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Fig. 1. Background on midbrain roof plate properties: schematic representation
of previous experiments that disclosed unexpected properties of the RP near
the isthmic organizer (IsO). (A-C) For simplicity, although the
midbrain RP is not yet differentiated from the neural folds at stage HH10, the
experimental manipulation performed at stage HH10 and its outcome on RP
patterning 1 or 2 days later are both represented on the same outline of a
stage HH10 chick neural tube. (A) Cells from a restricted isthmic domain (red
circle isthmic node, Isnode) extend to populate the midline on both sides of
the MH junction, marked by an arrow
(Millet et al., 1996;
Louvi et al., 2003;
Alexandre and Wassef, 2003).
(B) Interfering with local patterning at the Isnode prevents RP formation on
the adjacent midline (Alexandre and Wassef,
2003). (C) A lateral source of FGF8 created by insertion of a
FGF8-soaked bead or of an IsO transplant induces RP duplication/bifurcation
(Bally-Cuif and Wassef, 1994;
Crossley et al., 1996).
(D) The RP fails to develop on the dorsal midline of midbrain vesicles
whose AP axis has been inverted by a 180° rotation
(Marin and Puelles, 1994;
Alexandre and Wassef,
2003).
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