In Situ 3D-Imaging of the Inner Ear Synapses with a Cochlear Implant

Abstract

In recent years sensorineural hearing loss was found to affect not exclusively, nor at first, the sensory cells of the inner ear. The sensory cells’ synapses and subsequent neurites are initially damaged. Auditory synaptopathies also play an important role in cochlear implant (CI) care, as they can lead to a loss of physiological hearing in patients with residual hearing. These auditory synaptopathies and in general the cascades of hearing pathologies have been in the focus of research in recent years with the aim to develop more targeted and individually tailored therapeutics. In the current study, a method to examine implanted inner ears of guinea pigs was developed to examine the synapse level. For this purpose, the cochlea is made transparent and scanned with the implant in situ using confocal laser scanning microscopy. Three different preparation methods were compared to enable both an overview image of the cochlea for assessing the CI position and images of the synapses on the same specimen. The best results were achieved by dissection of the bony capsule of the cochlea.

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“…Classical histological sectioning methods used to study the anatomy of the cochlea require sophisticated techniques and the implant must be removed [ 41 , 42 ]. Currently, confocal microscopy dominates studies of cochlear anatomy because of its high resolution and ability to visualise immunolabeled structures of interest, such as hair cells, spiral ganglion neurons [ 37 , 39 , 48 , 62 ]. Nevertheless, the acquisition of the whole cochlea in confocal microscopy is very difficult due to the long acquisition time that can lead to photobleaching of the samples and the difficulty of working with large samples [ 48 , 63 ].…”

“…Currently, confocal microscopy dominates studies of cochlear anatomy because of its high resolution and ability to visualise immunolabeled structures of interest, such as hair cells, spiral ganglion neurons [ 37 , 39 , 48 , 62 ]. Nevertheless, the acquisition of the whole cochlea in confocal microscopy is very difficult due to the long acquisition time that can lead to photobleaching of the samples and the difficulty of working with large samples [ 48 , 63 ]. To solve these problems, methods of dissection of the organ of Corti or removal of the bone capsule are used.…”

“…Removal of the bone capsule allows preservation of the helical shape. Unfortunately, these sample preparation disrupted surrounding structures [ 40 , 48 ]. In addition, even implanted specimens can be imaged using confocal microscopy without big circumstances, but in some cases, the implant induces scattered radiation of the laser beam, and in addition, it is impossible to image through the CI which hinders the imaging of tissue that is behind the implant [ 12 , 48 ].…”

“…Unfortunately, these sample preparation disrupted surrounding structures [ 40 , 48 ]. In addition, even implanted specimens can be imaged using confocal microscopy without big circumstances, but in some cases, the implant induces scattered radiation of the laser beam, and in addition, it is impossible to image through the CI which hinders the imaging of tissue that is behind the implant [ 12 , 48 ]. Although the resolution is lower than that of confocal laser scanning microscopy, light-sheet microscopy acquisitions are easy and allowed complete visualisation of the NaKATPase alpha 3 immunolabeled spiral ganglion ( Figure 3 ).…”

“…These imaging investigations (confocal microscopy and light sheet) require transparent samples. With the development of these imaging techniques, numerous clearing protocols have been developed in recent years and adapted to cochlear clarification [ 46 , 47 , 48 ].…”