Immunoelectron microscopic and immunofluorescent localization of cytoskeletal and muscle-like contractile proteins in inner ear sensory hair cells.

The distribution of actin, alpha-actinin, fimbrin, tropomyosin and tubulin in the apical region of inner and outer hair cells was studied by immunofluorescent localization of antibodies to these proteins. The macromolecular distribution of actin and alpha-actinin was studied using post-embedding immunoelectron microscopic techniques. Actin is present in the stereocilia and cuticular plate of both inner and outer hair cells. Antibodies to actin were localized with fluorescence and colloidal gold. Colloidal gold particles were distributed uniformly over the stereocilia, stereocilia rootlets and cuticular plate. Fimbrin is present in the stereocilia and the cuticular plate. Immunofluorescent label was more intense over the cuticular plate of outer hair cells than over the cuticular plate of inner hair cells. Alpha-actinin is present in the cuticular plate only. At the ultrastructural level, antibodies to alpha-actinin were labeled throughout the cuticular plate, with larger accumulations of colloidal gold over the electron dense bodies in the cuticular plate, as well as over the electron dense region at the junctional complex. There was no label over the electron dense portion of the stereocilia rootlets. Tropomyosin is observed in the area of the stereocilia rootlets by immunofluorescent techniques, but like fimbrin, the antigenic sites of tropomyosin did not withstand processing for ultrastructural localization. Tubulin is not present in the apical region of inner or outer hair cells, although its presence could be documented in the hair cell body and in the supporting cells.