The Journal of Neuroscience, October 15, 2000, 20(20):7664-7671
Facial Visceral Motor Neurons Display Specific Rhombomere Origin
and Axon Pathfinding Behavior in the Chick
John
Jacob and
Sarah
Guthrie
Medical Research Council Centre for Developmental Neurobiology,
King's College, Guy's Campus, London SE1 1UL, United Kingdom
In the chick embryo, facial motor neurons comprise branchiomotor
and visceral motor subpopulations, which innervate branchial muscles
and parasympathetic ganglia, respectively. Although facial motor
neurons are known to develop within hindbrain rhombomere 4 (r4) and r5,
the precise origins of branchiomotor and visceral motor neuron
subpopulations are unclear. We investigated the organization and axon
pathfinding of these motor neurons using axonal tracing and rhombomere
transplantation in quail-chick chimeras. Our results 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
that a striking property of facial visceral motor neurons is the
ability of their axons to navigate back to appropriate ganglionic
targets in the periphery after heterotopic transplantation. These
results complement previous studies in which heterotopic facial
branchiomotor neurons sent axons to their correct, branchial arch,
target. By contrast, when trigeminal branchiomotor neurons were
transplanted heterotopically, we found that they were unable to
pathfind correctly, and instead projected to an inappropriate target
region. Thus, facial and trigeminal motor neuron populations have
different axon pathfinding characteristics.
Key words:
facial nerve; branchiomotor neuron; visceral motor
neuron; rhombomere; hindbrain; axon pathfinding
Copyright © 2000 Society for Neuroscience 0270-6474/00/20207664-08$05.00/0