THE DEVELOPMENT OF VESTIBULOCOCHLEAR EFFERENTS AND COCHLEAR AFFERENTSIN MICE

We have reinvestigated the embryonic development of the vestibulocochl ear system in mice using anterograde and retrograde tracing techniques . Our studies reveal that rhombomeres 4 and 5 include five motor neuro n populations. One of these, the abducens nucleus, will not be dealt w ith here. Rhombomere 4 gives rise to three of the remaining population s: the facial branchial motor neurons; the vestibular efferents; and t he cochlear efferents. The migration of the facial branchial motor neu rons away from the otic efferents is completed by 13.5 days post coitu m (dpc). Subsequently the otic efferents separate into the vestibular and cochlear efferents, and complete their migration by 14.5 dpc. In a ddition to their common origin, all three populations have perikarya t hat migrate via translocation through secondary processes, form a cont inuous column upon completion of their migrations, and form axonal tra cts that run in the internal facial genu. Some otic efferent axons tra vel with the facial branchial motor nerve from the internal facial gen u and exit the brain with that nerve. These data suggest that facial b ranchial motor neurons and otic efferents are derived from a common pr ecursor population and use similar cues for pathway recognition within the brain. In contrast, rhombomere 5 gives rise to the fourth populat ion to be considered here, the superior salivatory nucleus, a visceral motor neuron group. Other differences between this group and those de rived from rhombomere 4 include perikaryal migration as a result of tr anslocation first through primary processes and only then through seco ndary processes, a final location lateral to the branchial motor/otic efferent column, and axonal tracts that are completely segregated from those of the facial branchial and otic efferents throughout their cou rse inside the brain. Analysis of the peripheral distribution of the c ochlear efferents and afferents show that efferents reach the spiral g anglion at 12.5 dpc when postmitotic ganglion cells are migrating away from the cochlear anlage. The efferents begin to form the intragangli onic spiral bundle by 14.5 dpc and the inner spiral bundle by 16.5 dpc in the basal turn. They have extensive collaterals among supporting s ells of the greater epithelial ridge from 16.5 dpc onwards. Afferents and efferents in the basal turn of the cochlea extend through all thre e rows of outer hair cells by 18.5 dpc. Selective labeling of afferent fibers at 20.5 dpc (postnatal day 1) shows that although some afferen ts are still in early developmental stages, some type II spiral gangli on cells already extend for long distances along the outer hair cells, and some type I spiral ganglion cells end on a single inner hair cell . These data support previous evidence that in mice the early outgrowt h of afferent and efferent fibers is essentially achieved by birth. (C ) 1997 ISDN.