Shh may be a critical signal regulating ZPA function, but the genes involved in Shh signaling are under the control of several other factors that are needed for ZPA maintenance and function including Hand2 and Hoxb-8.
 Retinoic acid, an important signaling molecule needed throughout embryogenesis, acts through the Hox genes.
 It was originally postulated that retinoic acid acts to induce the Hoxb-8 gene, but this hypothesis has not been supported by genetic studies in mouse embryos lacking retinoic acid synthesis that still express Hoxb-8 in the limb.
Hoxb-8 signaling is active in the early embryo, beginning at the posterior end of the lateral plate mesoderm and extending to the anterior region.
 As Hoxb-8 spreads to more anterior regions, Shh is induced in the area that will become the ZPA.
 Shh is only induced in the anterior region because of signals from the AER.
 Experiments done by Heikinheimo et al.
show that when the AER is removed, beads that express FGF are sufficient to induce Shh signaling in the ZPA.
Thus, the likely signaling factor from the AER is FGF.

Additionally when the AER is removed, Shh is no longer expressed, and the ZPA can no longer be maintained.
 Acting in a positive feedback mechanism, FGF-4 is expressed near the ZPA.
FGF-4 acts to maintain Shh expression, while Shh acts to maintain FGF-4 expression.
 At the same time, Wnt-7a is expressed in the dorsal ectoderm, and provides further positive feedback to FGF-4 and Shh.
Without  this system, limbs and digits are either significantly reduced or missing.
Shh may be a critical signal regulating ZPA function, but the genes involved in Shh signaling are under the control of several other factors that are needed for ZPA maintenance and function including Hand2 and Hoxb-8.  Retinoic acid, an important signaling molecule needed throughout embryogenesis, acts through the Hox genes.  It was originally postulated that retinoic acid acts to induce the Hoxb-8 gene, but this hypothesis has not been supported by genetic studies in mouse embryos lacking retinoic acid synthesis that still express Hoxb-8 in the limb. Hoxb-8 signaling is active in the early embryo, beginning at the posterior end of the lateral plate mesoderm and extending to the anterior region.  As Hoxb-8 spreads to more anterior regions, Shh is induced in the area that will become the ZPA.  Shh is only induced in the anterior region because of signals from the AER.  Experiments done by Heikinheimo et al. show that when the AER is removed, beads that express FGF are sufficient to induce Shh signaling in the ZPA. Thus, the likely signaling factor from the AER is FGF.
Additionally when the AER is removed, Shh is no longer expressed, and the ZPA can no longer be maintained.  Acting in a positive feedback mechanism, FGF-4 is expressed near the ZPA. FGF-4 acts to maintain Shh expression, while Shh acts to maintain FGF-4 expression.  At the same time, Wnt-7a is expressed in the dorsal ectoderm, and provides further positive feedback to FGF-4 and Shh. Without  this system, limbs and digits are either significantly reduced or missing.
Zone of polarizing activity