Imaging the living inner ear using intravital confocal microscopy

The summating potential polarity encodes the ear health condition

The summating potential (SP), the DC potential which, along with the AC response, is produced when the hair cells convert the vibrational mechanical energy of sound into electrical signals, is the most enigmatic of the cochlear potentials because its polarity and function have remained elusive for more than seven decades. Despite the tremendous socioeconomic consequences of noise-induced hearing loss and the profound physiological importance of understanding how loud noise exposure impairs the hair cell receptor activation, the relationship between the SP and noise-induced hearing impairment remains poorly characterized. Here, I show that in normally hearing ears, the SP polarity is positive and its amplitude relative to the AC response grows exponentially across frequencies, and becomes negative and decreases exponentially across frequencies following noise-induced hearing injury. Since the SP is thought to be generated by K⁺ outflow down the gradient through the hair cell basolateral K⁺ channels, the SP polarity switch to negative values is consistent with a noise-induced shift in the operating point of the hair cells.

Intravital deep-tumor single-beam 3-photon, 4-photon, and harmonic microscopy

Three-photon excitation has recently been demonstrated as an effective method to perform intravital microscopy in deep, previously inaccessible regions of the mouse brain. The applicability of 3-photon excitation for deep imaging of other, more heterogeneous tissue types has been much less explored. In this work, we analyze the benefit of high-pulse-energy 1 MHz pulse-repetition-rate infrared excitation near 1300 and 1700 nm for in-depth imaging of tumorous and bone tissue. We show that this excitation regime provides a more than 2-fold increased imaging depth in tumor and bone tissue compared to the illumination conditions commonly used in 2-photon excitation, due to improved excitation confinement and reduced scattering. We also show that simultaneous 3- and 4-photon processes can be effectively induced with a single laser line, enabling the combined detection of blue to far-red fluorescence together with second and third harmonic generation without chromatic aberration, at excitation intensities compatible with live tissue imaging. Finally, we analyze photoperturbation thresholds in this excitation regime and derive setpoints for safe cell imaging. Together, these results indicate that infrared high-pulse-energy low-repetition-rate excitation opens novel perspectives for intravital deep-tissue microscopy of multiple parameters in strongly scattering tissues and organs.

Inner hair cell stereocilia are embedded in the tectorial membrane

Mammalian hearing depends on sound-evoked displacements of the stereocilia of inner hair cells (IHCs), which cause the endogenous mechanoelectrical transducer channels to conduct inward currents of cations including Ca²⁺. Due to their presumed lack of contacts with the overlaying tectorial membrane (TM), the putative stimulation mechanism for these stereocilia is by means of the viscous drag of the surrounding endolymph. However, despite numerous efforts to characterize the TM by electron microscopy and other techniques, the exact IHC stereocilia-TM relationship remains elusive. Here we show that Ca²⁺-rich filamentous structures, that we call Ca²⁺ ducts, connect the TM to the IHC stereocilia to enable mechanical stimulation by the TM while also ensuring the stereocilia access to TM Ca²⁺. Our results call for a reassessment of the stimulation mechanism for the IHC stereocilia and the TM role in hearing.

Multi-contrast focal modulation microscopy for in vivo imaging of thick biological tissues

In vivo high resolution imaging of biological tissues is desirable for a wide range of biomedical applications. Recently focal modulation microscopy (FMM) has been developed and an imaging depth comparable to multi-photon microscopy (MPM) and optical coherence microscopy (OCM) has been achieved. Here we report the first focal modulation microscope that is capable of performing real-time fluorescence and scattering imaging simultaneously on thick biological tissues. A novel spatiotemporal phase modulator (STPM) has been designed and integrated into such a microscope to achieve high performances in terms of imaging speed, contrast, effective spatial resolution, signal to noise ratio, and compatibility with multiple excitation wavelengths.