Supporting and membrane structures of human outer hair cells: evidence for an isometric contraction.

The supporting elements of the human organ of Corti express a number of cytokeratins, and it is obvious that the presence of cytokeratins provides mechanical stability to the framework of the entire organ of Corti. In the basal part of the outer hair cells and in the apical part of the Deiters cells, there is a significant expression of actin. In the human outer hair cells, the system of subsurface cisterns is not only restricted to the lateral cell wall but is also continuous to the basal region where the hair cells are closely attached to the supporting Deiters cells. In contrast to the well-known organization of the subsurface cisterns and pillar structures at the lateral outer hair cell wall of rodents, the pillars at the base of human outer hair cells can form channels. These channels allow a direct communication between the lumen of the subsurface cisterns and the intercellular space which separates outer hair cells and Deiters cells. At the apical part of the Deiters cells, a condensation of microfilaments is present which displays morphological similarities to actin fibers. These well-organized structural configurations between outer hair cells and Deiters cells imply that this region has special contractile properties. The human outer hair cells are therefore firmly stabilized not merely apically but also at the cell base, a situation which would permit a mainly isometric contraction under in vivo physiological conditions.