Fig. 9.

Challenging HMP with putative attractants. Lateral views of chick embryos 36 hours after surgery at HH18-19, rostral to the top, lateral to the right. Lbx1 expression is displayed in blue, quail-derived grafts in brown (A–C) and grafted cell pellets are marked with Celltracker Orange in red (D–F). (A) Rostrocaudal inversion of the occipital lateral mesoderm and ectoderm or (B) insertion of pharyngeal endoderm next to the caudal-most occipital somite or (C) insertion of mandibular arch mesenchyme plus overlying oral epithelium next to caudal-most occipital somites all do not divert TMP from their usual path (arrows). (D) Implanting RatB1 control cells next to the caudal-most occipital somite or elevating canonical Wnt signalling at this position by implanting cells secreting Wnt6 (E) or Wnt1 (F) also does not redirect HMP. (G, H) Implantation of Sdf1 loaded beads. Migratory limb muscle precursors at the caudal boundary of the limbs accumulated around the bead (G, arrowhead). (H) Sdf1-bead implantation next to the caudal-most occipital somites; HMP followed their normal path (arrow). (I, J) Implantation of Fgf8 loaded beads. When implanted at the interface of limb and flank level somites, the bead triggered the formation and emigration of migratory muscle precursors (I, arrowhead). (J) When implanted next to the caudal-most occipital somites, HMP were temporarily attracted by the bead (arrowheads) and then continued with their normal path (arrow). Thus, overriding forces prevent HMP deviation from their circumpharyngeal path. Abbreviations: FL, fore limb and occ, occipital lateral mesoderm.