Anatomical basis of a congenital hearing impairment: basilar papilla dysplasia in the Belgian Waterslager canary.

Abstract

Recent investigations into the anatomy of the inner ear of Belgian Waterslager canaries (BWC) have demonstrated myriad malformations associated with dysgenesis of the pars inferior of the otocyst. In those studies, the surface anatomy of BWC's basilar papilla and sacculus was examined utilizing scanning electron microscopy. In the present investigation, we utilized both light microscopy and transmission electron microscopy to describe the cross sectional anatomical pathology of the BWC's basilar papilla. Examination of the BWC's organ of Corti revealed numerous dysmorphologies: 1) hair cells from the tall hair cell region appeared broad and stunted, with deformed cuticular plates, abnormal stereocilia, and recognizable microvilli; 2) quantitative analysis of these hair cells revealed disproportionately large nuclei and abnormally short stereocilia; 3) hair cells from the short hair cell region of the papilla were absent, replaced by a population of large cells with electron-lucent cytoplasm; and 4) the tectorial membrane in the BWC papilla was narrow, covering only the area where the deformed tall hair cells were found. The malformations appeared to be more severe at the apex and midsection of the basilar papilla than at the base. These observations allow us to suggest a hypothesis to account for the distinct anatomofunctional hearing deficit observed in these birds. In addition, they further support our hypothesis that the inner ear of BWC is afflicted by a disorder similar to Scheibe's dysplasia, the most common inner ear defect associated with congenital hearing loss in humans.