The basilar papilla of the barn owl Tyto alba: a quantitative morphological SEM analysis

The barn owl has the longest basilar papilla (about 12 mm) of all bird species studied. In the apical half several morphological parameters change regularly: stereovillar number per hair cell, length of the stereovillar bundle and cell surface area decrease from basal to apical. Cell number across the papilla, width of stereovillar bundle, diameter of a single stereovillus and height of stereovilli increase in the same direction. The orientation of the stereovillar bundles' long axis is parallel (0 degrees) to the edges of the papilla at the neural and the abneural sides, whereas along the midline there is a zone with an orientation of 50 degrees towards the apex; apically, this change in orientation may increase up to 90 degrees. In the basal half of the owl papilla the situation is different: most parameters are fairly constant, e.g. stereovillar height, mean orientation of stereovillar bundles (0 degrees), bundle shape and stereovillar number per hair cell. As the basilar papilla of birds is known to be tonotopically organized, with the high frequency range being represented basally, the different organization of the basal half of the owl papilla may be a specialization related to the excellent high frequency hearing.