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Fig. 7. Models for the interactions between dHand, Gli3 and Tbx3,
in the limb-forming region at stages prior to Shh expression, that
refine limb position along the rostrocaudal axis of the embryo. (A) At stage
16, Tbx3 is expressed throughout the limb-forming region but
expression is more robust in the future posterior limb mesenchyme.
dHand is expressed throughout the limb-forming region. (B) At stage
17, Gli3 is expressed throughout the limb mesenchyme with higher
levels in the anterior. dHand expression is restricted to the
posterior. (C) Ectopic expression of Tbx3 in more anterior regions
leads to a repression of Gli3, resulting in a de-repression of
dHand and expansion of its expression domain. (D) A model
illustrating how the genetic antagonism between dHand and
Gli3 that positions the future ZPA may be mediated through
Tbx3. – and + symbols indicate, respectively, the
transcriptional repressor and activator function of each protein. In addition,
in the posterior limb, Tbx3 and dHand cooperate to induce
Shh expression and thereby specify the position of the ZPA in the
posterior limb mesenchyme. A, anterior; P, posterior.
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