Auditory stereocilia in the alligator lizard.

The normal anatomy of stereociliary tufts in the basilar papilla of the alligator lizard is described and demonstrated with scanning electron micrographs. Stereociliary tufts in the tectorial region differ from those in the free-standing region in several ways. Tectorial stereociliary tufts are short (less than 10 micron in height), unidirectional in orientation, and covered with a tectorial membrane. Free-standing stereociliary tufts are very tall (up to 38 micron in height), bidirectional in orientation and not covered by a tectorial membrane or any other tectorial substance. The heights of the stereociliary tufts along the length and across the width of the basilar papilla were measured in serial light and transmission electron micrographs. Free-standing stereociliary tufts decrease progressively in height along the length of the basilar membrane, being tallest at the apical end and shortest at the basal end. Tectorial stereociliary tufts do not increase progressively along the length of the basilar membrane but do increase progressively across the width of the basilar membrane, being shortest on the neural side. Free-standing stereociliary tufts are structurally simple being a prominent specialization of the lizard cochlea. Tectorial stereociliary tufts are structurally more complicated conforming more closely to the general anatomical pattern of vertebrate auditory hair cells.