TIME-COURSE OF NERVE-FIBER REGENERATION IN THE NOISE-DAMAGED MAMMALIAN COCHLEA

The time course of events which are essential for nerve-fiber regenera tion in the mammalian cochlea was determined using a group of chinchil las that had been exposed for 3.5 kr to an octave band of noise with a center frequency of 4 kHz and a sound pressure level of 108 dB. The a nimals recovered from 40 min (0 days) to 100 days at which times their inner ears were fixed and the organs of Corti prepared for phase-cont rast and bright-field microscopy as plastic-embedded flat preparations . Selected areas identified in the flat preparations were semi-thick a nd thin sectioned at radial or tangential angles for examination by br ight-field and transmission electron microscopy. The following time-or dered events appeared critical for nerve-fiber regeneration (1) The ar ea of the basilar membrane in which regeneration had a possibility of occurring showed signs of severe injury. Outer hair cells degenerated first followed by outer pillars, inner pillars, inner hair cells and o ther supporting cells; (2) Myelinated nerve fibers in the osseous spir al lamina became fragmented, starting at the distal ends of the fibers . This degeneration gradually extended back to Rosenthal's canal: (3) Fibrous processes, originating from Schwann-like cells in the osseous spiral lamina, extended laterally on the basilar membrane; (4) Schwann cells lined up medial to the habenulae perforata in the areas of seve rest damage, apparently ready to migrate through the habenulae onto th e basilar membrane; (5) Schwann-cell nuclei appeared on the basilar me mbrane beneath the developing layer of squamous epithelium which was i n the process of replacing the degenerated portion of the organ of Cor ti; (6) Regenerated nerve fibers with thin myelin sheaths or a simple investment of Schwann cell cytoplasm appeared in areas of total loss o f the organ of Corti, and (7) The myelin sheaths on the regenerated ne rve fibers gradually became thicker. (C) 1997 ISDN.