Evidence for hair cell regeneration in the papilla basilaris of the lizard Podarcis sicula.

Post embryonic hair cell production occurs continually throughout life in the vestibular and auditory sensory epithelium in fishes and amphibians and in the vestibular end organs of birds (Corwin, 1985, PNAS, USA 82: 3911-15; Roberson et al., 1992, Hear. Res., 57: 166-74; Stone et al., 1999, J. Neurocytol. 28: 863-76). In the mature hearing organ of birds and in the vestibular sensory epithelium of mammalian only after hair cell loss by treatment with ototoxic drugs or trauma, new hair cells reappeared in sensory epithelia (Cotanche, 1987, Hear. Res., 30: 181-96; Forge et al., 1993, Science 259: 1616-19; Zheng and Gao, 1997 J Neurosci., 17 (21): 8270-82). According to Bagger-Sjoback and Wersäll (1976), lizards treated with gentamicin for 21 days showed a severely damaged papilla basilaris. We investigated recovery phenomena in the sensory epithelium occurring in the gentamicin-treated lizard papilla basilaris by SEM. Using 5-bromodeoxyuridine we have studied the hair cells regeneration in the papilla basilaris both in normal conditions and at early and late time intervals following 3 doses of aminoglycoside gentamicin. Finally, in order to define if the regenerated cells become sensory cells, we also analysed the presence of calbindin, used as a marker of hair cells, by CLSM. The SEM and CLSM data are consistent: gentamicin induced damage of the hair cells in a dose-dependent manner and the two segments of papilla basilaris differed in their susceptibility to gentamicin. Initially the apical type A cells were relatively intact while the type B cell-population in the basal segment of the organ showed sensory hair fusions and cytoplasmic herniations. Successively a wide spread loss of hair of the sensory cells, 8 days after treatment, extrusion of entire hair cell was observed and the surface of the basal part of the organ instead was covered by supporting cells. The papilla basilaris, however, showed an apparently normal morphology after 28 days from treatment. Our immunocytochemistry studies showed that in the papilla basilaris of Podarcis sicula regeneration phenomena are present also in normal conditions and that regenerative processes, limited or slow in normal conditions, are very amplified by treatment with gentamicin. Therefore, in a different way to chick in which the regeneration occur only after treatment with ototoxic drugs or trauma, in some orders of reptiles, in the auditory organ, the regeneration is a spontaneous phenomenon also in normal conditions