Hair bundle heights in the utricle: differences between macular locations and hair cell types

Hair bundle structure is a major determinant of bundle mechanics and thus of a hair cell's ability to encode sound and head movement stimuli. Little quantitative information about bundle structure is available for vestibular organs. Here we characterize hair bundle heights in the utricle of a turtle, Trachemys scripta. We visualized bundles from the side using confocal images of utricular slices. We measured kinocilia and stereocilia heights and array length (distance from tall to short end of bundle), and we calculated a KS ratio (kinocilium height/height of the tallest stereocilia) and bundle slope (height fall-off from tall to short end of bundle). To ensure that our measurements reflect in vivo dimensions as closely as possible, we used fixed but undehydrated utricular slices, and we measured heights in three dimensions by tracing kinocilia and stereocilia through adjacent confocal sections. Bundle heights vary significantly with position on the utricular macula and with hair cell type. Type II hair cells are found throughout the macula. We identified four subgroups that differ in bundle structure: zone 1 (lateral extrastriola), striolar zone 2, striolar zone 3, and zone 4 (medial extrastriola). Type I hair cells are confined to striolar zone 3. They have taller stereocilia, longer arrays, lower KS ratios, and steeper slopes than do neighboring (zone 3) type II bundles. Models and experiments suggest that these location- and type-specific differences in bundle heights will yield parallel variations in bundle mechanics. Our data also raise the possibility that differences in bundle structure and mechanics will help explain location- and type-specific differences in the physiological profiles of utricular afferents, which have been reported in frogs and mammals.