A SIMPLE TECHNIQUE FOR INTRODUCING ANTEROGRADE AND RETROGRADE TRACERSINTO 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 t racer often result in incompete and inconsistent labeling, but large i njections can cause spurious labeling of brainstem structures due to d iffusion from perilymph to cerebrospinal fluid. Effective labeling wit h minimal artifact can, however, be achieved by a relatively simple me thod involving placement of a tracer-saturated pledglet of gelatin spo nge in the round window after rupture of its membrane. The gelatin spo nge simultaneously acts as a continuous-release vehicle for the tracer and prevents reflux of perilymph and tracer into the middle ear cavit y. Use of this technique produces labeling with a degree of intensity and anatomic detail that rivals that seen with more complicated method s of tracer placement.