Micro-architectures of the osseous spiral laminae and spiral limbus in the mouse cochlea: a scanning electron microscopic study on the morphological basis of the auditory mechanics.

Abstract

The auditory mechanics in the cochlea are closely related to the structural variations along the cochlear duct. In this study, the auditory teeth of the spiral limbus and the primary and secondary osseous spiral laminae in the mouse were examined along the entire course of the cochlear spiral by scanning electron microscopy (SEM) after removing the cochlear duct epithelium or the organic material with chemical maceration methods. The cochlear canal consisted of a hook and a spiral of one and half turns and the three-dimensional images and surface structures of the connective tissue forming the spiral limbus including the auditory teeth and the osseous spiral laminae are well demonstrated; the widths of the primary and secondary laminae, the spiral fissure between the two laminae, the tympanic lip, and the vestibular lip with the auditory teeth were measured, and the population density of the auditory teeth were determined in the hook and every half turn. The findings suggest that, in the cochlea the width of the basilar membrane increases in the hook and is almost constant in the spiral portion, the amount of fibers in the basilar membrane linearly decreases from the base to the apex, the displacement mode of the pillar cells during basilar membrane vibrations differs between the apical region lacking the bony edge of the tympanic lip and the other regions with bony edge, the connective tissue fibers from the auditory teeth are inserted to the bone surface of the primary lamina to support the teeth, the teeth on the marginal side of the vestibular lip maintain the stability of the tectorial membrane which is vibrating together with the basilar membrane in response to high frequency sounds on the basal side of the cochlear duct and low frequencies on the apical side, and the interdental cells on the Reissner membrane side maintain the size of the tectorial membrane which shows a linear increase in size from the base to the apex of the cochlear duct.