HAIR CELL MORPHOLOGY AND INNERVATION IN THE BASILAR PAPILLA OF THE EMU (DROMAIUS-NOVAEHOLLANDIAE)

The emu, being a member of the rather primitive bird group of the pala eognathid Ratitae, may reveal primitives features of the avian basilar papilla. There are, however, no qualitative differences with the papi llae of other birds such as the chicken or the starling. There are onl y quantitative differences in the continuous morphological gradients ( such as hair cell height, stereovillar height) from neural to abneural , and from the base to the apex of the papilla. Only few (about two in the emu) afferent terminals and on average one efferent fiber contact each hair cell. Along the abneural edge, there is a population of hai r cells that lack afferent innervation (short hair cells), suggesting that their function must lie in the papilla itself. There is thus a ge neral pattern in the structures of the avian basilar papilla. In detai l, however, a number of primitive characters were observed in the emu, as compared to advanced birds such as the starling and the barn owl. The hair cells are very densely packed and comparatively tall(up to 40 mu m in the apex). This anatomy correlates well with the good lower-f requency hearing (see Koppl and Manley, J. Acoust. Sec. Am. 101 (1997) 1574-1584). The afferent nerve fibers contacting the hair cells withi n the basilar papilla are rather thick, and there are a large number o f afferent fibers that contact more than one hair cell. The zone of ha ir cells without afferent innervation (short hair cells) along the abn eural edge of the basilar papilla is rather narrow in the emu. (C) 199 8 Elsevier Science B.V. All rights reserved.