Ultrastructural evidence that early synapse formation on central vestibular sensory neurons is independent of peripheral vestibular influences.

Migration and early differentiation of neurons of the tangential vestibular nucleus of the chick take place between embryonic days 5 and 8. In the absence of primary vestibular afferents (otocyst-ablation), a previous light microscope study documented that early developmental events still occurred, but the neurons failed to complete differentiation and to survive. In order to understand why these neurons undergo normal early development, we have repeated the vestibular deafferentation paradigm followed by ultrastructural observations on these neurons. We found that the ultrastructural events associated with migration and differentiation in the deafferented tangential nucleus were essentially normal from 5 to 8 days. Most important, longitudinal fibers, presumably of central, nonvestibular origins, formed the first synapses at the same time and sequence as observed in normal embryos. Thus vestibular sensory neurons receive their first input from central fibers, initiating events in the formation of a central vestibular circuitry without the influence of peripheral vestibular fibers or endorgan.