Auditory neuronal sizes after a unilateral conductive hearing loss.

The left external auditory meatus was removed in 4-day-old CBA/J mice; after killing at 45 days, serial sections of the cochleae and brain stem were prepared. From these, the cross-sectional areas of spiral ganglion neurons and of 14 auditory brain stem neuronal types were measured, using a total of 210 neurons of each of the 15 types from both the right and left sides. Nine neuronal types were significantly smaller (P less than 0.01) on the left side: spiral ganglion neurons; globular, small spherical, large spherical, octopus, multipolar, and granule cells of the ventral cochlear nucleus; Purkinje-like cells of the dorsal cochlear nucleus; and spindle cells of the lateral superior olivary nucleus. Two neuronal types were significantly smaller (P less than 0.01) on the right: principal cells of the medial nucleus of the trapezoid body (superior olivary complex), and spindle-shape principal neurons of the central nucleus of the inferior colliculus. The left ventral cochlear nucleus had significantly smaller volume (P less than 0.01) than the right but right and left dorsal cochlear nuclear volumes did not differ significantly (P greater than 0.05). Right and left sides were not significantly different (P greater than 0.05) for the following neuronal types: fusiform cells and coarse- and fine-Nissl deep cells of the dorsal cochlear nucleus, and rostral bipolar cells of the medial superior olivary nucleus. Neurons affected by unilateral conductive loss were not significantly different (P greater than 0.05) from the same cells in mice with bilateral conductive losses; neurons not affected by unilateral conductive loss were not significantly different (P greater than 0.05) from the same cells in normal mice.