Three-dimensional hard and soft tissue imaging of the human cochlea by scanning laser optical tomography (SLOT)

An affordable and adaptable Faraday isolator design for research

Presented here is an affordable Faraday isolator designed to account for conditions of high pulse energy and high total power lasers, with a clear input aperture allowing beam diameters up to 12 mm, and pulse energies and total powers limited by the dielectric crystal. This Faraday isolator is meant for laboratories with limited resources yet still need the features of commercial Faraday isolators. The design consists of a 3D printed mount housing seven permanent neodymium ring magnets, a Terbium Gallium Garnet (TGG) dielectric crystal, and two polarizing beam splitter cubes. Additionally, the design is customizable for different laser parameters, with the presented example isolator made for 1064 nm lasers. Measurements of the extinction ratio and a Stokes parameter analysis from different points within the isolator validated and characterized the constructed Faraday isolator. The final design had a measured minimum extinction ratio of 31 . 5 ± 0 . 3 dB and a maximum of 39 . 9 ± 0 . 2 dB depending on the polarization of back reflected light.

Anatomically and mechanically accurate scala tympani model for electrode insertion studies

The risk of insertion trauma in cochlear implantation is determined by the interplay between individual cochlear anatomy and electrode insertion mechanics. Whereas patient anatomy cannot be changed, new surgical techniques, devices for cochlear monitoring, drugs, and electrode array designs are continuously being developed and tested, to optimize the insertion mechanics and prevent trauma. Preclinical testing of these developments is a crucial step in feasibility testing and optimization for clinical application. Human cadaveric specimens allow for the best simulation of an intraoperative setting. However, their availability is limited and it is not possible to conduct repeated, controlled experiments on the same sample. A variety of artificial cochlear models have been developed for electrode insertion studies, but none of them were both anatomically and mechanically representative for surgical insertion into an individual cochlea. In this study, we developed anatomically representative models of the scala tympani for surgical insertion through the round window, based on microCT images of individual human cochleae. The models were produced in transparent material using commonly-available 3D printing technology at a desired scale. The anatomical and mechanical accuracy of the produced models was validated by comparison with human cadaveric cochleae. Mechanical evaluation was performed by recording insertion forces, counting the number of inserted electrodes and grading tactile feedback during manual insertion of a straight electrode by experienced cochlear implant surgeons. Our results demonstrated that the developed models were highly representative for the anatomy of the original cochleae and for the insertion mechanics in human cadaveric cochleae. The individual anatomy of the produced models had a significant impact on the insertion mechanics. The described models have a promising potential to accelerate preclinical development and testing of atraumatic insertion techniques, reducing the need for human cadaveric material. In addition, realistic models of the cochlea can be used for surgical training and preoperative planning of patient-tailored cochlear implantation surgery.

Three-dimensional mouse cochlea imaging based on the modified Sca/eS using confocal microscopy

The three-dimensional stria vascularis (SV) and cochlear blood vessel structure is essential for inner ear function. Here, modified Sca/eS, a sorbitol-based optical-clearing method, was reported to visualize SV and vascular structure in the intact mouse cochlea. Cochlear macrophages as well as perivascular-resident macrophage-like melanocytes were detected as GFP-positive cells of the CX3CR1^+/GFP mice. This study's method was effective in elucidating inner ear function under both physiological and pathological conditions.

Lightsheet Microscopy

In this paper, we review lightsheet (selective plane illumination) microscopy for mouse developmental biologists. There are different means of forming the illumination sheet, and we discuss these. We explain how we introduced the lightsheet microscope economically into our core facility and present our results on fixed and living samples. We also describe methods of clearing fixed samples for three-dimensional imaging and discuss the various means of preparing samples with particular reference to mouse cilia, adipose spheroids, and cochleae. © 2022 The Authors. Current Protocols published by Wiley Periodicals LLC.

Optimizing tissue clearing and imaging methods for human brain tissue

OBJECTIVES

To identify optimum sample conditions for human brains, we compared the clearing efficiency, antibody staining efficiency, and artifacts between fresh and cadaver samples.

METHODS

Fresh and cadaver samples were cleared using X-CLARITY™. Clearing efficiency and artifact levels were calculated using ImageJ, and antibody staining efficiency was evaluated after confocal microscopy imaging. Three staining methods were compared: 4-day staining (4DS), 11-day staining (11DS), and 4-day staining with a commercial kit (4DS-C). The optimum staining method was then selected by evaluating staining time, depth, method complexity, contamination, and cost.

