Morphological indications of hair cell neodifferentiation in the organ of corti of amikacin treated rat pups

Postnatal rats were treated with amikacin during the period of cochlear supra-normal sensitivity to ototoxic antibiotics, i.e. from day 3 to day 16 when the organ of Corti is achieving its maturation. Scanning and transmission electron microscopies were used to evaluate morphological changes in the organs of Corti at different post-treatment periods, up until postnatal day 90. A massive loss in outer (OHCs) and inner (IHCs) hair cells was observed from as early as day 17. A prominent feature in the apical part of cochleas at days 21 and 35 was the transient presence of small atypical cells in the region of pre-existing OHCs. These atypical cells did not have the appearance of degenerating OHCs, but had tuffs of microvilli reminiscent of nascent stereociliary bundles. A second striking observation was the presence of pear-shaped cells in the place normally occupied by IHCs. These cells had neither cuticular plate nor stereocilia, but their basal pole was contacted by both afferent and efferent fibers, as in the early stages of IHC maturation. Such efferent contacts are very rare in the adult cochlea. These findings, not previously reported in antibiotic poisoned cochleas of adult animals, suggest that an attempt at sensory cell neodifferentiation occurs in vivo in the developing organ of Corti intoxicated with aminoglycoside antibiotics.