Tetrapod Limb Development• Revisiting Axis specification• Limb Formation and Growth• Digit Formation

The tetrapod limb

stylopod

zeugopod

autopod

Limb Development in 4 dimensions• Proximal-Distal axis• Anterior Posterior axis• Dorsal-Lateral axis• Time

Limb bud Specification• Limb bud fate mapping

– Removal– Transplantation– Cell/Tissue tracking

• Limb Field• Limb Bud formation• Hox genes: Hox 6 and

Forelimb

Limb Bud: Lateral plate mesoderm

Multilimbs arise from dividing the bud

Induction of limb bud formation• Fgf8• Wnt2b/8c and Fgf10

Induction of Limb Bud

Forelimb or Hindlimb?• Tbx transcription factors: Tbx5 and Tbx4

Genetic Regulation of early limb formation

Hox6

Tbx5

Fgf10

For Forelimb

Proximal-Distal Axis: Anterior ectodermal ridge

AER and Limb bud development• AER major signaling center

for forming Limb.– Ensures plasticity of

mesenchyme– Maintains factors that

determine A-P axis– Contacts A-P/D-V axis

specifiers in order to inform cells how to differentiate

• AER at distal margin of forming limb bud.

• Makes direct contact with mesenchyme underneath (Progress zone-PZ).

AER and Limb development

Role of PZ in Limb growth/specification

Limb Growth

Reaction-diffusion model for proximal-distal limb specification

A-P axis: Sonic Hedgehog• Zone of polarizing activity (ZPA)• Shh expressed in ZPA

A-P axis: Sonic Hedgehog• Zone of polarizing activity (ZPA)• Shh expressed in ZPA

Polydactyly: Shh overexpression?

Digit formation

D-V axis: third and final axis• Wnt7a• Lim1-dorsal specification

regulator

Coordination of the 3 axes

Sculpting the hand• BMPs initiate cell death in interdigital, interior, and anterior-

posterior necrotic zones.

Joint Formation

Fgfs and dwarfism