Fig. 1. Mouse and chick facial branchiomotor neurones undergo different migratory pathways during development. (A,C,E,G) Ventral views of flat-mounted HH stage 23 to 24 chick hindbrains and (B,D,F,H) E11.0 to E11.5 mouse hindbrains. All the panels show the region of rhombomeres (r) 4 and 5 with the floor plate (fp) to the right (basal) and the r4 exit points to the left (alar). (A,B) Retrograde rhodamine-dextran labelling of facial branchiomotor (FBM) and visceromotor neurones (VMN) in chick (A) and mouse (B). (C-H) Expression patterns of neuronal markers in chick and mouse shown by in situ hybridisation. (C,D) Chick and mouse Hoxb1 label r4 progenitors along the dorsoventral axis. In mouse, an additional mouse Hoxb1-positive domain is detected in ventral r5 (arrow in D), whereas no equivalent expression of chick Hoxb1 is found in chick r5. The inset in D shows a transverse section of mouse Hoxb1 expression at the level of r4/r5. Note expression in the mantle layer lateral to the floor plate corresponding to migrating FBM neurones. (E) Chick Isl1 is expressed in chick in ventral r4 and r5, and in migrating FBM neurones within r4. (F) In mouse, a large stream of mouse Isl1-positive cells is present in ventral r4, r5 and rostral r6 (arrow in F). (G) Chick Phox2b is expressed at high levels in FBM and VMN neurones migrating laterally within r4 and r5, respectively. (H) In mouse, mouse Phox2b is expressed in ventral r4 and in the caudally migrating FBM population (arrow in H). G and H have an additional lateral Phox2b expression, which corresponds to the intermediate neural column expanding from r2 to r6.