RESULTS

Fresh samples outperformed cadaver samples in terms of the time and quality of clearing, artifacts, and 4',6-diamidino-2-phenylindole (DAPI) staining efficiency, but had a glial fibrillary acidic protein (GFAP) staining efficiency that was similar to that of cadaver samples. The penetration depth and DAPI staining improved in fresh samples as the incubation period lengthened. 4DS-C was the best method, with the deepest penetration. Human brain images containing blood vessels, cell nuclei, and astrocytes were visualized three-dimensionally. The chemical dye staining depth reached 800 µm and immunostaining depth exceeded 200 µm in 4 days.

CONCLUSIONS

We present optimized sample preparation and staining protocols for the visualization of three-dimensional macrostructure in the human brain.

Three-dimensional imaging of intact porcine cochlea using tissue clearing and custom-built light-sheet microscopy

Hearing loss is a prevalent disorder that affects people of all ages. On top of the existing hearing aids and cochlear implants, there is a growing effort to regenerate functional tissues and restore hearing. However, studying and evaluating these regenerative medicine approaches in a big animal model (e.g. pigs) whose anatomy, physiology, and organ size are similar to a human is challenging. In big animal models, the cochlea is bulky, intricate, and veiled in a dense and craggy otic capsule. These facts complicate 3D microscopic analysis that is vital in the cochlea, where structure-function relation is time and again manifested. To allow 3D imaging of an intact cochlea of newborn and juvenile pigs with a volume up to ∼ 250 mm3, we adapted the BoneClear tissue clearing technique, which renders the bone transparent. The transparent cochleae were then imaged with cellular resolution and in a timely fashion, which prevented bubble formation and tissue degradation, using an adaptive custom-built light-sheet fluorescence microscope. The adaptive light-sheet microscope compensated for deflections of the illumination beam by changing the angles of the beam and translating the detection objective while acquiring images. Using this combination of techniques, macroscopic and microscopic properties of the cochlea were extracted, including the density of hair cells, frequency maps, and lower frequency limits. Consequently, the proposed platform could support the growing effort to regenerate cochlear tissues and assist with basic research to advance cures for hearing impairments.

Scanning laser optical tomography in a neuropathic mouse model : Visualization of structural changes

BACKGROUND

In the field of hearing research a variety of imaging techniques are available to study molecular and cellular structures of the cochlea. Most of them are based on decalcifying, embedding, and cutting of the cochlea. By means of scanning laser optical tomography (SLOT), the complete cochlea can be visualized without cutting. The Ca_v1.3^-/- mice have already been extensively characterized and show structural changes in the inner ear. Therefore, they were used in this study as a model to investigate whether SLOT can detect structural differences in the murine cochlea.

MATERIALS AND METHODS

Whole undissected cochleae from Ca_v1.3^-/- and wild-type mice of various postnatal stages were immunostained and analyzed by SLOT. The results were compared to cochlea preparations that were immunostained and analyzed by fluorescence microscopy. In addition, cochlea preparations were stained with osmium tetraoxide.

RESULTS

Visualization by SLOT showed that the staining of nerve fibers at P27 in Ca_v1.3^-/- mice was almost absent compared to wild-type mice and earlier timepoints (P9). The analysis of cochlea preparations confirmed a reduction of the radial nerve fibers. In addition, a significantly reduced number of ribbon synapses per inner hair cell (IHC) at P20 and P27 in the apical part of the cochlea of Ca_v1.3^-/- mice was detected.

CONCLUSION

The visualization of whole non-dissected cochleae by SLOT is a suitable tool for the analysis of gross phenotypic changes, as demonstrated by means of the Ca_v1.3^-/- mouse model. For the analysis of finer structures of the cochlea, however, further methods must be used.

[Scanning laser optical tomography in a neuropathic mouse model : Visualization of structural changes. German version]

BACKGROUND

In the field of hearing research a variety of imaging techniques are available to study molecular and cellular structures of the cochlea. Most of them are based on decalcifying, embedding, and cutting of the cochlea. By means of scanning laser optical tomography (SLOT), the complete cochlea can be visualized without cutting. The Ca_v1.3^-/- mice have already been extensively characterized and show structural changes in the inner ear. Therefore, they were used in this study as a model to investigate whether SLOT can detect structural differences in the murine cochlea.

MATERIALS AND METHODS

Whole undissected cochleae from Ca_v1.3^-/- and wildtype mice of various postnatal stages were immunostained and analyzed by SLOT. The results were compared to cochlea preparations that were immunostained and analyzed by fluorescence microscopy. In addition, cochlea preparations were stained with osmium tetraoxide.

