Morphological and morphometric study of the superior vestibular nerve trunk in guinea pigs

The guinea pig has been chosen as a research model for otologic or neuropathic studies due to the relative ease of the cochlea, cochlear nerve, and vestibular nerve dissection. Little data have been reported on the normality of these nerves. The vestibular nerve is composed of the superior vestibular, inferior vestibular, and branch nerves. This study aimed to study the microscopic anatomy of the superior vestibular nerve (SVN) of guinea pigs using light microscopy and to search for normality patterns for use in experimental models in basic otologic research. We used eight male albino guinea pigs (Cavia porcellus, English strain), weighing between 400 and 500 g. After anesthetizing, the animals were perfused with a fixative solution of 2.5% glutaraldehyde. Dissection was performed by the access method to the temporal bone, coming to the rock and exposing the cochlea and vestibular nerve. The NVS fragments were removed, postfixed in osmium tetroxide, and embedded in the epoxy plastic resin Poly/Bed 812® (Polysciences Inc., Warrington, PA). Semi-thin transverse serial sections (0.5 μm) were made using a microtome MT6000-XL, RMC, Inc. and stained with toluidine blue. Morphology and morphometry were described and evaluated using the KS 400 application (Kontron 2.0, EchingBei, Munich, Germany) by macro, a computer program specially designed and developed for the study of the VIII nerve. The SVN was found to be devoid of epineurium, with only a thin conjunctive tissue layer. The myelin sheath of guinea pigs is relatively thin compared to the sensory and motor nerves found in mammals. The average fascicular area SVN was 0.19 ± 0.05 mm² , with the largest area found to be 0.24 mm² and the lowest was 0.12 mm² . The average number of fibers was 5,753.00 ± 538 fibers. The density of myelinated fibers reached 32,316.08 ± 11,375.29 fibers/mm² . Its diameter ranged from 1.0 to 9 μm and its peak was 3 μm. The measured results confirm the results of another study, indicating that the methodology is appropriate and reproducible. These findings are important for the evaluation of injured nerves in experimental models of peripheral neuropathy and basic ear disease.