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Fig. 4. Anterior neural patterning by Nodal signaling and antagonism in the
mouse embryo. Blue shading indicates regions expressing Nodal
and/or Gdf3; red shading indicates regions expressing the Nodal
antagonists Lefty1 and Cer1. Shortly after implantation,
Nodal is expressed throughout the epiblast [5.25 days post-coitum
(dpc)], and induces formation of the anterior visceral endoderm (AVE; red) at
the distal end of the egg cylinder at 5.5 dpc; note that the initial
appearance of the AVE is already slightly asymmetric, with a bias towards the
prospective anterior side (Yamamoto et
al., 2004). Nodal signaling is also required for the movement of
the AVE (purple arrow) to the anterior side (5.75 dpc), where the expression
of Nodal antagonists (Lefty1, Cer1) by the AVE is essential for the
specification of anterior neural identity in the adjacent epiblast.
Conversely, Nodal signaling is required for the generation of axial
mesendoderm (orange) by the anterior primitive streak during gastrulation (7.5
dpc); in turn, the axial mesendoderm produces signaling factors (black arrows)
that are essential for forebrain maintenance and ventral neural tube
patterning.
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