of
equivalent levels produced no elongation (Ninomiya et al. 2004 ). It is possible that
cells could respond to this gradient of Tgfb signaling through changes in cadherin-
and protocadherin-mediated cell adhesion during intercalation (Brieher and
Gumbiner 1994 ; Kraft et al. 2012 ). Similar results have been obtained by opposing
gradients of Nodal and BMP activity in zebrafish blastoderm explants (Xu et al.
2014 ), suggesting an important role for integration of the these pathways in control-
ling elongation. Temporal changes in Nodal signaling are also correlated with the
onset of mediolateral intercalation behavior, further suggesting that the local activity
of Wnt/PCP may be cued by differences in Tgfb activity.
Tgfb signaling, most likely through Nodal and Nodal-related proteins, is also
involved in inducing ingression behavior and bottle cell formation by activating
EMT. This can be shown in Xenopus by overexpression of Nodal in animal caps,
which results in ectopic bottle cell formation (Lustig et al. 1996 ; Agius et al. 2000 ;
Kurth and Hausen 2000 ). Also, loss of Nodal signaling dramatically reduces EMT-
like cell ingression in zebrafish embryos (Keller et al. 2008 ). Movement of the
hypoblast in amniotes, which expresses Nodal antagonists, is also highly correlated
with primitive streak formation in the epiblast formed by those cells left behind.
This Nodal-controlled ingression may also be self-regulating, as ingression may
facilitate recruitment of additional cells to the streak (see above), which would
expose more cells to Nodal signals, further triggering more EMT and ingression
(Voiculescu et al. 2014 ).
Signaling by FGFs also plays a key role in regulating convergent extension and
mediolateral intercalation. FGF signaling can regulate brachyury (t) expression,
which in turn can activate wnt11 (Tada and Smith 2000 ). In addition to its role in
morphogenesis, FGF signaling also has a well-known role in mesoderm formation
(Chap. 7 ). Experiments with cytoplasmic antagonists of FGF signaling, Sprouty/
Spry and Spred, indicate that maternal Sprys antagonize convergent extension
behavior by blocking PKC and calcium outputs of FGF signaling (Sivak et al. 2005 ).
Later during gastrulation, zygotic Spred proteins accumulate and antagonize Mapk
signaling, allowing the morphogenetic signals to predominate after medoderm
specification has occurred (Sivak et al. 2005 ). Additionally, FGF regulation of con-
vergent extension is controlled by the Nodal related 3.1 protein through an atypical
and little characterized mechanism (Yokota et al. 2003 ).
6 Vertebrate Axial Patterning: From Egg to Asymmetry