The cochlear apex demystified: Implications from synchrotron radiation phase-contrast imaging and microscopy for cochlear implantation.

Due to the complex organization of the human cochlear apex, further analysis of the tonotopic relationship between the organ of Corti (OC) and spiral ganglion (SG) is required in relation to cochlear implantation. In this study, the human SG nerve fiber organization and ultrastructure were assessed using semi-thin light microscopy sectioning and three-dimensional (3D) synchrotron radiation phase-contrast imaging (SR-PCI). A fresh human temporal bone underwent high-resolution SR-PCI with a dual-detector system. Orthogonal sectioning, cropping, and tissue segmentation were used to create high-resolution 3D reconstructions. Peripheral dendrites were traced from the basilar membrane to the SG, and a tonotopic map was constructed using Greenwood's function. Results were compared and validated against novel high-resolution microscopy data of a sectioned human cochlea. Only the basal and initial middle turn of the cochlea displayed a well-defined Rosenthal's canal (RC), and after 450 degrees, this converged into a central modiolar space. The OC and SG tonotopic maps remained closely aligned for angular depths up to approximately 650 degrees, after which the SG frequencies became significantly more spatially compact relative to the OC. In the central modiolus, the apical 1.37 mm of the SG contained over four octaves of tonotopic representation. In comparison, the compressed apical SG represented 9.6 mm of the OC (28% of the overall length) over the same tonotopic range. These results were validated with microscopy, which revealed that this apical SG contained around 8000 neurons and represented 960 inner hair cells along the OC. This is the first study to present the detailed cellular organization and 3D tonotopic arrangement of the human SG within the central modiolus. For low frequency stimulation, rate-based coding may be required to augment tonotopic mapping in the compressed SG regions. In addition, the OC tonotopic map has significantly less compression and could potentially be targeted directly for place-based coding.