RHOMBOMERE-SPECIFIC ORIGIN OF BRANCHIAL AND VISCERAL MOTONEURONS OF THE FACIAL-NERVE IN THE RAT EMBRYO

The goal of this study was to localize selectively the facial nerve br anchial and visceral motoneurons in the rat embryo hindbrain. This was achieved by injecting dextran amines into the peripheral facial nerve on embryos maintained in an artificial cerebrospinal fluid. Sprague-D awley rat embryos 13, 14, and 15 days old (E13, E14, E15) were obtaine d by cesarean section. Branchial motoneurons were first labeled at E13 . They were close to the midline and migrated from rhombomere (r) 4 to ward r5 and r6. By E15, they had migrated caudally and ventrolaterally into the former location of r6. Most of them had reached their ''adul t'' position by E15. Another group of motoneurons, the accessory facia l nucleus, was found in r4 at E13 and in corresponding regions at late r stages. Visceral motoneurons were labeled from the periphery at all stages. At E13, they were mainly in r5 but also in r2, r3, r4, and r6. At E14, most of them had migrated laterally, and, by E15, they were i n the prospective parvocellular reticular formation. They could be div ided into two subgroups: a more rostral one with fibers that made loop s close to the midline and a more caudal one with fibers that went dir ectly to the exit. The findings presented here show that most branchia l and visceral motoneurons of the facial nerve are born in different a nd specific rhombomeres. Interestingly, developmental genes are expres sed specifically in these rhombomeres and could be involved in the gen esis of the facial and superior salivatory nuclei. (C) 1996 Wiley-Liss , Inc.