Primary cilia of inner ear hair cells do not respond to mechanical stimuli

Hair cells, the sensory cells of the inner ear, carry a bundle of 30–300 specialized actin-based microvillus-like projections called stereocilia. Tiny filamentous ‘tip links’ connect the stereocilia and transmit the force produced by sub-nanometer movements to Ca2+-permeable mechanoelectrical transduction (MET) channels at the tips of stereocilia—likely encoded by TMC1 and TMC2 genes. Adjacent to the tallest stereocilia is the kinocilium—a single nonmotile, (9+2) microtubule-based true cilium. In chicken hair cells, kinocilia are connected to stereocilia by the same proteins as the tip links, raising the question of whether kinocilia also have MET channels. Indeed, TMC1 protein has been reported in kinocilia of mouse hair cells. In developing zebrafish hair cells, moreover, Ca2+ imaging experiments suggest that, prior to the onset of the normal MET, kinocilia mediate a mechanosensitivity elicited by reverse …