A simple technique for introducing anterograde and retrograde tracers into the vestibular and cochlear sensory organs

The standard method for labeling the afferent and efferent innervation of the cochlear and vestibular sensory organs is by microinjection of tracer substances into the labyrinth. Injection of small amounts of tracer often result in incomplete and inconsistent labeling, but large injections can cause spurious labeling of brainstem structures due to diffusion from perilymph to cerebrospinal fluid. Effective labeling with minimal artifact can, however, be achieved by a relatively simple method involving placement of a tracer-saturated pledglet of gelatin sponge in the round window after rupture of its membrane. The gelatin sponge simultaneously acts as a continuous-release vehicle for the tracer and prevents reflux of perilymph and tracer into the middle ear cavity. Use of this technique produces labeling with a degree of intensity and anatomic detail that rivals that seen with more complicated methods of tracer placement.