Ultrastructure of the Basilar Papilla in the Bullfrog

The basilar papilla is one of two auditory organs found within the inner ear of most species of amphibians. It is thought to be homologous to the cochlea of higher vertebrates. We have used the techniques of light and electron microscopy to study this organ. The papilla is a short, cartilaginous, tubular extension of the saccule, terminated at the end by a thin, flat, contact membrane that separates the endolymphatic and perilymphatic spaces. Hair cells and supporting cells are arranged semicircumferentially in parallel rows along a portion of the tube. The tectorial membrane hangs from a cord that spans a diameter of the tube. From the surface of each hair cell there projects a bundle of sensory hairs, consisting of a number of stereocilia and a single kinocilium located at one side of the bundle. All hair cells are oriented in the same direction: the kinocilium is located at that side of the hair cell nearest to the contact membrane. About 350 myelinated nerve fibers innervate approximately 60 hair cells. Only one type of synapse has been found, an afferent connection between hair cells and nerve terminals. Notably absent are synapses of the sort commonly associated with efferent innervation.