Chemistry of Granular and Closely Related Regions of the Cochlear Nucleus

Abstract

Granule cells of the cochlear nucleus have been described as having very small somata, about 6-10 urn diameter, containing a thin rim of cytoplasm around the nucleus, and 2-4 dendrites (Brawer et al., 1974; Kane, 1974; Mugnaini et al., 1980a,b; Weedman et al., 1996). They are located in many parts of the cochlear nucleus but are especially concentrated in granular regions, which we will define as collections of densely packed granule cells readily recognized in Nissl-stained sections. Most granular regions are located near the boundaries of the cochlear nucleus or its subdivisions: the anteroventral cochlear nucleus (AVCN), posteroventral cochlear nucleus (PVCN), and dorsal cochlear nucleus (DCN). The fusiform soma layer of the DCN is exceptional in that it lies within a subdivision and contains prominent large fusiform (pyramidal) cells as well as densely packed granule cells (Brawer et al., 1974; Mugnaini et al., 1980b; Wouterlood & Mugnaini, 1984; Osen, 1988). The small cell cap, or marginal zone, borders some granular regions and contains granule cells as well as many non-granule small cells, some of which have dendrites that enter the overlying granular layer (Brawer et al., 1974; Cant, 1993; Benson & Brown, 1990). It is sometimes described together with granular regions as a small cell or marginal shell (Zhao et al., 1995), but we will not include it in our consideration of granular regions because it lacks densely packed granule cells. The cochlear nucleus granular regions resemble those of the cerebellar cortex but with less dense packing. Our unpublished measurements in cat of DNA concentrations, which should be proportional to density of cell nuclei, gave values in the granular region dorsolateral to AVCN about a third of those in the cerebellar flocculus granular layer and about three times those in the AVCN.