Lecture 15 Vertebrate limb development

Anterior

Posterior

Dorsal

Ventral

4 because they develop with bilateral symmetry

Somites

The limb buds grow out into the surrounding skin, thickening the skin to form the AER which is important for future patterning of the limb bud.

2 T-box TF

• Tbx5 is only expressed in the forelimb buds to specify wing identity • Pitx1 and Tbx4 are only expressed in the hindlimb buds to specify leg identity • Tbx5 represses Tbx4 which is why Tbx4 isn’t expressed in the forelimb bud • Pitx1, which is restricted to the P limb bud, induces expression of Tbx4

• Early in embryogenesis there is a gradient of RA set up from P  A • This sets up the netted expression of different TF i.e. Hox genes that specify the AP axis • Hox genes feedback to give a new local zones of RA in these regions • RA in combination with the Hox code specifies the expression of different Tbx TF that generate limb identity

Zones of FGF expression

Proliferation

In the lateral plate mesoderm (outside of the paraxial mesoderm, next to somites 18-20

The hindlimb is also generated from lateral plate mesoderm, but in somites 26-28

• A bead soaked in FGF, mimics the action of the limb field to induce ectopic development of limbs i.e. it resulted in a 3rd limb, showing the outgrowth of a limb in a responsive field is promoted by FGF.

• There is an initial burst of FGF8 in intermediate mesoderm. • Then, Tbx TF (4/5) have be induced. This induces TF that act cell-autonomously on cells to upregulate the FGF10 encoding gene. • The entire lateral plate mesoderm, that sits under the surface ectoderm, expresses and translates FGF10 in the mesoderm. • FGF10 is a secreted molecule that acts on adjacent AER to induce these cells to upregulate a 2nd TF called FGF8. • Domains of FGF10 expression are restricted along AP by Wnt2b. • Wnt3a is induced in the limb field • FGF8 acts on the progress zone

This is a reciprocal signalling interaction as FGF10 signals to surface ectoderm so it starts to upregulate FGF8 that signals back to lateral plate mesoderm. This slowly restricts cells and gives more specification over time.

• Made up primarily of mesenchymal cells - Expresses FGF10

• Expresses FGF8

• Expresses Shh (morphogen) - Found posterior of the limb bud

Suggests the AER is driving outgrowth of the limb AER then acts on the progress zone. The longer the progenitor cell sits as a proliferating cell in the PZ, the more distal the fate. The TIMING of when a cell differentiates is key to its fate

Initial progenitors are present for all the skeletal derivatives. Under influence of Hox code a domain of RA forms in flank where the limb bud originates. Over time FGF8 is induced in the AER. Sets up 2 opposing gradients of the 2 antagonistic signals (RA/FGF8). Cells encountering increased RA become more proximal.

Flies

Distalless