Fine structural aspects on auditory hair cell degeneration in the budgerigar, Melopsittacus undulatus, as induced by kanamycin.

The effects of kanamycin, an antibiotic of the aminoglycoside group, on the auditory sensory epithelium of the budgerigar, Melopsittacus undulatus, were examined using both scanning and transmission electron microscopes. Results show that the threshold of the auditory brainstem response increased in birds treated with kanamycin 200 mg/kg for 49 days. While the auditory sensory epithelium of the normal budgerigar consists of short and tall hair cells, and supporting cells, following kanamycin administration, the middle to proximal region of the epithelium of the inner ear showed degenerative changes, with the other parts remaining apparently intact. In the damaged region, the short hair cells were flattened, and the tall ones became heterogeneous in shape. Both types of cells contained many dense bodies in their cytoplasm; they were rounded in shape and homogeneously dense. Severely degenerated tall hair cells also contained many large vacuoles with heterogeneous contents. Because the dense bodies and large vacuoles were positive for acid phosphatase reaction, they were respectively judged to be primary lysosomes and secondary lysosomes containing degenerating cell debris. Most supporting cells in the impaired region were lower in cytoplasmic electron density, and their apical surface became enlarged in area. Some flattened short hair cells were situated on the apical part of the swollen supporting cells. This finding suggests that the short hair cells are pressed toward the scala media by the supporting cells.