Zone of polarizing activity

Zone of Polarizing Activity

The Zone of Polarizing Activity (ZPA) is a region located at the posterior margin of the limb bud that plays a crucial role in the early development of limbs in vertebrates. It is responsible for the anterior-posterior organization (thumb to little finger axis) of the limb.

Function

The ZPA functions by secreting a protein called Sonic hedgehog (Shh), which acts as a morphogen to control the growth and patterning of the limb. The concentration gradient of Shh across the limb bud determines the identity of the digits in a dose-dependent manner. High concentrations of Shh induce the formation of posterior digits, while lower concentrations are required for the formation of anterior digits.

Discovery

The ZPA was first identified in the 1960s by John Saunders and his student John Fallon. They discovered that grafting an additional ZPA to the anterior margin of a chick limb bud resulted in a mirror-image duplication of the digits.

Role in Developmental Disorders

Abnormalities in the ZPA or its signaling pathway can lead to limb malformations such as polydactyly (extra digits) and syndactyly (fused digits). Understanding the function of the ZPA and its signaling pathway can therefore provide insights into the causes of these developmental disorders.

See Also

• Limb development
• Morphogen
• Sonic hedgehog