Quantitative TEM analysis of the barn owl basilar papilla.

The morphology of the barn owl's basilar papilla was quantitatively analyzed using TEM methods. The hair-cell (HC) parameters studied in the basal two-thirds of the papilla are remarkably constant. This large portion represents an extended high frequency area, or fovea [Köppl et al. (1993) J. Comp. Physiol. A 171, 695-704]. In the apical third of the papilla, in contrast, these parameters change regularly, as they do in other avian species. The HC in the most neural position remain morphologically more similar along the entire length of the papilla than do neighbouring cell rows. In the behaviourally most important frequency range (4-9 kHz), the afferent innervation of these neural HC is very dense and is reminiscent of the situation in mammals. Differences in HC morphology also indicate a specialization of the extreme apex of the papilla in the barn owl. Avian HC morphology is not correlated with a specific place along the basilar papilla but rather with the best frequency. Based on the body of recent quantitative morphological data on avian HC structure, a modified definition of HC types in birds is suggested (while keeping introduced terms): THC (tall hair cells) are defined as all those HC with afferent (and normally also efferent) innervation. SHC (short hair cells) are the (more specialized) HC without afferent innervation; obviously their function is restricted to the papilla itself.