EFFECTS OF STREPTOMYCIN ON DEVELOPMENT OF THE APICAL STRUCTURES OF HAIR-CELLS IN THE CHICK BASILAR PAPILLA

A single dose of streptomycin (166 mg/kg egg weight) was given to chick embryos 7-15 days after the beginning of incubation. Embryos were fixed 4-12 days later, and the hair cells examined by scanning and transmission electron microscopy. The highest proportion of abnormalities was produced by injections on or before the 11th day of incubation, later injections affecting a substantially smaller proportion of embryos. This suggests the possibility of a critical period for streptomycin ototoxicity in the chick. In addition to the normal signs of cellular degeneration, the most striking abnormality was a massive expansion in the apical surface, sometimes by a factor of 20, in area. The organisation of stereocilia was also commonly affected. The stereocilia could be broken up into multiple small separate bundles, and often there was a wide separation between the kinocilium and the stereocilia. Stereocilia tended to be reduced in number, fused, and either of abnormally large, small, or irregular diameters. Structures with the appearance of stereociliar cores often lay horizontally within the surface of the cuticular plate, sometimes running for 15-mu-m or more. Sometimes abnormal 'stereocilia' were expressed around the extreme margins of the cuticular plate. In addition, adjacent hair cells could show very different developmental stages, as though the development of some cells had been arrested. With all these changes, the short hair cells in the centre of a papillar cross-section tended to be the most affected, with the tall hair cells and the short hair cells on the extreme inferior (i.e. abneural) edge being least affected. It is suggested that the streptomycin alters the balance of the different aspects of development of the hair cells. It might therefore be possible to use ototoxicity as a way of analysing hair-cell development.