Apical ectodermal ridge

Apical Ectodermal Ridge

Diagram of a limb bud showing the apical ectodermal ridge (AER) at the distal tip.

The apical ectodermal ridge (AER) is a critical structure in the embryonic development of vertebrate limbs. It is a thickened area of ectoderm at the distal end of the limb bud and plays a crucial role in the outgrowth and patterning of the limb.

Structure

The AER is a specialized epithelial structure that forms at the boundary between the dorsal and ventral ectoderm of the limb bud. It is characterized by a ridge of columnar epithelial cells that express specific signaling molecules essential for limb development.

Function

The primary function of the AER is to maintain the underlying mesenchyme in a proliferative state, allowing for the continued growth of the limb bud. It achieves this by secreting fibroblast growth factors (FGFs), which are crucial for the proliferation and survival of mesenchymal cells.

Signaling

The AER interacts with the underlying mesenchyme through a complex network of signaling pathways. The FGFs produced by the AER activate receptors in the mesenchyme, promoting cell division and preventing apoptosis. This interaction is essential for the proper patterning and elongation of the limb.

Developmental Role

During limb development, the AER is responsible for the proximal-distal axis formation. It ensures that the limb grows in the correct orientation and that the various segments of the limb, such as the humerus, radius, and ulna, develop in the correct sequence.

Clinical Significance

Disruptions in AER function can lead to congenital limb malformations. For example, the absence or malfunction of the AER can result in conditions such as amelia (absence of limbs) or phocomelia (shortened limbs).

Related Pages

• Limb development
• Ectoderm
• Mesenchyme
• Fibroblast growth factor