DiI reveals a prenatal arrival of efferents at the differentiating otocyst of mice.

We have reinvestigated the time of arrival of efferent fibers at the developing otocyst of mice employing diffusion of the lipophilic dye DiI in fixed tissue. In contrast to almost all previous reports, our data indicate a prenatal arrival of efferent fibers. A few efferent fibers were found to enter the eighth nerve root at embryonic day (ED) 10 1/2. Retrogradely labelled efferent cell bodies were at this stage coextensive with those of the facial motor nucleus, but started to segregate by ED 12. In contrast to retrogradely labelled facial motor neurons, labelled efferent neurons were bilaterally distributed in the hindbrain with a few projecting to both otocysts as early as ED 12. Anterograde labelling from the brain showed efferent fibers in the vestibular ganglion by ED 11. Invasion of the future vestibular sensory epithelia started by ED 12. Growth cones of efferent fibers had also reached the future cochlear sensory epithelium but invasion was only achieved by a few filopodia at this stage. The early arrival of efferents at the future sensory epithelia demonstrated here may allow an as yet unexplored interaction of efferent fibers with the proliferating and/or differentiating hair cells.