ORGANIZATION AND DEVELOPMENT OF FACIAL MOTOR-NEURONS IN THE KREISLER MUTANT MOUSE

The adult facial nerve contains the axons from two populations of effe rent neurons. First, the branchiomotor efferent neurons that innervate the muscles of the second arch. These neurons project out of the hind brain in the motor root and form the facial motor nuclei. Second, the preganglionic efferent neurons that innervate the submandibular and pt erygopalatine ganglia. These neurons project from the hindbrain via th e intermediate nerve and form the superior salivatory nucleus. The mot or neurons of the facial nerve are known to originate within rhombomer es 4 and 5. In the kreisler mouse mutant there is a specific disruptio n of the hindbrain rhombomeres 5 and 6 appear to be absent. To investi gate changes in the organization of the facial motor neurons in this m utant, we have used lipophilic dyes to trace the facial motor componen ts both retrogradely and anterogradely. As expected, facial motor neur ons are missing from rhombomere 5 in this mutant. in addition, the los s of these neurons correlates with the specific loss of the superior s alivatory nucleus. In contrast, the branchiomeric neurons, that origin ate in rhombomere 4, appear to develop normally. This includes the cau dal migration of their cell bodies forming the genu of the facial nerv e. Our studies confirm that rhombomeres are critical to hindbrain deve lopment and that they are the fundamental unit at which motor neurons are specified.