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. 2016 May 19;4(2):18. doi: 10.3390/jdb4020018

Figure 3.

Figure 3

Schematic model for the acquisition of forelimb-forming fields during evolution. Light blue, orange and green bars represent the pharyngeal mesoderm (ph), ventral mesoderm posterior to the pharynx (vmp)/ALPM and PLPM, respectively (see Figure 2 for details). The ventral mesoderm (VM)/LPM is regionalized into the ALPM and PLPM in lampreys and gnathostomes, but not in amphioxus, and retinoic acid signaling seems to be involved in this process. Nested expression of Hox genes (pink bars) is observed in lampreys and gnathostomes, but not in amphioxus. Blue regions in the embryos are expression domains of Tbx5 orthologs (BlTbx4/5, LjTbx4/5, Tbx5) in the VM/LPM. In amphioxus, BlTbx4/5 (blue) appears in both ph and vmp, similar to cardiac markers (AmphiNkx2.5-tin/AmphiTbx20; see Figure 2). In lampreys, LjTbx4/5 expression (blue) is expressed in the ALPM; it is not expressed in the unseparated PLPM. In gnathostomes, retinoic acid signaling, as well as Hox proteins, directly activates expression of Tbx5 (blue) in the somatic layer of the PLPM to induce forelimb formation [11,49,50]. See text for additional references and details.