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Fig. 10. Model of nodal regulation and dorsal-ventral axis specification
in the sea urchin embryo. (A) Integration of signaling and maternal
transcription factor inputs by cis-regulatory elements of the nodal
gene. (B) Starting at the 32-64 cell-stage, nodal expression
is initiated throughout most of the presumptive ectoderm by combinatorial
maternal inputs from p38 MAP kinase and maternal Univin, as well as from
signals emanating from the vegetal pole. Unidentified repressors prevent
expression of nodal and zygotic expression of univin in the
animal pole domain. These signals are transduced by maternal transcription
factors such as Smad, homeodomain, Oct, bZIP families and require SoxB1 and
TCF, resulting in a broad initial expression of nodal in the
ectoderm. Expression of nodal and zygotic expression of
univin at the vegetal pole is prevented by the
ß-catenin-mediated downregulation of SoxB1. (C) Starting at the
very early blastula stage, an endogenous ventral-dorsal redox gradient,
possibly related to an asymmetric distribution of mitochondria in the egg
and/or early embryo and acting on bZIP and other redox-sensitive transcription
factors, results in a slightly increased expression of nodal on the
presumptive ventral side, thereby reinforcing the Nodal autoregulatory loop.
This slight asymmetry in the expression of nodal is translated into a
corresponding asymmetry in the expression of Lefty, which starts
downregulating Nodal signaling on the presumptive dorsal side. (D)
During blastula stages, Nodal autoregulation and Lefty-mediated lateral
inhibition then establish a robust reaction diffusion system which results in
the sharp restriction of nodal expression to the ventral
territory.
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