PATHFINDING BY CRANIAL NERVE-VII (FACIAL) MOTOR-NEURONS IN THE CHICK HINDBRAIN

Cranial nerve VII (facial) motorneurons begin extending axons through rhombomeres 4 and 5 (R4 and R5) in the chick hindbrain on the second d ay of incubation. Without crossing the midline, facial motorneuron axo ns extend laterally from a ventromedial cell body location. All facial motorneuron axons leave the hindbrain through a discrete exit site in R4. To examine the importance of the exit site in R4 on motorneuron p athfinding, we ablated R4 before motorneuron axonogenesis. We find tha t mechanisms intrinsic to R5 direct the initial lateral orientation of R5 motorneuron axons. Upon reaching a particular lateral position, al l R5 motorneuron axons must turn. In normal embryos the axons all turn rostrally to reach the nerve exit in R4. In embryos with R4 ablated, sometimes the axons turn rostrally and sometimes they turn caudally. A model combining permissive fields and chemotropic cues is presented t o account for our observations.