RESULTS

Visualization by SLOT showed that the staining of nerve fibers at P27 in Ca_v1.3^-/- mice was almost absent compared to wildtype mice and earlier timepoints (P9). The analysis of cochlea preparations confirmed a reduction of the radial nerve fibers. In addition, a significantly reduced number of ribbon synapses per inner hair cell (IHC) at P20 and P27 in the apical part of the cochlea of Ca_v1.3^-/- mice was detected.

CONCLUSION

The visualization of whole non-dissected cochleae by SLOT is a suitable tool for the analysis of gross phenotypic changes, as demonstrated by means of the Ca_v1.3^-/- mouse model. For the analysis of finer structures of the cochlea, however, further methods must be used.

Applicability of vital staining and tissue clearing to vascular anatomy and melanocytes' evaluation of temporal bone in six laboratory species

The purpose of the present study was to define the applicability of tissue clearing to the field of otology. We combined tissue clearing with vital staining perfusion via a pumping system to examine the vascular anatomy of temporal bones in laboratory animals. We used six different types of species including Korean wild mouse, mouse, Mongolian gerbil, hamsters and Guinea pigs. A mixture of Alcian blue reagent and 4% paraformaldehyde was circulated throughout the entire circulatory system of the animal via a perfusion pump system. Transparency images were obtained from the temporal bones according to the protocol of the SunHyun 3D Imaging Kit. In examining the inner surface of the tympanic membrane, flaccid part (pars flaccida) was positioned along the entire marginal area in Guinea pig. In the Guinea pig, unlike the other species, the cortical bone of the mastoid (bullae) was easily removed using cold instruments, allowing a direct approach to the enclosed structures. The distribution and pattern of cochlea melanocytes were compared among the species. "Mobius strip"-like accumulated melanocytes in vestibules were shown in both the Korean wild mouse and mouse. The collateral blood supply to the cochlea in six different species was checked in various pattern. Combining dye infusion with tissue-clearing techniques, we documented the middle ear and transparent inner ear structures in six different species. The information and associated images will help other researchers to develop hypotheses and design experimental investigations.

Cellular cartography of the organ of Corti based on optical tissue clearing and machine learning

The highly organized spatial arrangement of sensory hair cells in the organ of Corti is essential for inner ear function. Here, we report a new analytical pipeline, based on optical clearing of tissue, for the construction of a single-cell resolution map of the organ of Corti. A sorbitol-based optical clearing method enabled imaging of the entire cochlea at subcellular resolution. High-fidelity detection and analysis of all hair cell positions along the entire longitudinal axis of the organ of Corti were performed automatically by machine learning–based pattern recognition. Application of this method to samples from young, adult, and noise-exposed mice extracted essential information regarding cellular pathology, including longitudinal and radial spatial characteristics of cell loss, implying that multiple mechanisms underlie clustered cell loss. Our method of cellular mapping is effective for system-level phenotyping of the organ of Corti under both physiological and pathological conditions.

Enabling second harmonic generation as a contrast mechanism for optical projection tomography (OPT) and scanning laser optical tomography (SLOT)

Volumetric imaging of connective tissue provides insights into the structure of biological tissue. Second harmonic generation (SHG) microscopy has become a standard method to image collagen rich tissue like skin or cornea. Due to the non-centrosymmetric architecture, no additional label is needed and tissue can be visualized noninvasively. Thus, SHG microscopy enables the investigation of collagen associated diseases, providing high resolution images and a field of view of several hundreds of μm. However, the in toto visualization of larger samples is limited to the working distance of the objective and the integration time of the microscope setup, which can sum up to several hours and days. A faster imaging technique for samples in the mesoscopic range is scanning laser optical tomography (SLOT), which provides linear fluorescence, scattering and absorption as intrinsic contrast mechanisms. Due to the advantages of SHG and the reduced measurement time of SLOT, the integration of SHG in SLOT would be a great extension. This way SHG measurements could be performed faster on large samples, providing isotropic resolution and simultaneous acquisition of all other contrast mechanisms available, such as fluorescence and absorption. SLOT is based on the principle of computed tomography, which requires the rotation of the sample. The SHG signal, however, depends strongly on the sample orientation and the polarization of the laser, which results in SHG intensity fluctuation during sample rotation and prevents successful 3D reconstruction. In this paper we investigate the angular dependence of the SHG signal by simulation and experiment and found a way to eliminate reconstruction artifacts caused by this angular dependence in SHG-SLOT data. This way, it is now possible to visualize samples in the mesoscopic range using SHG-SLOT, with isotropic resolution and in correlation to other contrast mechanisms as absorption, fluorescence and scattering.