Development of the Tetrapod Limb

Limb Anatomy

The Limb Bud

Hox Gene Specification of Limb Skeleton Identity

  • From proximal to distal: Hox genes in the limb

Determining What Kind of Limb to Form and Where to Put It

  • Specifying the limb fields
  • Induction of the early limb bud

Outgrowth: Generating the Proximal-Distal Axis of the Limb

  • The apical ectodermal ridge
  • Specifying the limb mesoderm: Determining the proximal-distal polarity
  • Turing’s model: A reaction-diffusion mechanism of proximal-distal limb development

Specifying the Anterior-Posterior Axis

  • Sonic hedgehog defines a zone of polarizing activity
  • Specifying digit identity by Sonic hedgehog
  • Sonic hedgehog and FGFs: Another positive feedback loop
  • Hox genes are part of the regulatory network specifying digit identity

Generating the Dorsal-Ventral Axis

Cell Death and the Formation of Digits and Joints

  • Sculpting the autopod
  • Forming the joints

Evolution by Altering Limb Signaling Centers