Figure 1

Segmentation in vertebrates is governed by divergent molecular mechanisms: somites, rhombomeres and pharyngeal arches. (A) A whole mount phalloidin staining of a stage 11 chick embryo viewed dorsally. The somites are readily visible as iterated blocks on either side of the neural tube. (B) An image of a dissected hindbrain region of chick embryo at 3 days of development. The roofplate has been removed to show the rhombomeres. (C) A longitudinal section through the pharyngeal region of a dogfish embryo. The embryo has been stained with an anti-laminin antibody. The pharyngeal segments are clearly shown. (D) Schematic diagram of somitogenesis. As the embryonic axis elongates, the presomitic mesoderm is patterned by opposing gradients of FGF8/Wnt activity (high posterior-low anterior) and retinoic acid (RA; low posterior-high anterior). Cycling expression of Notch pathway components (NICD = Notch Intracellular Domain) and of the transcription factor Mesp2 acts as a pacemaker in this process. (E) Diagram of rhombomere (r) formation. R identity is regulated by a posteriorly increasing gradient of RA and by FGF8 signalling from the midbrain-hindbrain boundary. R boundary formation involves Notch activation (NICD) and they are maintained through cell sorting driven by alternating expression of EphA4 (in r3/5) and ephrin ligands (in r2, 4, 6). (F) The formation of all pharyngeal arches depends on FGF signalling, that of the posterior arches also on Wnt and RA signalling and on the transcription factor Tbx1. In all panels anterior is to the top.