Solitary hair cells are distributed throughout the extramacular epithelium in the bullfrog's saccule

The frog inner ear contains eight sensory organs that provide sensitivities to auditory, vestibular, and ground-borne vibrational stimuli. The saccule in bullfrogs is responsible for detecting ground- and airborne vibrations and is used for studies of hair cell physiology, development, and regeneration. Based on hair bundle morphology, a number of hair cell types have been defined in this organ. Using immunocytochemistry, vital labeling, and electron microscopy, we have characterized a new hair cell type in the bullfrog saccule. A monoclonal antibody that is specific to hair cells revealed that a population of solitary hair cells exists outside the sensory macula in what was previously thought to be nonsensory epithelium. We call these extramacular hair cells. There are 80-100 extramacular hair cells in both tadpole and adult saccules, which extend up to 1 mm from the edge of the sensory macula. The extramacular hair cells have spherical cell bodies and small apical surfaces. Even in adults, the hair bundles of the extramacular cells appear immature, with a long kinocilium (6-9 microm) and short stereocilia (0.5-2 microm). At least 90% of extramacular hair cells are likely to be innervated as demonstrated by labeling of nerve fibers with an antineurofilament antibody. The extramacular hair cells may differentiate in regionsjust beyond the edge of the macula at an early stage in development and then be pushed out via the interstitial growth of the epithelium that surrounds the macula. It is also possible that they may be produced from cell divisions in the extramacular epithelium that has not been considered capable of giving rise to hair cells.