The cytoarchitecture of the dorsal cochlear nucleus (DCN) was compared in 3- and 26-mo.-old C57BL/6 mice. The effects of genetically controlled progressive hearing loss present in the CNS in this mouse strain were analyzed with Nissl-stained and Golgi-impregnated material. The DCN was divided into the superficial molecular, an intermediate fusiform-granule and the deep polymorphic layers. The molecular layer (ML) consisted of many fibers and a few small ovoid to spherical, fusiform and granule cells. The fusiform-granule layer (FL) contained large fusiform and many granule cells. Most FL fusiform cells were oriented with their long axes perpendicular to the DCN surface and were present as small aggregations or individually. Cartwheel cells were adjacent to the FL fusiform cells. The deep polymorphic layer (PL) contained spherical, fusiform, granule and multipolar neurons. The granule cells formed a dorsal cap of the DCN. From this cap, sheets of granule cells separated the DCN from the posterior ventral cochlear nucleus (PVCN) and from the brainstem. The internal organization, neuronal location, orientation and morphology were similar in both age groups. The granule cells had 4-5 primary dendrites, varicosities and few to no dendritic appendages. The FL fusiform cells displayed different dendritic morphology in the 2 ages. One or two elaborate primary ML apical dendrites in the 3-mo.-old mice were covered with spikelike dendritic spines. The basal 1 or 2 PL dendrites were less elaborate and had few dendrite spines. FL fusiform neurons in 26-mo.-old mice had regular dendritic varicosities and fewer spines which were short and stumpy. Basal dendrites had varicosities and interruptions. Cartwheel neurons in 3-mo.-old mice had elaborate ML dendritic trees covered with dendritic spines. In 26-mo.-old mice the dendrites had many varicosities and fewer short blunted dendritic spines. Large multipolar neurons in older mice had thinner dendrites with more varicosities than were in the 3-mo.-old group. In both age groups, multipolar cells had few dendritic spines distally limited. Small and large spherical cells had 2-5 primary dendrites with varicosities, little higher-order branching and spines. Fusiform cells had 1 or 2 primary dendrites, little secondary branching and few to no spines. Minor degenerative changes were noted in spherical and fusiform cells in the 2 age groups. These included dendritic varicosities, interruptions and some irregularities of somata surface. Degenerative changes present in the cochlea had significant effects on a limited population of DCN neurons. The neuronal morphology and architecture of the DCN in C57BL/6 mouse is similar to other mammalian species.
