Apical cochlear nerve exposed to perilymph in the gerbil and rat

There is considerable controversy regarding the origin and composition of perilymph in the scala vestibuli and scala tympani and the barriers and transport mechanisms between them. To elucidate the anatomical separation between perilymph and the extracellular fluid which surrounds the cochlear nerve in Mongolian gerbils (Meriones unguiculatus) and Sprague Dawley rats (Rattus rattus) we examined the modiolus, osseous spiral lamina and the bone of the perilymphatic scalae using light and electron microscopy. Although the cochlear nerve, spiral ganglion and their extracellular fluid are separated from perilymph by the bone of the modiolus in the middle and basal turns, no bone separates these neural structures from perilymph in the apical turn in the two species examined. Instead, the spiral ganglion and axonal elements of the apical turn were covered by a continuation of the bone lining cell layer of the scala tympani. Gaps between lining cells appeared to provide direct communication between the perilymphatic fluid and the extracellular fluid investing the cochlear nerve. Various authors have described openings in modiolar bone in both the scala vestibuli and the scala tympani. While the bone and cellular covering of the modiolus in the basal middle turns of the cochlea is not presumed to be a complete barrier to fluid exchange between the two scalae and the modiolar canal it can be expected to impose some limitation on the rate of passive diffusion. Therefore our data indicates that in the apical turn of the Mongolian gerbil and Sprague-Dawley rat there may be a more significant communication between perilymph and the extracellular fluid and neural elements of the apical modiolar canal than previously reported.