Regenerated nerve fibers in the noise-damaged chinchilla cochlea are not efferent

Nerve-fiber regeneration in the chinchilla cochlea following a traumatic noise exposure was systematically described by Bohne and Harding (1992). However, their study did not determine the origin of the regenerated nerve fibers (RNFs). In the present study, 23 chinchillas were exposed for 12 h to a 0.5 kHz octave band of noise at 120 dB SPL. After a 3-month or 1-year recovery period, their right cochleas were incubated to demonstrate acetylcholinesterase (AChE) activity and then briefly counterstained with Neutral Red or OsO 4. Their left cochleas were fixed with OsO 4 and dissected using a combined organ of Corti (OC)/modiolus technique that preserved both structures for high-resolution microscopy. All cochleas were prepared as plastic-embedded flat preparations. Damage was located in the basal two-thirds of the cochlea and generally consisted of multiple lesions in the OC, often involving total degeneration of one or more OC segments (i.e., OC wipeouts). The OC wipeouts were separated from one another by areas which contained some identifiable cells of the OC (i.e., OC remnants). Most RNFs were found in OC wipeouts adjacent to OC remnants. In those animals (83%) with significant OC damage, 13 (100%) 3-month-recovery chinchillas had 1–96 RNFs while 6 (86%) 1-year-recovery chinchillas had 7–62 RNFs. In the AChE-stained cochleas, none of the RNFs were AChE-positive, but normal AChE-positive fibers were found in the undamaged apical turn. A variable number of surviving spiral ganglion cells was present in those regions of Rosenthal's canal that had originally innervated the missing hair cells in the OC wipeouts and remnants. It is concluded that RNFs are not part of the efferent cochlear system and therefore, most likely belong to the afferent system.