ANATOMICAL CORRELATES OF THE PASSIVE PROPERTIES UNDERLYING THE DEVELOPMENTAL SHIFT IN THE FREQUENCY MAP OF THE MAMMALIAN COCHLEA

As the cochlea develops, the cells in the basal cochlea become sensiti ve to progressively higher frequencies. To identify features of cochle ar morphology that may underlie the place code shift, measurements of infant and adult gerbil cochleas were made at both the light and elect ron microscopic levels. The measurements included areas of the cochlea r duct, basilar membrane, and organ of Corti, height and width of the basilar membrane, thickness of the tympanic cover layer, thickness of the upper and lower basilar membrane fiber bands, and optical density of the basilar membrane. The results indicated that basilar membrane d imensions do not change as the place code shifts and that regions that code for the roughly the same frequency (e.g., approximate to 11.2 kH z) at different ages can have basilar membranes of very different dime nsions. In contrast, the size of the organ of Corti and the thickness of fiber bands inside the basilar membrane do change in ways consisten t with the shift in the frequency map.