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What controls limb patterning?

Asked by Trilok 

Limb buds appear in the 4th week of gestation. Further limb development depends upon interplay of genes and factors. Bold letters used below can be used for mnemonic purpose. For grasping this concept, you need to visualize the limb in anatomic position:

a. Postero-anterior (PA) i.e. radio-ulnar development: ZPA (Zone of Polarizing Activity)
  • Expresses: SHH (Sonic Hedge Hog) gene
b. Elongation (proximal-distal development): AER (Apical Ectodermal Ridge)
  • Expresses: FGF (predominantly FGF 8) 
c. Palmar-Dorsal development: PZ (Progress zone)
  • Depends on WNT7a activation of Lmx1b
AER: Elongation (FGF)
Terrasigillata at English Wikipedia, CC BY-SA 3.0, via Wikimedia Commons