Interbulbar axonal collateralization and morphology of anterior olfactory nucleus neurons in the rat

The organizational patterns of the bilateral projections of the anterior olfactory nucleus (AON) to the main olfactory bulb (MOB) were defined in the rat with Golgi staining, HRP tracing-methods and fluorescent dyes. Three issues were addressed: (1) description of the morphology of the AON-neurons projecting to the MOB, (2) quantitative analysis of the bilateral pathways arising in different AON subdivisions and (3) ultrastructural identification of AON to MOB channels. The cytoarchitectural features of the AON as recognized in Golgi preparations were correlated with its neural architecture as revealed by retrograde HRP-tracing from the MOB. The following cell types were determined: (1) pyramidal like neurons typified by a lack of basal dendrites and a sparse covering with long spines (pars externa), (2) fusiform shaped cells with bipolar dendritic arborisations (pars medialis) and (3) densely spined fusiform, pyramidal, and polygonal neurons (pars ventroposterior, lateralis and dorsalis) with a tendency of radial orientation of their apical dendrites. In addition, in the more caudal parts of the pars ventroposterior there were neurons with tertiary dendritic processes oriented nearly parallel to the molecular layer. Quantitative analysis of AON neurons projecting to the MOB showed that the pars externa neurons project exclusively to the contralateral MOB while pars medialis neurons project almost exclusively to the ipsilateral MOB. All subdivisions of the AON which establish specific termination patterns within the MOB, participated in about equal portion in the ipsilateral projections to the MOB. The highest proportion of the bilaterally projecting neurons were found in the dorsal subdivision, followed by the lateral and ventroposterior subdivisions. The postsynaptic targets of the AON to MOB channel are the spinous processes and varicosities of the proximal and distal-most dendrites of granule cells. The boutons derived from AON projection neurons contained clear spherical vesicles and established exclusively asymmetric synaptic junctions.