The microenvironment of the cochlea is maintained by the barrier between the systemic circulation and the fluids inside the stria vascularis. However, the mechanisms that control the permeability of the intrastrial fluid-blood barrier remain largely unknown. The barrier comprises endothelial cells connected to each other by tight junctions and an underlying basement membrane. In a recent study, we found that the intrastrial fluid-blood barrier also includes a large number of perivascular cells with both macrophage and melanocyte characteristics. The perivascular-resident macrophage-like melanocytes (PVM/Ms) are in close contact with vessels through cytoplasmic processes. Here we demonstrate that PVM/Ms have an important role in maintaining the integrity of the intrastrial fluid-blood barrier and hearing function. Using a cell culture-based in vitro model and a genetically induced PVM/M-depleted animal model, we show that absence of PVM/Ms increases the permeability of the intrastrial fluid-blood barrier to both lowand high-molecular-weight tracers. The increased permeability is caused by decreased expression of pigment epithelial-derived factor, which regulates expression of several tight junction-associated proteins instrumental to barrier integrity. When tested for endocochlear potential and auditory brainstem response, PVM/ M-depleted animals show substantial drop in endocochlear potential with accompanying hearing loss. Our results demonstrate a critical role for PVM/Ms in regulating the permeability of the intrastrial fluid-blood barrier for establishing a normal endocochlear potential hearing threshold. mouse cochlea | paracellular permeability | tight junction | capillary T he intrastrial fluid-blood barrier separates the stria vascularis (SV) from peripheral circulation. The integrity of the barrier is critical for maintaining inner ear homeostasis, especially for sustaining the endocochlear potential (EP), an essential driving force for hearing function (1-4). Disruption of the barrier is closely associated with a number of hearing disorders, including autoimmune inner ear disease, noise-induced hearing loss, agerelated hearing loss, and several genetically linked diseases (5-10). Despite the importance of the intrastrial fluid-blood barrier, little is understood about regulation of the barrier and the mechanisms that control its permeability.In the classic view, the intrastrial fluid-blood barrier comprises basement membrane and endothelial cells (ECs) that connect to each other with tight junctions (11) to form a diffusion barrier that prevents most blood-borne substances from entering the ear (2). In a recent study, we found that the intrastrial fluid-blood barrier also includes a large number of pericytes and perivascular-resident macrophage-like melanocytes (PVM/Ms) (12, 13). The PVM/Ms are not observed in other capillary regions such as in capillary beds of the spiral ligament. The PVM/Ms are in close contact with vessels through cytoplasmic processes. The structural complexity of PVM/Ms' capillar...

It is commonly believed that the exceptional sensitivity of mammalian hearing depends on outer hair cells which generate forces for amplifying sound-induced basilar membrane vibrations, yet how cellular forces amplify vibrations is poorly understood. In this study, by measuring subnanometer vibrations directly from the reticular lamina at the apical ends of outer hair cells and from the basilar membrane using a custom-built heterodyne low-coherence interferometer, we demonstrate in living mouse cochleae that the soundinduced reticular lamina vibration is substantially larger than the basilar membrane vibration not only at the best frequency but surprisingly also at low frequencies. The phase relation of reticular lamina to basilar membrane vibration changes with frequency by up to 180 degrees from ∼135 degrees at low frequencies to ∼-45 degrees at the best frequency. The magnitude and phase differences between reticular lamina and basilar membrane vibrations are absent in postmortem cochleae. These results indicate that outer hair cells do not amplify the basilar membrane vibration directly through a local feedback as commonly expected; instead, they actively vibrate the reticular lamina over a broad frequency range. The outer hair cell-driven reticular lamina vibration collaboratively interacts with the basilar membrane traveling wave primarily through the cochlear fluid, which boosts peak responses at the best-frequency location and consequently enhances hearing sensitivity and frequency selectivity.cochlea | outer hair cells | hearing | cochlear amplifier | interferometry A s the auditory sensory organ, the mammalian cochlea is able to detect soft sounds at levels as low as ∼20 μPa, equivalent to eardrum vibrations of <1-pm displacement (1), which is >100-fold smaller than the diameter of a hydrogen atom. The cochlea's remarkable sensitivity is commonly attributed to a micromechanical feedback system, which amplifies soft sounds using forces generated by outer hair cells (2-10). In addition to active bundle movements (2), mammalian outer hair cells are capable of changing their lengths in response to electrical stimulation in vitro (11)(12)(13)(14). This electromechanical force production is termed outer hair cell electromotility or somatic motility, which is produced by the membrane protein, prestin (15). Transgenic mice without prestin or with altered prestin suffer from severe hearing loss (4, 16). Targeted inactivation of prestin over well-defined cochlear segments dramatically reduces the traveling wave's peak response (17). Recent micromechanical measurements in sensitive living mouse cochleae showed (18) that electrical stimulation of outer hair cells induces vigorous vibrations of the reticular lamina at the apical surfaces of the outer hair cells. Whereas this experiment demonstrates that outer hair cells are capable of generating substantial forces to vibrate cochlear microstructures in vivo, the lack of frequency selectivity of the electrically induced reticular lamina vibration, however, is incons...

