The development of laminar and tonotopic organization in the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) exemplifies the exquisite organization of the central nervous system in its segregation of cell types, specific distribution of afferent and efferent fibers, and distribution of neurotransmitters. Virtually all of these characteristics are arranged according to the tonotopic and/or laminar organization of the DCN. This study used immunocytochemical, histochemical, and quantitative techniques to examine some molecular factors related to development of the tonotopic and laminar organization of the DCN, including the role of glia, lectin labeling of glycoconjugates, and the development of one apparently non-laminar input (glycine).; Results suggest that changing patterns of glycoconjugates labeled by peanut agglutinin lectin correspond temporally and spatially to ingrowth of axons. A detailed quantitative study showed that labeling of these glycoconjugates diminished in intensity following the proliferation of neurites immunoreactive for glutamic acid decarboxylase. Glia, immunoreactive for glial fibrillary acidic protein, have processes present at birth that are continuous with the adjacent secondary rhombic lip. By postnatal day two, immunoreactive glial processes are oriented trans-strially throughout the DCN in planes parallel to the tonotopic planes of primary afferent axons. These data suggest that glia are related to the tonotopic organization of the DCN. Finally, glycine-like immunoreactive cell bodies and puncta are observed homogeneously throughout the DCN from birth through adulthood. Data on glycine-like immunoreactivity lend anatomical support to physiological studies suggesting that glycine mediates inhibition early in the development of the cochlear nucleus.; These data suggest that the development of lamination and tonotopic organization are complex events resulting from the interaction of various developmental mechanisms. For example, glia may be instrumental in determining the tonotopic organization of primary afferent axons in the DCN, but glycoconjugates may determine the extent of growth of these axons. It appears that these glycoconjugate "barriers" are not immutable, however, since some neurites increase their immunoreactivity for glutamic acid decarboxylase prior to diminished glycoconjugate labeling. Finally, although virtually every DCN input studied to date follows a laminar pattern of development, glycine is an exception. Since glycine shows an exceptional and homogeneous distribution, it may prove significant for further study.