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Fig. 8. A model of consecutive step of the neural crest specification.
(A) A proposed model of sequential steps of neural crest specification.
(Phase I) During gastrulation, midline BMP antagonists inhibit BMP4
expression and specify the neural plate (orange). After this, X-TSK
expression is repressed in the presumptive neural plate (yellow), while it is
maintained in the lateral ectoderm by BMP4 signaling. (Phase II) After stage
12.5, X-TSK expression is upregulated in the presumptive neural crest
region, which has intermediate levels of BMP signaling. (siRNA) In X-TSK
depleted embryo, BMP signaling levels (green) are increased in the TSK
expressing-region. Under these conditions, epidermal fates are expanded up to
the neural plate margin (blue), while the neural crest region (yellow) is
repressed. (B) Molecular network of neural crest specification. X-TSK
inhibits BMP4 (red asterisk) and modulates Notch signaling via direct binding
to X-delta-1 (purple asterisk). Notch signaling can regulate the expression of
BMP4, and BMP and Notch signaling interact to control the expression
of Hairy2A and Msx1. Timing of Notch activation alters its
effect on BMP4 expression (see
Glavic et al., 2004) (black
asterisk). Finally, caudalizing signals such as XWnt-8 converge on
this network downstream of Msx1 to control the activation of
Pax3 expression and neural crest cell specification within the
Msx1-expressing region. The interactions indicated by `1' and `2'
have been reported previously (Glavic et
al., 2004; Monsoro-Burq et
al., 2005).
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