How does Shh end up expressed only in the posterior bit of the limb bud What

How does shh end up expressed only in the posterior

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How does Shh end up expressed only in the posterior bit of the limb bud?What does it do once it is expressed?
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Feedback between the AER and the ZPA in the forelimb bud: induction of SHH Newly induced AER secretes FGF8 into underlying mesenchyme Posterior limb bud mesenchyme expresses hoxb-8; FGF-8 is coming from the AER; where they overlap – induction of Shh (only in posterior) ZPA is formed/induced; Shh maintains FGF8s’ expression in the AER and FGF8 and FGF4s maintain Shh expression… It’s more complicated: other proteins are necessary as well
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Nature Reviews Genetics 4, 133-143 (February 2003) , DOI 10.1007/978-3-662-43784-1 Limb outgrowth
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Digit formation : Shh-secreting cells form digits 4 and 5, and contribute to the specification of digits 2 and 3 in the mouse limb digit I is Shh independent; digit II possibly depends on Shh concentration (as gradient is formed); digit III depends on Shh concentration and the duration of Shh exposure; and digit IV and V depend on the duration of high levels of Shh signaling autocrine and paracrine exposure to Shh
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