In addition to wings in chicks and forelimbs in mice, pectoral fins in zebrafish serve as a model to study vertebrate limb formation. Despite fin and limb developmental processes share many similarities, they exhibit significant differences, one of which is the AER maintenance. While in birds and mammals the limb AER persists until the end of digit-patterning stage and eventually regresses, the fin AER transforms into an extended structure, named the apical ectodermal fold (AEF). After the AER-AEF transition at 36 hours post fertilization, the AEF is located distal to the circumferential blood vessels of the fin bud. The AEF potentially functions as an inhibitor to fin outgrowth since removing the AEF results in the formation of a new AER and subsequently a new AEF. In addition, repeated AF removal leads to excessive elongation of the fin mesenchyme, potentially because of prolonged exposure of AER signals to the fin mesenchyme. Recently, the AER, which has long been thought to consist of only ectodermal cells, in fact composes of both mesodermal and ectodermal cells in zebrafish.