[1] The position of the limb bud, and hence the AER, is specified by the expression boundaries of Hox genes in the embryonic trunk.

[6][7] By E10, this thickening is more noticeable since the epithelium now consists of two layers and becomes confined to the ventral-distal margin of the bud although it is not detectable in living specimens using light microscope or by scanning electron microscopy (SEM).

[8] Between E10.5-11, a linear and compact AER with a polystratified epithelial structure (3-4 layers) has formed and positioned itself at the distal dorso-ventral boundary of the bud.

[6][8][9][10] After reaching its maximum height, the AER in mouse limb buds flattens and eventually become indistinguishable from the dorsal and ventral ectoderm.

[13] 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,[14] they exhibit significant differences, one of which is the AER maintenance.

[15] After the AER-AEF transition at 36 hours post fertilization, the AEF is located distal to the circumferential blood vessels of the fin bud.

Complex communication ensues as AER-secreted FGFs and ZPA-secreted Shh initiate and regulate Hox gene expression in the developing limb bud.

Though many of the finer details remain to be resolved, a number of significant connections between Hox gene expression and the impact on limb development have been discovered.

This means that although Shh signaling is required, its effects change over time as the mesoderm is primed to respond to it differently.

These three phases of regulation reveal a mechanism by which natural selection can independently modify each of the three limb segments – the stylopod, the zeugopod, and the autopod.

Implantation of sufficient beads can induce formation of a 'normal' additional limb at an arbitrary location in the embryo.

In contrast, transplantation of cells from the progress zone of a developing arm to replace the progress zone of a developing leg will produce a limb with leg structures proximally (femur, knee) and arm structures distally (hand, fingers).