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Fig. 8. Summary and interpretation. The left diagrams indicate the initial
condition; the right diagrams indicate how it evolves with time. (A) In
the caudal midbrain, FGF8 maintains expression of Lmx1b and
Wnt1 in a wide domain that is therefore competent to differentiate
into RP (Chizhikov and Millen,
2004b). However, FGF8 prevents RP maturation by inhibiting the
transcription of maturation markers or by inducing cell apoptosis. Conversely,
short-range homeogenetic mechanisms (arrows), mediated in part by GDF7, induce
RP extension. When the system equilibrates, a RP progress zone forms at short
distance from the FGF8 source. Our previous studies
(Louvi et al., 2003;
Alexandre and Wassef, 2003)
indicate that growth, but also convergent extension, may increase the distance
between the FGF8 source and the progress zone, leading to RP extension
posteriorly. Interestingly, although elsewhere in the neural tube RP
differentiation is confined to the competent neural folds, FGF8 releases RP
progression from the neural fold environment by inducing a RP competent
territory. (B) Lateral extension of the RP under the influence of a
FGF8-soaked bead depends on similar mechanisms. (C) The same
interactions could induce a short local RP segment near FGF8 beads. FGF8
signaling rapidly induces high levels of Lmx1b and Wnt1
before decreasing. Because Gdf7 expression is slowly induced by
Lmx1b and Wnt1, a patch of Gdf7-expressing cells
may be induced whose extension would follow the FGF8 decreasing gradient or
respond to convergent extension. (D) We find that changing the polarity
of dorsal midbrain fragments results in the maintenance of high levels of
Wnt1 and induction of Lmx1b expression in the transplants.
The transplant thus becomes competent for RP homeogenetic induction from the
adjacent host RP. Long-range DV regulations probably also contribute to RP
induction by reorganizing tissue polarity in the transplant and upregulating
dorsally the expression of competence factors. The asterisks indicate the
sites where homeogenetic RP induction was initiated.
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