Apical Ectodermal Ridge (AER)
We have described earlier that the AER persists at the tip until the last phalangeal cartilage begins differentiation. The following experiments have demonstrated that the AER is formed and then maintained under the influence of limb mesoderm:
1) When limb mesoderm is grafted under the ectoderm of any region of an early embryo it induces the formation of an AER in the overlying ectoderm.
2) When limb bud mesoderm is removed or separated from the AER by a barrier (e.g. a thin sheet of mica) or is replaced by non-limb mesoderm the AER flattens and disappears. It is believed that limb mesoderm secretes some substance called AER Maintenance Factor (AER MF) that sustains the integrity of the AER.