Mammalian cochlear outer hair cells density evaluated by means of an optical tweezer

We report on the first individual measurements of guinea pig's cochlear outer hair cells densities. Cells were isolated in vitro and manipulated with an optical tweezer. They were levitated in an upward laser beam coaxially trapping the cells. Then they were released by switching off the laser and let fall down in upright position. Measuring their speed and using the Stokes' law, we calculated their mean density. In our experimental frame, the results suggest that the density of the cellular body (between the basal nucleus and the apical cuticular plate) remains quasi constant whatever the cells' length. This implies that density variation of the cellular body does not participate in an intrinsic tuning mechanism.