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Evidence for a Highly Elastic Shell-Core Organization of Cochlear Outer Hair Cells by Local Membrane Indentation

Alexandra Zelenskaya ^*, Jacques Boutet de Monvel ^*, Devrim Pesen , Manfred Radmacher , Jan H. Hoh  and Mats Ulfendahl ^*

^* Department of Clinical Neuroscience and Center for Hearing and Communication Research, Karolinska Institutet, SE-171 76 Stockholm, Sweden; Department of Physiology, Johns Hopkins School of Medicine, Baltimore, Maryland 21205; and Institut für Biophysik Universität Bremen, 28359 Bremen, Germany

Correspondence: Address reprint requests to Mats Ulfendahl, Center for Hearing and Communication Research, M1-ENT, Karolinska University Hospital, SE-17176 Stockholm, Sweden. Tel.: 46-85177-6306; Fax: 46 8 301876; E-mail: mats.ulfendahl{at}cfh.ki.se.

Cochlear outer hair cells (OHCs) are thought to play an essential role in the high sensitivity and sharp frequency selectivity of the hearing organ by generating forces that amplify the vibrations of this organ at frequencies up to several tens of kHz. This tuning process depends on the mechanical properties of the cochlear partition, which OHC activity has been proposed to modulate on a cycle-by-cycle basis. OHCs have a specialized shell-core ultrastructure believed to be important for the mechanics of these cells and for their unique electromotility properties. Here we use atomic force microscopy to investigate the mechanical properties of isolated living OHCs and to show that indentation mechanics of their membrane is consistent with a shell-core organization. Indentations of OHCs are also found to be highly nonhysteretic at deformation rates of more than 40 µm/s, which suggests the OHC lateral wall is a highly elastic structure, with little viscous dissipation, as would appear to be required in view of the very rapid changes in shape and mechanics OHCs are believed to undergo in vivo.

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