Facial visceral motor neurons display specific rhombomere origin and axon pathfinding behavior in the chick

In the chick embryo, facial motor neurons comprise branchiomotor and viscer al motor subpopulations, which innervate branchial muscles and parasympathe tic ganglia, respectively. Although facial motor neurons are known to devel op within hindbrain rhombomere 4 (r4) and r5, the precise origins of branch iomotor and visceral motor neuron subpopulations are unclear. We investigat ed the organization and axon pathfinding of these motor neurons using axona l tracing and rhombomere transplantation in quail-chick chimeras. Our resul ts show that a large majority of branchiomotor neurons originate in r4 but that a cohort of these neurons undergoes a caudal migration from r4 into r5 . By contrast, visceral motor neurons develop exclusively in r5. We found t hat a striking property of facial visceral motor neurons is the ability of their axons to navigate back to appropriate ganglionic targets in the perip hery after heterotopic transplantation. These results complement previous s tudies in which heterotopic facial branchiomotor neurons sent axons to thei r correct, branchial arch, target. By contrast, when trigeminal branchiomot or neurons were transplanted heterotopically, we found that they were unabl e to pathfind correctly, and instead projected to an inappropriate target r egion. Thus, facial and trigeminal motor neuron populations have different axon pathfinding characteristics.