Hair Cell Recovery in Normal and Mitotically Blocked Cultures of the Bullfrog Saccule

Hair cells, the sensory receptors of the vertebrate inner ear, are lost with age and are highly susceptible to damage from disease, noise, and ototoxic drugs. This is a profound health problem because a significant proportion of the human population suffers from deafness or balance disorders resulting directly from hair cell loss. We now know that mitotic division of supporting cell progenitors and the differentiation of the resulting progeny into new hair cells and supporting cells causes regeneration of hair cells in nonmammalian vertebrates. Recent studies have shown that hair cell recovery after aminoglycoside-induced damage in the vestibular organs also can occur by nonmitotic mechanisms. Using immunocytochemical markers against the hair cell and supporting cell phenotypes, we have used confocal and electron microscopy to examine the fate of damaged hair cells and the origin of immature hair cells after gentamicin treatment in mitotically blocked cultures of the bullfrog saccule. Lethally damaged hair cells that undergo apoptotic cell death are replaced by scar formations created by the expansion of neighboring supporting cells. Sublethally damaged hair cells, after losing their hair bundles, remain in the sensory epithelium for prolonged periods, acquiring supporting cell-like morphology and immunoreactivity. These modes of damage appear to be mutually exclusive, implying that sublethally damaged hair cells repair their bundles. Transitional cells, coexpressing hair cell and supporting cell markers, are seen in or near scar formations. These cells have morphology and immunoreactivity similar to that of sublethally damaged hair cells. Ultrastructural analysis also reveals that immature hair cells have autophagic vacuoles, implying that they originated from damaged hair cells rather than from supporting cells. Supporting cells also did not decrease in number during hair cell recovery, implying that these cells did not undergo phenotypic conversion into hair cells without an intervening mitotic event.