The apical ectodermal ridge (AER) is a specialized structure located at the tip of developing limb buds in vertebrate embryos. It plays a crucial role in limb development by signaling to the underlying mesoderm, promoting the growth and patterning of the limb. The AER produces growth factors that help regulate the outgrowth of the limb and the formation of digits. The AER is essential for proper limb formation, as it influences the differentiation of cells in the mesoderm beneath it. Disruption of the AER can lead to limb malformations, highlighting its importance in embryonic development. Understanding the AER contributes to the broader field of developmental biology.