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Reference.org
Apical ectodermal ridge
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<p>The apical ectodermal ridge (AER) is a structure that forms from the <a href="/facts/Ectoderm/lg5p0jTL">ectodermal</a> cells at the distal end of each <a href="/facts/Limb_bud/HzU60WeN">limb bud</a> and acts as a major signaling center to ensure proper development of a limb. After the limb bud induces AER formation, the AER and limb <a href="/facts/Mesenchyme/3QDGRD1C">mesenchyme</a>—including the <a href="/facts/Zone_of_polarizing_activity/IaPgTGJM">zone of polarizing activity</a> (ZPA)—continue to communicate with each other to direct further <a href="/facts/Limb_development/vl63Sgff">limb development</a>. </p><p>The position of the limb bud, and hence the AER, is specified by the expression boundaries of <a href="/facts/Hox_gene/kP1yVyDY">Hox genes</a> in the embryonic trunk. At these positions, the induction of cell outgrowth is thought to be mediated by a <a href="/facts/Positive_feedback/eFGatCpF">positive feedback</a> loop of <a href="/facts/Fibroblast_growth_factor/eC48LP3g">fibroblast growth factors</a> (FGFs) between the <a href="/facts/Intermediate_mesoderm/e7WkJRp8">intermediate mesoderm</a>, the <a href="/facts/Lateral_plate_mesoderm/6p6NkUuW">lateral plate mesoderm</a> and the <a href="/facts/Surface_ectoderm/TT0zdd6m">surface ectoderm</a>. <a href="/facts/FGF8/ufaC4qhS">FGF8</a> in the intermediate mesoderm signals to the lateral mesoderm, restricting the expression of <a href="/facts/FGF10/5kGoGsRE">FGF10</a> through intermediate <a href="/facts/Wnt_signaling_pathway/7wxpKrjy">Wnt</a> signals. Then, FGF10 in the lateral plate mesoderm signals to the surface ectoderm to create the AER, which expresses FGF8. </p><p>The AER is known to express <a href="/facts/FGF2/5CDdKMbk">FGF2</a>, <a href="/facts/FGF4/DFg3nVtj">FGF4</a>, <a href="/facts/FGF8/ufaC4qhS">FGF8</a>, and <a href="/facts/FGF9/PSE4JtKJ">FGF9</a>, while the limb bud mesenchyme expresses <a href="/facts/FGF2/5CDdKMbk">FGF2</a> and <a href="/facts/FGF10/5kGoGsRE">FGF10</a>. Embryo manipulation experiments have shown that some of these FGFs alone are sufficient for mimicking the AER. </p>