Abstract
The ultrastructure of the apical end of the hair bundle of the vestibular hair cell in the guinea pig was studied by transmission electron microscopy using osmium and aldehyde-osmium fixed specimens. Stereo-kinociliar bonds were a regular finding in both cristae and maculae. The bond unit consisted of two parts: extracellular, interciliar filaments of macromolecular size and an intracellular structure, bound to the plasma membrane within the kinocilia. In the cristae, the stereo-kinociliar bonding was usually effected by only one bond unit between a single stereocilium and the kinocilium; in the maculae the units were generally paired and several stereocilia were coupled to each kinocilium.