called the apical ectodermal ridge (AER). Research by Saunders and Gasseling in 1948 identified the AER and its subsequent involvement in proximal distal outgrowth. Twenty years later, the same group did transplantation studies in chick limb bud and identified the ZPA. In 1993, Todt and Fallon showed that the AER and ZPA are dependent on each other.

Patterning

Patterning along the limb bud requires signals from many sources. Specifically,

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