Figure 2.

(A) Prior to limb bud outgrowth, the polarized and columnar somatopleural cells in the lateral plate mesoderm, shown in yellow, undergo an EMT transition followed by mesenchymal proliferation. (B) Simultaneously with this mesenchymal proliferation the limb bud outgrowth is formed through different possible mechanisms. (1) A proliferation gradient with higher proliferation rates towards the developing apical ectodermal ridge, (2) migration of mesenchymal cells from flanking regions into the limb field and (3) phase separation of the more fluidized limb cells from the flanking cells.