Continuous hair cell turnover in the inner ear vestibular organs of a mammal, the Daubenton's bat (Myotis daubentonii).
In both humans and mice the number of hair cells in the inner ear sensory epithelia declines with age, indicating cell death (Park et al. 1987; Rosenhall 1973). However, recent reports demonstrate the ability of the vestibular sensory epithelia to regenerate after injury (Forge et al. 1993, 1998; Kuntz and Oesterle 1998; Li and Forge 1997; Rubel et al. 1995; Tanyeri et al. 1995). Still, a continuous hair cell turnover in the vestibular epithelia has not previously been demonstrated in mature mammals. Bats are the only flying mammals, and they are known to live to a higher age than animals of equal size. The maximum age of many species is 20 years, with average lifespans of 4-6 years (Schober and Grimmberger 1989). Further, the young are fully developed and able to fly at the age of 2 months, and thus the vestibular organs are thought to be differentiated at that age. Consequently, long-lived mammals such as bats might compensate for the loss of hair cells by producing new hair cells in their postembryonic life. Here we show that the utricular macula of adult Daubenton's bats (more than 6 months old) contains innervated immature hair cells as well as apoptotic hair cells, which strongly indicates a continuous turnover of hair cells, as previously demonstrated in birds.