Regeneration of the mammalian vestibular sensory epithelium following gentamicin-induced damage

Objectives: The aims of this study are (1) to investigate if significant long-term recovery of mature hair bundle (MHB) numbers takes place following gentamicin-induced damage to the mammalian vestibular sensory epithelium and (2) to assess if the different MHB types in the vestibular sensory epithelium have a different susceptibility to ototoxic damage. Methods: Gentamicin (8 mg in 0.1-mL sterile water) was injected transtympanically into one ear of guinea pigs, the contralateral ear acting as a control. The animals were killed at 4 days, 4 weeks, and 3 and 10 months post-treatment and the utricles (n = 38) were extracted from both ears. Mature hair bundle and immature-looking hair bundle (IHB) densities on the surface of the utricle were determined using scanning electron microscopy. Results: The MHB density showed a significant decline between 4 days and 4 weeks post-treatment. There was greater loss of type I MHBs (tallest stereocilia comparable in height to the kinocilium) than type II MHBs (kinocilium taller than the tallest stereocilia). A significant increase in IHB density was seen at 4 weeks post-treatment, after which it declined rapidly. A significant but incomplete recovery in MHB density (to 66% of control value) was seen in the striolar region at 10 months post-treatment, and these were composed mainly of type II MHBs. Conclusions: It would appear that the mature mammalian vestibular sensory epithelium does have the capacity for long-term recovery of MHB numbers following gentamicin-induced damage, but this is limited and does not result in complete restoration of the epithelium. Type I MHBs are more susceptible to ototoxic damage than type II MHBs.