Volumetric and histological changes in the cochlear nuclei of visually deprived rats: a possible morphological basis for intermodal sensory compensation.

The cochlear nucleus of 90 day-old rats was studied to determine whether bilateral visual deafferentation at birth induced compensatory structural changes in the auditory system. Computer-assisted reconstructions of the cochlear nucleus were used to demonstrate the three-dimensional organization of the nucleus and for calculating volumes of the subdivisions. In enucleated rats the volume of the granular region of the nucleus (GCD) increased 22.6% relative to controls, and the volume of the anterior ventral nucleus (AVCN) also increased by 10.0%. The posterior ventral nucleus (PVCN) and dorsal nucleus (DCN) were not affected. Since the volume change in GCD was substantial, this region was subjected to further analysis. Two types of granular cells were identified in the GCD of both control and enucleated rats with Kluver-Barrera staining. Type 1 cells contained an ovoid dark nucleus with clumpy chromatin and possessed only a very thin rim of cytoplasm. Type 2 cells were larger with pale spheroidal nuclei and with a more prominent cytoplasmic rim. There was an obvious stratification of these two cell types in the superficial layers over the lateral aspect of the AVCN in both groups. In contrast to this, the subpeduncular corner was not distinctively laminated and contained predominantly Type 2 cells. In enucleated animals the nuclear size of Type 1 cells increased by 3.8% while the size of Type 2 nuclei was not affected. Granule cell numbers increased by 28% in the visually deafferented rats. The GCD seems to be more highly developed in species which are dependent on non-visual modalities for spatial location. The regional hypertrophy of the GCD and AVCN seen in the present study may indicate that intermodal sensory compensation is taking place in the auditory system of visually deafferented rats.