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Fig. 7. Induction of floor-plate character in `area a' cells through a cooperation
between Shh and Nodal signalling. (A-F) Shh and Nodal are
co-expressed in the HH stage 4+ prechordal mesoderm as it migrates underneath
`area a'. Sections of HH stage 4 (A,B), 4+ (C,D) and 5 (E,F) embryos processed
for mRNA in situ hybridisation with probes against Shh (A,C,E) or the
chick Nodal gene Nr1 (B,D,E). The axial level of the sections, just
anterior to Hensen's node, is indicated in the schematics to the left of each
pair of images. At HH stage 4, no expression of either Shh (A) or
Nodal (B) is observed in `area a' or the underlying tissue.
Approximately 1 hour later, at HH stage 4+, Shh (C) and
Nodal (D) are co-expressed in the nascent prechordal mesoderm lying
underneath `area a' (white arrowheads in C and D). At HH stage 5, after the
nascent prechordal mesoderm has passed underneath `area a', no expression of
either Shh or Nodal is seen in the axial mesoderm (white
arrowheads in E,F) while `area a'-derived cells of the neural midline now
exhibit strong expression of Shh (black arrowhead in E). (G-Z) Nodal
potentiates Shh signalling to induce floor plate character in `area a'
explants in vitro. (G,L,Q,V) Exposure to 2 nM Shh induces expression of Shh,
Netrin1 and HNF3ß in `area a' explants. (H,M,R,W) Exposure to
0.2 nM Shh is insufficient to induce floor-plate markers. (I,N,S,X) Exposure
to Nodal alone can result in a very weak induction of ventral midline markers
(I,N and inset in S), although sometimes elicits no ventral midline
differentiation (S,X). (J,O,T,Y) Exposure of explants to a combination of 0.2
nM Shh and Nodal results in strong induction of ventral midline markers.
(K,P,U,Z) Control medium does not elicit ventral midline differentiation.
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