High-resolution three-dimensional imaging of the lateral plasma membrane of cochlear outer hair cells by atomic force microscopy

The outer hair cells (OHCs) from the mammalian organ of Corti are assumed to enhance the sensitivity and the selectivity of the cochlea via an electromotile response to sound stimulation. These OHC mechanical changes feed energy back into the cochlea before completion of the transduction process by inner hair cells. OHC electromotility is thought to depend on specific transmembrane motor proteins. Electron microscopy has been used previously to image the OHC lateral plasma membrane, where voltage sensors and motors are located. A very specific and regular organization of membrane particles has been described, together with an equally specific submembraneous meshwork of cytoskeleton anchored to the plasma membrane. To confirm and extend these observations, we have used, for the first time on the OHC lateral wall, atomic force microscopy (AFM). As a result of an improved tapping mode technique as well as the unique ultrastructural organization of the OHC plasma membrane, we have obtained high-resolution three-dimensional (3D) images of a markedly enhanced quality, allowing high-resolution 3D imaging. Tapping-mode AFM confirmed the presence of regularly aligned particles (presumably transmembrane proteins) on both faces of the OHC plasma membrane. It also revealed the presence of markedly different membrane domains, smooth and undulating. The differences between these zones probably are due to local differences in cytoskeleton-membrane interactions. Moreover, 3D reconstructions allowed us to distinguish between globular and pore-like particles, a distinction that may be of great functional significance.