Olfactory projections in a chondrostean fish, Acipenser baeri: an experimental study.

The presence of extrabulbar primary olfactory projections has been well established in teleosts. In order to investigate the phylogeny of these projections and to compare their targets with those of the secondary olfactory projections, the connections of the olfactory epithelium and olfactory bulb were studied by means of tract-tracing methods in a chondrostean, Acipenser baeri. Primary olfactory projections mainly extend to the glomerular layer of the olfactory bulb, but a significant number of extrabulbar efferent fibers course to various telencephalic and diencephalic regions. Both extrabulbar primary and olfactory bulb projections course in diffuse pathways. Extensive overlap was observed between the targets of these extrabulbar primary olfactory fibers and those of the secondary efferent projections of the olfactory bulb, though the latter were more numerous and reached additional targets. Tracer application to the olfactory bulb also revealed a number of bulbopetal neurons in the ipsilateral and contralateral telencephalic areas, as well as crossed interbulbar projections. The presence in a chondrostean of important extrabulbar primary projections and their extensive overlap with secondary olfactory projections suggest that such projections are a derived characteristic of bony fishes.