Thyroid hormone is necessary for cochlear development and auditory function, but the factors that control these processes are poorly understood. Previous evidence indicated that in mice, the serum supply of thyroid hormone is augmented within the cochlea itself by type 2 deiodinase, which amplifies the level of T(3), the active form of thyroid hormone, before the onset of hearing. We now report that type 3 deiodinase, a thyroid hormone-inactivating enzyme encoded by Dio3, is expressed in the immature cochlea before type 2 deiodinase. Dio3-/- mice display auditory deficits and accelerated cochlear differentiation, contrasting with the retardation caused by deletion of type 2 deiodinase. The Dio3 mRNA expression pattern in the greater epithelial ridge, stria vascularis, and spiral ganglion partly overlaps with that of thyroid hormone receptor beta (TRbeta), the T(3) receptor that is primarily responsible for auditory development. The proposal that type 3 deiodinase prevents premature stimulation of TRbeta was supported by deleting TRbeta, which converted the Dio3-/- cochlear phenotype from one of accelerated to one of delayed differentiation. The results indicate a protective role for type 3 deiodinase in hearing. The auditory system illustrates the considerable extent to which tissues can autoregulate their developmental response to thyroid hormone through both type 2 and 3 deiodinases.

Otoacoustic emissions, sounds generated by the inner ear, are widely used for diagnosing hearing disorders and studying cochlear mechanics. However, it remains unclear how emissions travel from their generation sites to the cochlear base. The prevailing view is that emissions reach the cochlear base via a backward-traveling wave, a slow-propagating transverse wave, along the cochlear partition. A different view is that emissions propagate to the cochlear base via the cochlear fluids as a compressional wave, a fast longitudinal wave. These theories were experimentally tested in this study by measuring basilar membrane (BM) vibrations at the cubic distortion product (DP) frequency from two longitudinal locations with a laser interferometer. Generation sites of DPs were varied by changing frequencies of primary tones while keeping the frequency ratio constant. Here, we show that BM vibration amplitude and phase at the DP frequency are very similar to responses evoked by external tones. Importantly, the BM vibration phase at a basal location leads that at a more apical location, indicating a traveling wave that propagates in the forward direction. These data are in conflict with the backwardtraveling-wave theory but are consistent with the idea that the emission comes out of the cochlea predominantly through compressional waves in the cochlear fluids. basilar membrane vibration ͉ cochlear traveling wave ͉ laser interferometer ͉ otoacoustic emission K emp (1, 2) discovered that the cochlea can generate sounds that are transmitted to the external ear canal through the middle ear. Such otoacoustic emissions (OAEs) are considered a by-product of normal cochlear performance (3), relying on a putative cochlear amplifier (4-6). Because OAEs can provide information on the health status of the cochlea and can be noninvasively measured, they are increasingly used for diagnosing auditory disorders and as a research tool for studying cochlear physiology (3). However, the applications of OAEs have been limited because the generation and transmission mechanisms are not clear.There are two different theories about OAE propagation from the generation site to the cochlear base. Propagation may either be dominated by a traveling wave moving in the backward direction (1, 7-9) or occur through a compressional wave in the cochlear fluids (10-16). Backward-traveling waves are slow and vibrate in the transverse direction, whereas compressional waves are longitudinal and travel at the speed of sound in water.Backward-traveling waves were first postulated in Kemp's original reports (1, 2). Support for this theory comes from indirect measurements, primarily of the delay between the stimulus and the appearance of the emission in the ear canal (17). Although the interpretation of such measurements is not unambiguous, it is commonly believed that the backward delay is the same as the forward delay. Consequently, the emission delay would be twice the forward delay (7, 9, 18). However, a direct proof of the theory requires backward waves to be me...

Auditory sensory outer hair cells are thought to amplify sound-induced basilar membrane vibration through a feedback mechanism to enhance hearing sensitivity. For optimal amplification, the outer hair cell-generated force must act on the basilar membrane at an appropriate time at every cycle. However, the temporal relationship between the outer hair cell-driven reticular lamina vibration and the basilar membrane vibration remains unclear. By measuring sub-nanometer vibrations directly from outer hair cells using a custom-built heterodyne low-coherence interferometer, we demonstrate in living gerbil cochleae that the reticular lamina vibration occurs after, not before, the basilar membrane vibration. Both tone- and click-induced responses indicate that the reticular lamina and basilar membrane vibrate in opposite directions at the cochlear base and they oscillate in phase near the best-frequency location. Our results suggest that outer hair cells enhance hearing sensitivity through a global hydromechanical mechanism, rather than through a local mechanical feedback as commonly supposed.