DOES THE ORGAN OF CORTI ATTEMPT TO DIFFERENTIATE NEW HAIR-CELLS AFTERANTIBIOTIC INTOXICATION IN RAT PUPS

In the adult mammalian cochlea, post-injury hair cell losses are consi dered to be irreversible. Recent studies in cochlear explants of embry onic rodents show that the organ of Corti can replace lost hair cells after injury. We have investigated this topic in vivo during the perio d of cochlear development. Rat pups were treated with a daily subcutan eous injection of 500 mg/kg amikacin for eight consecutive days betwee n postnatal day 9 (PND 9) and PND 16. During this period the organ of Corti is not fully mature, but hair cells are hyper-sensitive to amino glycoside antibiotics. Scanning and transmission electron microscopy w as used to evaluate morphological changes in the organs of Corti durin g the treatment and at different post-treatment periods, up until PND 90. A massive loss in outer and inner hair cells was observed at least as early as PND 14. A prominent feature in the apical part of cochlea s al PND 21 and 35 was the transient presence of small atypical cells in the region of pre-existing outer hair cells. These atypical cells h ad tufts of microvilli reminiscent of nascent stereociliary bundles. A second striking observation was the replacement of degenerating inner hair cells by pear-shaped supporting cells throughout the cochlea. Th ese cells were covered with long microvilli, and their basal pole was contacted by both afferent and efferent fibers, as in the early stages of inner hair cell maturation. At PND 55 and 90, these features were not clearly observed due to further cytological changes in the organ o f Corti. It is possible that an attempt at hair cell neodifferentiatio n could occur in vivo after an amikacin treatment in the rat during th e period of cochlear hyper-sensitivity to antibiotic. (C) 1997 ISDN.