Fig. 7. RA action during presomite stages in anterior regions. (A) At early gastrula stages, RA signals from the anterior endoderm (ae) and node (n) to specify axial mesodermal cells emerging from the node (arrows 1 and 2). (B) During the stabilization step RA signals from the prechordal mesendoderm (pme) to restrict the extent of posterior pme (arrow 3); during this stage other signalling pathways pattern the overlying forebrain primordium (red arrows). (C) In the absence of RA, pme is extended posteriorly (yellow arrow) and altered signals are transferred to the overlying neuroectoderm (red dashed arrows). ae, anterior endoderm; pme, prechordal mesendoderm; a-me, axial mesoderm; pame, paraxial mesoderm; anp, anterior neural plate; pop, posterior neural plate; n, node.

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